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Cannabis remains one of the most widely abused drugs worldwide, with a substantial number of users in many Western societies. Epidemiological studies reveal that most users are young individuals either in their teens or early twenties. The most prevalent pattern of use is intermittent. Traditional use of cannabis is also prevalent in many countries. The main acute adverse effects of cannabis are cognitive impairment, especially of attention and memory, psychomotor impairment, and a possible increased risk of traffic accidents. The main physical and psychological health effects of chronic, heavy cannabis use, especially daily use, over many years, are most commonly respiratory disease, dependence and subtle cognitive impairment. It is estimated that approximately 10 per cent of persons who try cannabis progress to daily use, with a further 20 to 30 per cent using it on a weekly basis. Cannabis use during pregnancy is likely to result in shortened gestation and low-birth-weight babies. Other potential dangers of chronic heavy use that remain to be confirmed include an increased risk of developing cancer, a decline in occupational performance, impaired educational attainment in adolescents and birth defects. While it is known that underachieving adolescents are more likely to smoke cannabis, a cause-effect relationship has not been established. There is considerable anecdotal information regarding the therapeutic usefulness of smoking cannabis. In recent times, the most intense interest has been directed towards the prevention of weight loss in patients with acquired immunodeficiency syndrome (AIDS), although management of pain, prevention of emesis and control of glaucoma and movement disorders still command attention. Of these potential uses, treatment of cachexia in AIDS patients and of chronic pain seems most worthy of scientific validation. Efforts to develop synthetic cannabinoid derivatives should remain a primary goal for treatment of most such disorders. The discovery of an endogenous cannabinoid system provides a means not only of developing new therapeutic agents but also of establishing the biological consequences of long-term cannabis exposure on the brain. The localization of cannabinoid receptors in brain areas associated with some of the most prominent pharmacological effects of cannabis, that is, cognition, pain perception and movement, provides an insight into the physiological importance of those receptors. The identification of anandamide, the endogenous ligand for the cannabinoid receptor, provided credence for a naturally occurring endogenous cannabinoid system in the brain. This endogenous system adapts to chronic exposure to tetrahydrocannabinol (THC) by a reduction in the number of receptors, the physiological consequence of which remains to be established.
