The health and psychological consequences of cannabis use - chapter 5

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5. The acute effects of cannabis intoxication

5.1 Psychological and physical effects

Any attempt to summarise the acute effects of cannabis, or of any psychoactive drug, is necessarily an oversimplification. The effects experienced by the user will depend upon: the dose, the mode of administration, the user's prior experience with the drug, any concurrent drug use, and the "set" - the user's expectations, mood state and attitudes towards drug effects - and "setting" - the social environment in which the drug is used (Jaffe, 1985). The following descriptions of the typical effects of cannabis are made with this qualification in mind.

The major motive for the widespread recreational use of cannabis is the experience of a subjective "high", an altered state of consciousness which is characterised by: emotional changes, such as mild euphoria and relaxation; perceptual alterations, such as time distortion, and; intensification of ordinary sensory experiences, such as eating, watching films, listening to music, and engaging in sex (Jaffe, 1985; Tart, 1970). When used in a social setting, the "high" is often accompanied by infectious laughter, talkativeness, and increased sociability.

Cognitive changes are usually marked during a "high". These include an impaired short-term memory, and a loosening of associations, which make it possible for the user to become lost in pleasant reverie and fantasy, while making it difficult for the user to sustain goal-directed mental activity. Motor skills, reaction time and motor coordination are also affected, so many forms of skilled psychomotor activity are impaired while the user is intoxicated (Jaffe, 1985).

Not all the effects of cannabis intoxication are welcomed by users. Some users report unpleasant psychological reactions, ranging from a feeling of anxiety to frank panic reactions, and a fear of going mad to depressed mood (Smith, 1968; Weil, 1970; Thomas, 1993). These effects are most often reported by naive users who are unfamiliar with the effects of cannabis, and by some patients given THC for therapeutic purposes. More experienced users may also report these effects on occasion, especially after the oral ingestion of cannabis when the effects may be more pronounced and of longer duration than those usually experienced after smoking cannabis. These effects can usually be successfully prevented by adequate preparation of users about the type of effects they may experience. If these effects develop they can be managed by reassurance and support (Smith, 1968; Weil, 1970). Psychotic symptoms, such as delusions and hallucinations, are very rare experiences that occur at very high doses of THC, and perhaps in susceptible individuals at lower doses (Smith, 1968; Thomas, 1993; Weil, 1970).

The inhalation of marijuana smoke, or the ingestion of THC, the psychoactive derivative of cannabis, has a number of bodily effects. Among these the most dependable are the effects on the heart and vascular system. The most immediate effect of cannabis use by all routes of administration is an increase in heart rate of 20-50 per cent over baseline which occurs within a few minutes to a quarter of an hour and lasts for up to three hours (Huber et al, 1988; Jones, 1984). Changes in blood pressure also occur which depend upon posture: blood pressure is increased while the person is sitting, and decreases while standing. A sudden change from a recumbent posture may produce postural hypotension and fainting, an effect which may explain the feeling of "light-headedness" and faintness that is often the earliest indication of intoxication in naive users (Maykut, 1984). Increases are also observed in the production of the catecholamine norepinephrine, although these lag behind the cardiovascular changes, and their mechanisms are not well understood (Hardman and Hosko, 1976).

In healthy young users these cardiovascular effects are unlikely to be of any clinical significance. They may, however, magnify anxiety in naive users. The cannabis-induced tachycardia and postural hypotension may contribute to the panic attacks sometimes experienced by naive users (Weil, 1970) who may mistakenly interpret the palpitations, and the feeling of faintness, as symptoms of serious misadventure, magnifying pre-existing anxiety in a positive feedback cycle that leads to a panic attack.