The smoke from a cannabis joint contains over 400 chemicals, including 61 cannabinoids (of which the most potent psychoactive agent is delta 9-tetrahydrocannabinol, THC) and the same tars, irritants, carcinogens and carbon monoxide as tobacco smoke. Cannabinoids are readily absorbed when cannabis is smoked or taken by mouth and quickly reaches the brain, but are then concentrated in fatty tissues from which they are slowly released. Cannabinoids can be detected in the urine for up to 30 days after a single joint of cannabis.

Used recreationally, cannabis produces a ‘high’ but also impairs memory, cognition, perception and psychomotor performance. Impairment in complex tasks can persist for over 24 hours after a single joint, with implications for automobile and train driving, aircraft piloting, machine operating, and academic performance. In chronic users cognition and memory deficits have been shown to persist even after 6 weeks’ abstinence. Adverse psychiatric effects include panic and psychotic reactions, aggravation of schizophrenia with increased risk of violence, dependence and withdrawal effects on discontinuation. Health risks of chronic use include bronchitis, emphysema, probably lung cancer and aggravation of heart disease.

Implications for policy are discussed: difficulties include the high prevalence of cannabis use, community care of psychiatric patients, long-lasting effects on performance, slow elimination and lack of correlation between body fluid concentration of cannabinoids and degree of intoxication or performance impairment.