The health and psychological consequences of cannabis use - chapter 4


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4. Cannabis the drug

4.5 Dosage

A typical joint contains between 0.5g and 1.0g of cannabis plant matter, which varies in THC content between 5mg and 150mg (i.e. typically between 1 per cent and 15 per cent THC). Not all of the available THC is ingested; the actual amount of THC delivered in the smoke has been estimated at 20 per cent to 70 per cent of that in the cigarette (Hawks, 1982), with the rest being lost through combustion or escaping in sidestream smoke. The bioavailability of THC from marijuana cigarettes (the fraction of THC in the cigarette which reaches the bloodstream) has been reported to range between 5 per cent and 24 per cent (mean 18.6 per cent) (Ohlsson et al, 1980). For all these reasons, the actual dose of THC that is absorbed when cannabis is smoked is not easily estimated.

In general, only a small amount of smoked cannabis (e.g. 2mg to 3mg of available THC) is required to produce a brief pleasurable high for the occasional user, and a single joint may be sufficient for two or three individuals. A heavy smoker may consume five or more joints per day, while heavy users in Jamaica, for example, may consume up to 420mg THC per day (Ghodse, 1986). In clinical trials designed to assess the therapeutic potential of THC, single doses have ranged up to 20mg in capsule form. In human experimental research, THC doses of 10mg, 20mg and 25mg have been administered as low, medium and high doses (Barnett et al 1985; Perez-Reyes et al 1982).

Perez-Reyes et al (1974) determined the amount of THC required to produce the desired effects by slow intravenous administration. They estimated that the threshold for perception of an effect was 1.5mg while a peak social "high" required 2-3mg THC. These levels did not differ between frequent and infrequent users, so Perez-Reyes et al concluded that tolerance or sensitivity to the perceived high does not develop.