The health and psychological consequences of cannabis use - chapter 7

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7. The psychological effects of chronic cannabis use

7.3 Is there a cannabis dependence syndrome?

7.3.1 The significance of dependence

If there is a cannabis dependence syndrome, it has important implications for both cannabis users and public health (Edwards, 1982). First, people who currently use cannabis, and young adults who are considering whether to use it, should make decisions which are informed by an appraisal of the risk of their becoming dependent on the drug. If there is a risk of dependence, and cannabis continues to be regarded as a drug that does not produce dependence, such decisions cannot be informed.

Second, if there is a cannabis dependence syndrome, then persons who become dependent on cannabis place themselves at an increased risk of experiencing any adverse health effects attributable to cannabis use. Dependent cannabis users typically smoke two or more cannabis cigarettes daily over many years, putting themselves at risk of the pulmonary hazards of smoking. A chronic state of cannabis intoxication could place them at increased risk of accidents, and the THC they absorb may accumulate in their bodies, placing them at increased risk of experiencing any adverse health effects of THC (Edwards, 1982).

Third, although a dependent pattern of cannabis use may be rare in comparison with the more prevalent pattern of experimental and intermittent use, it may nonetheless have public health significance because of the widespread experimentation with cannabis in many Western societies. The public health significance of cannabis dependence would also increase if the prevalence of use substantially increased as a result of changes in the availability of the drug.

7.3.2 The nature of dependence

For much of the 1960s and 1970s the apparent absence of tolerance to the effects of cannabis, and of a withdrawal syndrome analogous to that seen in alcohol and opioid dependence, supported the consensus of informed opinion that cannabis was not a drug of dependence. Expert views on the nature of dependence changed during the late 1970s and early 1980s, when the more liberal definition of drug dependence embodied in Edwards and Gross's (1976) alcohol dependence syndrome was extended to all psychoactive drugs (Edwards et al, 1981). The drug dependence syndrome reduced the emphasis upon tolerance and withdrawal, and attached greater importance to symptoms of a compulsion to use, a narrowing of the drug using repertoire, rapid reinstatement of dependence after abstinence, and the high salience of drug use in the user's life. This new conception influenced the development of the Third Revised Edition of the Diagnostic and Statistical Manual of the American Psychiatric Association (1987) (DSM-III-R), which reduced the importance of tolerance and withdrawal symptoms in favour of a greater emphasis upon continued use of a drug in the face of its adverse effects.

Top of page Drug dependence in DSM-III-R

"Psychoactive substance use disorders" include all forms of drug and alcohol dependence in DSM-III-R (American Psychiatric Association, 1987; Kosten et al, 1987). "The essential feature of this disorder is a cluster of cognitive, behavioral and physiologic symptoms that indicate that the person has impaired control of psychoactive substance use and continues use of the substance despite adverse consequences" (p166). A diagnosis of psychoactive substance dependence is made if any three of the nine criteria listed below have been present for one month or longer:
  1. the substance is often taken in larger amounts or over a longer period than the person intended;
  2. there is a persistent desire or one or more unsuccessful efforts to cut down or control substance use;
  3. a great deal of time is spent in activities necessary to get the substance (e.g., theft), taking the substance..., or recovering from its effects;
  4. frequent intoxication or withdrawal symptoms when expected to fulfil major role obligations at work, school, or home..., or when substance use is physically hazardous...;
  5. important social, occupational, or recreational activities given up or reduced because of substance use;
  6. continued substance use despite knowledge of having a persistent or recurrent social, psychological, or physical problem that is caused or exacerbated by the use of the substance;
  7. marked tolerance;
  8. characteristic withdrawal symptoms;
  9. substance often taken to relieve or avoid withdrawal symptoms" (American Psychiatric Association, 1987, pp167-8).

Criteria 8 and 9, are not required for the dependence syndromes of cannabis, hallucinogens and PCP to be diagnosed.

These criteria may seem to conflict with community conceptions of drug dependence, in that they explicitly include tobacco smoking as a form of drug dependence, and could conceivably include caffeine dependence (among heavy coffee drinkers). The fact that these forms of drug taking are not usually be regarded as producing drug dependence is less a reason for rejecting these diagnostic criteria than a signal of the need to persuade the community to adopt a broader conception of drug dependence, which reduces the emphasis upon "physical" dependence as evidenced by the occurrence of a marked withdrawal syndrome on abstinence.

Top of page Cannabis tolerance and withdrawal: experimental evidence

Although tolerance and withdrawal symptoms are not required within DSM-III-R, there is evidence that both can occur under certain conditions of dosing with cannabinoids. This should not be surprising since, as Hollister (1986) has observed, cannabis "would have been an exceptional centrally acting drug if tolerance/dependence were not one of its properties" (p9). Yet for many years it was believed that there was little tolerance to cannabis and no withdrawal syndrome. The predominant recreational pattern of intermittent use in the community, and the use of low doses of THC and short dosage schedules in laboratory studies, contributed to this belief (Hollister, 1986), as did the expectation that if there was a cannabis withdrawal syndrome, it would be as readily recognised as the opioid withdrawal syndrome (Edwards, 1982).

Since the middle 1970s evidence has emerged from human and animal studies that chronic administration of high doses of THC results in the development of marked tolerance to a wide variety of cannabinoid effects, such as cardiovascular effects, and to the subjective high in humans (Compton, Dewey, and Martin, 1990; Fehr and Kalant, 1983; Hollister, 1986; Jones, Benowitz, and Herning, 1981; National Academy of Science, 1982). Moreover, the abrupt cessation of chronic high doses of THC generally produces a mild withdrawal syndrome like that produced by other long-acting sedative drugs (Compton et al, 1990; Jones and Benowitz, 1976; Jones et al, 1981).

Jones and Benowitz (1976) provided convincing evidence in humans of the development of tolerance to the cardiovascular and subjective effects of THC. They conducted human laboratory studies of the effects of high doses of THC (210 mg per day) administered orally over a period of 30 days on a fixed dosing schedule to healthy male volunteers who had an extensive history of cannabis use. Clinical observations of the subjects showed that as the duration of the high dose regimen increased, there was a decline in the positive effects of intoxication, and in the subjects' ratings of the "high". There was a marked deterioration in the subjects' social functioning according to nurses' ratings during the early days of the high dose regimen, but there was almost complete recovery to baseline levels by the end of the dosing period. There was similar evidence of recovery in cognitive and psychomotor performance in the course of the high dose regimen.

The most convincing evidence of tolerance came from observations of the cardiovascular and subjective effects of smoking a marijuana cigarette at various points during the study. The magnitude of both the cardiovascular and subjective responses to smoking a single "joint" decreased with the length of time subjects had received a high dose of THC. After a few days of high doses of THC, the increased heart rate was replaced by a normal, and in some cases a slowed, heart rate. Similarly, self-ratings indicated that the "high" produced by the cigarette all but disappeared in the course of the high dose regimen.

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Similar observations of tolerance to the subjective effects of cannabis have been made by Georgotas and Zeidenberg (1979). They studied five healthy male marijuana smokers over a four-week period, in which they smoked an average of 10 joints per day, providing an average daily dose of 210mg of THC. In the course of this experiment, subjects rapidly developed tolerance to the drug's effects:

Although initially they found the marijuana to be of good quality, they now found it much weaker and inferior to what they were getting outside. They felt it did not make them as high as often as they were accustomed (p429).

An abstinence syndrome has been observed in monkeys maintained on a schedule of chronic high doses of THC. Its symptoms consisted of: "yawning, anorexia, piloerection, irritability, tremors and photophobia" (Jones and Benowtiz, 1976). Similar symptoms were observed by Jones and Benowitz (1976) after their subjects were abruptly withdrawn from high doses of THC. Within six hours of withdrawal subjects complained of "inner unrest", and by 12 hours, "increased activity, irritability, insomnia, and restlessness were reported by the subjects and obvious to staff" (p632). Common symptoms reported were " `hot flashes', sweating, rhinorrhea, loose stools, hiccups and anorexia" (p632) which many subjects compared to a bout of influenza. These symptoms were reduced by the resumption of marijuana use (Jones et al, 1981).

Georgotas and Zeidenberg (1979) reported similar withdrawal phenomena in their long-term dosing study. During the first week of a four-week wash-out period after four weeks of receiving 210mg of cannabis a day, the subjects "became very irritable, uncooperative, resistant, and at times hostile ... their desire for food decreased dramatically and they had serious sleeping difficulties" (p430). These effects disappeared during the final three weeks of the wash out. These studies suggest that tolerance can develop to cannabis's effects and that a withdrawal syndrome can occur on abstinence under certain conditions, namely, chronic administration of doses as low as 10 mg per day for 10 days (Jones et al, 1981).

The results of laboratory studies have received suggestive support from a small number of studies of heavy cannabis users. Weller and Halikas (1982), for example, found that the self-reported positive effects of cannabis use diminished over a five to six-year period in regular users of cannabis. The average reduction in the frequency of experiencing the positive effects was small, perhaps because only 27 per cent were daily users, but they were consistent and included some of the symptoms reported in laboratory studies.

The laboratory and observational studies raise the following questions: How relevant are these observations to contemporary cannabis users? How often does sufficient tolerance to cannabis develop for users to experience a withdrawal syndrome? How often is cannabis used to relieve or avoid withdrawal symptoms, and if so, does such behaviour play any role in maintaining use and producing dependence? These questions remain unanswered (Edwards, 1982; Jones, 1984), although (as will be seen below) there is clinical and observational evidence that some heavy chronic users experience tolerance and withdrawal symptoms, and that some use cannabis to control these symptoms.

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7.3.3 Clinical and observational evidence on dependence

There has not been an organised program of research on the cannabis dependence syndrome comparable to that undertaken on the alcohol and the opiate dependence syndromes. Instead, its existence and characteristics have had to be inferred from a diverse body of research studies. This comprises: limited data on the prevalence and characteristics of persons seeking professional help in dealing with their cannabis use and associated problems; a small number of observational studies of problems reported by non-treatment samples of long-term cannabis users; and a very small and recent literature examining the validity of the cannabis dependence syndrome, usually as part of larger investigations of the validity of the substance dependence syndromes embodied in DSM-III-R and other classification systems.

During the 1980s evidence began to emerge that there had been an increase in the number of persons seeking help with cannabis as their major drug problem. Jones (1984), for example, reported that 35,000 patients sought treatment in the United States in 1981 for drug problems in which "cannabis was their primary drug" (p703), an increase of 50 per cent over three years. Many of these patients behaved "as if they were addicted to cannabis" and they presented "some of the same problems as do compulsive users of other drugs" (p711). More recently, Roffman and colleagues (1988) have reported a strong response to a series of community advertisements offering help to people who wanted to stop using marijuana.

Sweden, which has had a long history of hashish use, has also experienced an increase in numbers of heavy hashish users presenting to treatment services for assistance with problems caused by its use (Engstrom et al, 1985). Tunving et al (1988) have described their experience treating approximately 100 individuals per year who presented to Swedish treatment services requesting help in controlling their cannabis use. Although no data were reported on the proportion of these individuals who satisfied the DSM-III-R criteria for cannabis dependence, these patients typically complained of symptoms which arguably would meet some of its criteria. They reported, for example, that they had been unable to stop using cannabis after having made several unsuccessful attempts to stop or cut down, that they were frequently intoxicated, often every day, and that they continued to use despite suffering adverse effects which they recognised were connected with their cannabis use, such as sleeplessness, depression, diminished ability to concentrate and memorise, and blunting of emotions. Hannifin (1988) and Miller and Gold (1989) have reported similar behaviour patterns among cannabis users who have sought assistance.

In Australia, there are indications that some heavy cannabis users request help in controlling their use. Didcott et al (1988), for example, reported on the characteristics of 3,462 clients seen in 12 residential treatment services in New South Wales in 1985 and 1986. They found that cannabis was identified as the "primary drug problem" by 25 per cent of clients seen, second only to the opioid drugs, which were so identified by 73 per cent of clients. Just over half of all clients (52 per cent), the majority of whom were polydrug users, identified their cannabis use as "a problem". The prevalence of cannabis use as a principal drug problem was lower in a 1992 National Census of Clients of Australian Treatment Service Agencies (Chen, Mattick and Bailey, 1993). In this census cannabis use was the main drug problem for 6 per cent of the 5,259 clients, fifth in order of importance behind alcohol (52 per cent), opiates (26 per cent), tobacco (9 per cent) and opiate/polydrug problems (7 per cent).

Suggestive evidence of cannabis dependence has emerged from a small number of observational studies of regular cannabis users. Weller, Halikas and Morse (1984), for example, followed up a cohort of 100 regular marijuana users who were first identified in 1970-1971, and assessed them for alcohol and marijuana abuse using Feighner's criteria for alcoholism and an analogous set of criteria for marijuana (see Weller and Halikas, 1980). Their concept of abuse would arguably have included most cases of dependence. They were able to interview 97 of their subjects about the amount and frequency of alcohol and marijuana use, and their experience of problems related to the use of both drugs. According to Feighner's criteria, 9 per cent of subjects were alcoholic and 9 per cent were "abusers" of marijuana, with 2 per cent qualifying for both diagnoses. The most common symptoms reported among those classified as marijuana abusers were feeling "addicted", a history of failed attempts to limit use, early morning use, and traffic arrests related to marijuana use.

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Hendin et al (1987) reported on the experiences of 150 long-term daily cannabis users who had been recruited through newspaper advertisements. Although they did not explicitly inquire about the symptoms of a cannabis dependence syndrome, substantial proportions of their sample reported experiencing various adverse effects of long-term use, despite which they continued to use cannabis. These included: impaired memory (67 per cent); an impaired ability to concentrate on complex tasks (49 per cent); difficulty getting things done (48 per cent); or thinking clearly (43 per cent); reduced energy (43 per cent); ill health (36 per cent); and accidents (23 per cent). Substantial minorities reported that it had impeded their educational (31 per cent), and career achievements (28 per cent), and half of the sample reported that they would like to cut down or stop their use.

These findings have been broadly supported by Kandel and Davies (1992) and by Stephens and Roffman (1993). Kandel and Davies reported on the characteristic problems reported by near daily cannabis users (aged 28-29 years) who were identified in a prospective study of the consequences of adolescent drug use. The major adverse consequences of use were: subjectively experienced cognitive deficits; reduced energy; depression; and problems with spouse. Stephens and Roffman's sample of users answering an advertisement offering assistance in quitting cannabis complained of: "feeling bad about using"; procrastinating because of their use; memory impairment; loss of self-esteem; withdrawal symptoms; and spouse complaints about their use. In the absence of control groups, however, it is impossible to be certain that the prevalence of these symptoms is higher than in the community, and that they were not present prior to cannabis use, as has been reported in some longitudinal studies (e.g. Shedler and Block, 1990).

The most direct support for the validity of the cannabis abuse dependence syndrome comes from a series of studies of the validity of diagnostic criteria for substance dependence. Kosten et al (1987) tested the extent to which the DSM-III-R psychoactive substance dependence disorders for alcohol, cannabis, cocaine, hallucinogens, opioids, sedatives and stimulants constituted syndromes. A sample of 83 persons (41 from an inpatient psychiatric unit and 42 from an outpatient substance abuse treatment unit) was interviewed using a standardised psychiatric interview schedule to elicit the symptoms of drug dependence as defined in DSM-III-R for each of the drug classes. Multiple diagnoses were allowed, so many individuals qualified for more than one type of drug dependence.

There was consistent support for a unidimensional dependence syndrome for alcohol, cocaine and opiates. The results were more equivocal in the case of the cannabis dependence syndrome. All the items were moderately positively correlated, had good internal consistency, and seemed to comprise a Guttman scale, but a Principal Components Analysis of the cannabis items suggested that (unlike alcohol, cocaine and heroin, all of which had a single underlying factor) there seemed to be three independent dimensions of dependence: compulsion indicated by impaired social activity attributable to drug use, preoccupation with drug use, giving up other interests, and using more than intended; inability to stop use, indicated by not being able to cut down the amount used, rapid reinstatement after abstinence, and tolerance to drug effects; and withdrawal identified by withdrawal symptoms, use of cannabis to relieve withdrawal symptoms, and continued use despite problems.

Two more recent studies on much larger samples have provided stronger support for the concept of a cannabis dependence syndrome. Newcombe (1992) reported factor analyses of 29 questionnaire items designed to measure DSM-III-R abuse and dependence for a community sample of 614 young adults reporting on their use of alcohol, cocaine, and cannabis. He reported a strong common factor for all three drug types which accounted for 36 per cent to 40 per cent of the item variance. Rounsaville, Bryant, Babor, Kranzler and Kadden (1993) report the results of factor analyses of items designed to assess dependence in each of three diagnostic systems (DSM-III-R. DSM-IV and ICD-10) for each of six drug classes (alcohol, cocaine, marijuana, opiates, sedatives and stimulants). Their sample comprised 521 persons recruited from inpatient and outpatient drug treatment, psychiatric treatment services, and the general community. They found that a single common factor explained the variation between diagnostic criteria for all diagnostic systems, and for all drug types.

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7.3.4 Epidemiological evidence on cannabis abuse and dependence

The best evidence on the prevalence of cannabis abuse and dependence in the community comes from the Epidemiological Catchment Area (ECA) study (Robins and Regier, 1991) which involved face-to-face interviews with 20,000 Americans in five catchment areas: Baltimore, Maryland; Los Angeles, California; New Haven, Connecticut; Durham, North Carolina; and St Louis, Missouri. A standardised and validated clinical interview schedule was used to elicit a history of psychiatric symptoms found in 40 major psychiatric diagnoses, including drug abuse and dependence. This information was used to diagnose the presence or absence of a DSM-III diagnosis of drug dependence (Anthony and Helzer, 1991). Although not a true random sample of the American population, it is the best available data on the prevalence of different types of drug dependence and their correlates in a non-treatment population.

Illicit drug use was defined as "any non-prescription psychoactive agents other than tobacco, alcohol and caffeine, or inappropriate use of prescription drugs" (Anthony and Helzer, 1991, p116). To exclude individuals who had only briefly experimented with illicit drugs, individuals had to have used an illicit drug on more than five occasions before they were asked about any symptoms of drug dependence. The focus of the interview schedule was on the "consequent psychiatric symptoms and behavioral changes that constitute the syndromes of drug abuse and dependence" (p117).

The criteria used to define drug abuse and dependence were derived from the DSM-III, which divided symptoms of abuse and dependence into four main groups: (1) tolerance to drug effects; (2) withdrawal symptoms; (3) pathological patterns of use; and (4) impairments in social and occupational functioning due to drug use. Drug abuse required a pattern of pathological use and impaired functioning. In the case of cannabis, a diagnosis of dependence required pathological use, or impaired social functioning, in addition to either signs of tolerance or withdrawal. The problem had to have been present for at least one month, although there was no requirement that all criteria had to be met within the same period of time. In reporting the results Anthony and Helzer report the prevalence of abuse and/or dependence combined for all drug types.

Illicit drug use was relatively common in the sample, with 36 per cent of persons having used at least one illicit drug. Cannabis was the most commonly used illicit drug, having been used by 76 per cent of those who had used any illicit drug more than five times. Drug abuse and dependence were relatively common, with 6.2 per cent of the population qualifying for such a diagnosis. Cannabis abuse and/or dependence was the most common form of abuse and/or dependence, with 4.4 per cent of the population being so diagnosed compared with 1.7 per cent for stimulants, 1.2 per cent for sedatives, and 0.7 per cent for opioid drugs. Two-thirds of cases of cannabis abuse and/or dependence had used cannabis within the past year, and half had used within the past month. "Almost two-fifths (38 per cent) of those with a lifetime history of cannabis abuse and/or dependence reported active problems in the prior year" (Anthony and Helzer, 1991, p123)

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When DSM-III-R diagnoses of dependence and abuse were approximated, three fifths of those with a diagnosis of dependence and/or abuse met the criteria for dependence. The proportion of current users who were dependent increased with age, from 57 per cent in the 18-29 year age group to 82 per cent in the 45-64 year age group, reflecting the remission of less severe drug abuse problems with age. Only a minority of those who had a diagnosis of abuse and/or dependence (20 per cent of men and 28 per cent of women) had mentioned their drug problem to a health professional, even though 60-70 per cent had sought medical treatment in the previous month. There were predictable age and gender differentials in prevalence of drug abuse and/or dependence. Men had higher prevalence than women (7.7 per cent versus 4.8 per cent). This was largely due to differences in exposure to illicit drugs, since the prevalence of a diagnosis of abuse and/or dependence among persons who had used an illicit drug more than five times was the about the same for men and women (21 per cent and 19 per cent). The highest prevalence of abuse and/or dependence (13.5 per cent) was in the 18-29 year age group (16.0 per cent among men and 10.9 per cent among women), declining steeply thereafter in both sexes.

It is difficult to make clear inferences about the prevalence of cannabis dependence in the community from the ECA study, because DSM-III rather than DSM-III-R criteria were used, and the data on the prevalence of drug abuse and/or dependence have not been broken down either by abuse and dependence or by drug class. The first of these problems may not be too serious, since studies comparing DSM-III and DSM-III-R criteria (e.g. Rounsaville et al, 1987) suggest that there is reasonable agreement between a DSM-III diagnosis of abuse or dependence and DSM-III-R dependence, in the case of cannabis dependence. Any disagreements in diagnosis seem to be in the direction of DSM-III-R identifying more cases as dependent than DSM-III, suggesting that any errors in the prevalence of drug abuse in the ECA study will be in the direction of underestimation.

The absence of detailed ECA reports on the separate prevalence of drug abuse and dependence is more difficult to circumvent. If we assume that any differences between drug types in the proportion of users who became dependent would have been reported (and hence that the ratio of cases of dependence to abuse for cannabis is 3:2), then the prevalence of cannabis dependence in the USA in 1982-1983 would have been 2.6 per cent of the population. If we also assume that the ratio of cases of cannabis dependence to cases of cannabis abuse was the same for men and women, then 3.2 per cent of men and 2.0 per cent of women would have been diagnosed as cannabis dependent.

Similar estimates of the population prevalence of cannabis dependence were produced by a community survey of psychiatric disorder conducted in Christchurch, New Zealand, in 1986, using the same sampling strategy and diagnostic interview as the ECA study (Wells et al, 1992). This survey used the DIS to diagnose a restricted range of DSM-III diagnoses in a community sample of 1,498 adults aged 18-64 years of age. The prevalence of having used cannabis on five or more occasions was 15.5 per cent, remarkably close to that of the ECA estimate, as was the proportion who met DSM-III criteria for marijuana abuse or dependence, namely 4.7 per cent. The fact that this survey largely replicated the ECA findings for most other diagnoses, including alcohol abuse and dependence, enhances confidence in the validity of the ECA study findings.

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7.3.5 The risk of cannabis dependence

It is important to put the existence of a cannabis dependence syndrome into perspective to avoid a falsely alarmist impression that all cannabis users run a high risk of becoming dependent upon cannabis. A variety of estimates suggest that the crude risk is small, and probably more like that for alcohol rather than nicotine or the opioids. Other data suggests that certain characteristics of users increase the risk of dependence developing, although in most cases it is impossible to place quantitative estimates on the latter risks.

As with all drugs of dependence, persons who use cannabis on a daily basis over periods of weeks to months are at greatest risk of becoming dependent upon it. The ECA data suggested that approximately half of those who used any illicit drug on a daily basis satisfied DSM-III criteria for abuse or dependence (Anthony and Helzer, 1991). Since this estimate was based upon drug abuse and dependence for all drug types, including opioids, it probably overestimates the risks of dependence among daily cannabis users. Kandel and Davis (1992) estimated the risk of dependence among near daily cannabis (according to approximated DSM-III criteria) at one in three.

The risk of developing dependence among less frequent users of cannabis, including experimental and occasional users, would be substantially less than that for daily users. A number of reasonably consistent estimates of the risks of a broader spectrum of users becoming dependent on cannabis can be obtained from recent studies. A crude estimate from the ECA study was that approximately 20 per cent of persons who used any illicit drug more than five times met DSM-III criteria for drug abuse and dependence at some time. The specific rate of abuse and dependence for cannabis (calculated by dividing the proportion who met criteria for abuse and dependence by the proportion who had used the drug more than five times) was 29 per cent. A more conservative estimate which removed cases of abuse (40 per cent) from the overall estimate of cannabis abuse and dependence would be that 17 per cent of those who used cannabis more than five times would meet DSM-III criteria for dependence.

Estimates derived from a number of other studies suggest that the ECA estimates of the risk of dependence are reasonable. The crude percentage of cases of dependence and abuse among persons who had used cannabis five or more times in the Christchurch epidemiology study (Wells et al, 1992) was 30 per cent, while an estimate derived from Newcombe's community survey of young adults was 25 per cent of those who had ever used cannabis. A comparable estimate can be derived from Kandel and Davies' (1992) study of near daily cannabis users. [This was done by multiplying the ECA estimate of the proportion of daily users who met criteria for abuse and dependence (50 per cent) by the proportion of near daily users in Kandel and Davis sample (44 per cent), and adding this to the ECA estimate of the proportion of non-daily illicit drug users who met the criteria (30 per cent) multiplied by their proportion in the Kandel and Davies sample (55 per cent)]. On Kandel and Davies data the estimated rate of abuse and dependence among those who had used cannabis 10 or more times was 39 per cent, the higher rate reflecting the higher number of times of use required to be counted as a cannabis user in Kandel and Davies study (10 times versus five times in ECA). A lower estimate of 12 per cent for DSM-III-R cannabis dependence was obtained by McGee and colleagues (1993) in a prospective study of 18-year-old youth in Dunedin, New Zealand. A lower estimate was to be expected given the youth of the sample, and the fact that the estimate is the proportion of dependent users among those who had ever used cannabis.

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Although one would not want to claim a great deal of precision for any of these individual estimates of the risk of cannabis dependence, it is reassuring that they are within a range of 12-37 per cent, and that the estimates vary in predictable ways with the ages of the samples and the stringency of the criteria used in defining cannabis use. The reasonable consistency of the estimates suggests the following rules of thumb about the risks of cannabis dependence. For those who have ever used cannabis, the risks of developing dependence is probably of the order of one chance in 10. The risk of dependence rises with the frequency of cannabis use, as it does with all drugs, so that among those who use the drug more than a few times the risk of developing dependence is in the range of from one in five to one in three. The range of the estimates reflects variations in the number of occasions of use that is taken to reflect more than simple experimentation, with the general rule being that the more often the drug has been used, and the longer the period of use, the higher is the risk of becoming dependent. Although there have been few formal comparisons of the dependence potential of cannabis with that of other drugs, these risks are probably more like those associated with alcohol than those associated with tobacco and opiates (Woody, Cottler and Cacciola, 1993).

Apart from frequency of use, other risk factors have been identified in the series of prospective studies of adolescent illicit drug use reviewed above. These include the following factors which have been shown to predict continued use and more intensive involvement with illicit drugs: poor academic achievement; deviant behaviour in childhood and adolescence; nonconformity and rebelliousness; personal distress and maladjustment; poor parental relationships; earlier use; and a parental history of drug and alcohol problems (Brook et al, 1992; Kandel and Davies, 1992; Newcombe, 1992; Shedler and Block, 1990). For most of these variables it is difficult to attach any quantitative estimates to the increased risk of dependence, because they have been measured in different ways in different studies.

These overall statements of the risks of cannabis dependence ignore the fact that the risk of dependence is not equally distributed in the population. The ECA study suggested that men have a higher risk of developing dependence than women, and that the risk was highest among the younger 18-29 year old cohort. In both cases, however, the most likely explanation was the different rates of exposure to cannabis among men and women, and among younger and older persons (Anthony and Helzer, 1991). When this was controlled by looking at the rates of dependence among daily users of the drug among men and women and younger and older persons, the differences in the risk of dependence largely disappeared (Anthony and Helzer, 1991).

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7.3.6 The consequences of cannabis dependence

Another important issue that needs to be considered when placing the risks of cannabis dependence into perspective is that of the consequences of developing dependence. How easy or difficult is it for those who decide to stop using cannabis to achieve and maintain abstinence? This question is difficult to answer in the absence of systematic research on the natural history of cannabis dependence. The following are reasonable inferences about what the rate of remission might be. First, cannabis dependence resembles alcohol dependence in the risk of dependence, and the similarity in the age and gender distributions of heaviest use, and abuse, and dependence. It seems reasonable then to suppose that there is likely to be a high rate of remission without treatment in cannabis dependence, as there is in as in alcohol dependence in the community (Helzer, Burnham and McEvoy, 1991). The large discrepancy between the ECA estimates of cannabis abuse and dependence in the community, and the proportions of cannabis users among drug users seeking treatment provides indirect support for this inference. Kandel and Davies' (1992) findings provide more direct support. They found that 44 per cent of those who had used cannabis more than 10 times became near daily users for an average period of three years. Yet by age 28-29, less than 15 per cent of those who had ever been daily users were still daily users, indicating a very high rate of remission during the 20s.

Among those who develop cannabis dependence, how disruptive to everyday life and functioning is it? This is even more difficult to answer. All that can be said with confidence is that there are some cannabis users who are sufficiently troubled by the consequences of their dependence to seek assistance. The experience of Roffman and colleagues suggests that this number may be increased if more effort was made to attract dependent cannabis users into treatment. Among the population of cannabis dependent persons seeking treatment, the major complaints have been the loss of control over their drug use, cognitive and motivational impairments which interfere with occupational performance, lowered self-esteem and depression, and the complaints of spouses and partners (see above). There is no doubt that some dependent cannabis users report impaired performance and a reduced enjoyment of everyday life, but more detailed research is necessary to make a better judgment about how common this is, and how severe the impairment typically produced by cannabis dependence is.

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7.3.7 The treatment of cannabis dependence

Given the widespread scepticism about the existence of a cannabis dependence syndrome, the question of what should be done to assist those who present for help with their cannabis use has largely been ignored (see Kleber, 1989). Indeed, Stephens and Roffman (1993) have suggested that there is a widespread view among drug and alcohol treatment practitioners that cannabis dependence does not require treatment because the withdrawal syndrome is so mild that most users can quit without assistance. Although, as argued above, it is likely that rates of remission without treatment are substantial, the fact that many users succeed without professional assistance does not mean we should ignore requests for assistance from those who are unable to stop on their own. As with persons who are nicotine dependent, those dependent cannabis users who have repeatedly failed in attempts to stop their cannabis use need professional assistance to do so. But what types of treatment should be offered?

There is not a lot of information on which to base useful recommendations. The available literature largely consists of treatment suggestions based upon personal experience, or upon clinical wisdom derived from opinions about the best forms of treatment for other related forms of dependence, such as alcohol and tobacco (e.g. de Silva, DuPont, and Russell, 1981). Jones (1984), for example, suggested that because cannabis was usually smoked in social settings, the treatment for cannabis dependence should be based upon principles derived from successful forms of treatment for nicotine dependence. Such treatment would include: assisted cessation of cannabis use accompanied by education about the acute and chronic effects of the drug; social skills training in resisting the social cues for cannabis use; and the mobilisation of peer support to maintain abstinence through self-help groups.

Others have preferred to adopt approaches adapted from those developed to treat alcohol dependence. Hannifin (1988), in arguing for the concept of "cannabism" by analogy to "alcoholism", implied that it be managed in much the same way. Miller and his colleagues (Miller and Gold, 1989; Miller, Gold and Pottash, 1989) have recommended a treatment model based upon the preferred form of treatment for alcohol dependence in the United States, namely, detoxification, a 12-step program delivered during an extended inpatient stay, and enrolment in Alcoholics Anonymous or Narcotics Anonymous after discharge. Stephens and Roffman (1993) and Zweben and O'Connell (1992) have suggested eclectic approaches combining management of withdrawal, relapse prevention methods, and enrolment in 12-step programs. Tunving et al (1988) have described their experience with a similar eclectic outpatient program for cannabis users in Sweden. De Silva et al (1981) provide short accounts of a variety of treatment approaches for marijuana dependent adolescents.

There have been very few controlled evaluations of the effectiveness of these recommendations. Smith et al (1988) reported a simple pre-treatment and post-treatment comparison of cannabis use among patients who received outpatient aversion therapy and group self-management counselling. They found good self-reported rates of abstinence, but these were obtained from telephone interviews conducted by the therapists who delivered the treatment. Roffman et al (1988) have reported a randomised controlled trial comparing group based relapse prevention or social support. Subjects were 120 men and women (average age 32 years with an average history of 16 years marijuana use) who had answered advertisements publicising a treatment program for adults seeking help to stop using marijuana. Their results at one month follow-up were much less positive than those of Smith et al: only 30 per cent of their patients were still abstinent, although 75 per cent had set abstinence as a treatment goal. By the end of a year the abstinence rate had dropped to 17 per cent. Results were a little more positive when evaluated in terms of average number of days of use, and in problems experienced, suggesting that the outcome of cannabis cessation treatment is much like that for alcohol and tobacco (Heather and Tebbutt, 1989).

Much more research is clearly required before sensible advice can be given about the best ways to achieve abstinence from cannabis. In the absence of better evidence of treatment effectiveness, those who offer treatment for cannabis dependence should avoid replicating experience in the alcohol field, where intensive and expensive forms of inpatient treatment have been widely adopted in the absence of any good evidence that they are more effective than less intensive outpatient forms of treatment (Heather and Tebbut, 1989; Miller and Hester, 1986).

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7.3.8 Conclusions

In 1982 Edwards reviewed the available evidence on the question of whether there was a cannabis dependence syndrome as defined by the 1981 World Health Organisation criteria. Although he argued that there was good evidence of tolerance and a withdrawal syndrome, there was insufficient evidence bearing on the criteria of compulsion, narrowing of repertoire, reinstatement after abstinence, use to relieve or prevent withdrawal symptoms and salience of cannabis use. He added that although tolerance and withdrawal were insufficient to prove the existence of a dependence syndrome, they nonetheless constituted "grounds for believing that such a syndrome may exist" (p38). Until these issues were resolved, he concluded, the question remained "very open".

On the basis of evidence gathered since Edwards wrote, we conclude that there probably is a cannabis dependence syndrome like that defined in DSM-III-R which occurs in heavy chronic users of cannabis. There is good experimental evidence that chronic heavy cannabis use can produce tolerance and withdrawal symptoms, and some clinical and epidemiological evidence that some heavy cannabis users experience problems controlling their cannabis use, and continue to use despite the experience of adverse personal consequences of use. There is reasonable observational evidence that there is a cannabis dependence syndrome like that for alcohol, cocaine and opioid dependence. If the estimates of drug dependence from the ECA study are approximately correct, cannabis dependence is the most common form of dependence on illicit drugs, reflecting its high prevalence of use in the community. The risk of developing the syndrome is probably of the order of: one chance in ten among those who ever use the drug; between one in five and one in three among those who use more than a few times; and around one in two among those who become daily users of the drug.

Recognition of the cannabis dependence syndrome has been delayed because of its apparent rarity in Western societies, which reflects a number of factors. First, heavy daily cannabis use has been relatively uncommon by comparison with the intermittent use of small quantities of cannabis. Second, until recently there have been few individuals who have presented requesting assistance for cannabis related problems. This may have been because it is easier to stop using cannabis than opioids or alcohol without specialist assistance, or it may be that the impact of cannabis dependence on the user is not as transparently adverse as that of alcohol or opioid problems to users and their families. Third, an overemphasis on the occurrence of tolerance and a withdrawal syndrome in the past has hindered its recognition in those individuals who have presented for treatment. Fourth, cannabis dependence (which is widespread among opioid dependent persons) has been perceived to be a less serious problem than dependence on alcohol, opioids and stimulants, which have accordingly been given priority in treatment (Hannifin, 1988).

Given the widespread use of cannabis, and its continued reputation as a drug which is free of the risk of dependence, the clinical features of cannabis dependence deserve to be better delineated and studied. This would enable its prevalence to be better estimated, and individuals with this dependence to be better recognised and treated. Treatment should probably be on the same principles as what is effective for other forms of dependence. Treatment for tobacco dependence may provide a better model than treatment for alcohol dependence, although this area is in need of research.

Although cannabis dependence is likely to be a larger problem than previously thought, we should be wary of over-estimating its social and public health importance. It will be most common in the minority of heavy chronic cannabis users. Even in this group, the prevalence of drug-related problems may be relatively low by comparison with those of alcohol dependence, and the rate of remission without formal treatment is likely to be high. While acknowledging the existence of the syndrome, we should avoid exaggerating its prevalence and the severity of its adverse effects on individuals. Better research on the experiences of long-term cannabis users should provide more precise estimates of the risk.