Many young adults in developed countries report a history of experimentation with cannabis. Approximately 10 percent of those who try cannabis become daily users, and an additional 20 to 30 percent use the drug weekly over prolonged periods. These patterns of usage are of concern because of the adverse health effects associated with heavy cannabis use, and the associated use of alcohol, tobacco and other drugs. Hall and Solowij review the current literature on the adverse health effects of acute and chronic use of cannabis.

The principal psychoactive compound in cannabis is d-9-tetrahydrocannabinol (THC). Marijuana contains between 0.5 and 5 percent THC and hashish contains between 2 and 20 percent. Cannabis is usually smoked, because users can maximize THC absorption in the lungs by inhaling deeply and holding their breath. Immediate effects include euphoria and relaxation, intense sensory perceptions and time distortion. Other effects include talkativeness, impaired motor skills and dysfunction in attention and short-term memory. Ingestion of cannabis elevates both heart rate and blood pressure, but these findings are considered negligible in healthy young adults, as tolerance to these cardiovascular effects develops. The most unpleasant side effects are anxiety and panic reactions, and these are often cited by users as reasons for discontinuing the drug. To date, cannabis poisoning has not resulted in any confirmed deaths. The most serious consequence of cannabis use may be that of driving while impaired. However, studies on the effects of cannabis use on driving performance have been equivocal.

Chronic use of cannabis affects multiple body systems, specifically the respiratory, immune and reproductive systems. Heavy cannabis smoking increases the symptoms associated with chronic bronchitis, a problem that appears to be independent of, but synergistic with, the effects of tobacco. Histopathologic changes that precede lung cancer and cancers of the aerodigestive tract have been reported in young adults who are heavy users. Animal studies indicate that cannabis smoke may be carcinogenic, both directly and through its adverse effects on the immune system, but these findings may be uncertain since the doses of THC used in these studies were very high. Other animal studies show that THC decreases testosterone production and generally impairs fertility in both males and females; however, these results have not been replicated in humans. Cannabis use in pregnancy does not seem to increase the risk of birth defects; however, birth weight seems to be lower, and children exposed to cannabis in utero appear to have behavior and development difficulties, especially in attention, memory and higher cognitive functioning. These children also appear to have an increased risk of developing certain types of cancer and leukemia.

Chronic use is also associated with subtle impairments of higher cognitive functions, such as memory and attention. Prolonged use of cannabis leads to a dependence syndrome similar to that of alcohol, affecting approximately one in 10 users. In adolescents, heavy cannabis use is associated with academic underachievement, job instability, problems in family relationships, involvement in crime and use of other illicit substances.

The authors conclude that there is increasing evidence of adverse health effects associated with cannabis use. They emphasize that much remains to be understood about its long-term effects on humans. Studies of cannabis users are complicated by the co-existence of other variables, especially alcohol use and other behavior risk factors. Patients should be advised about the possible health hazards of cannabis use, particularly the risks associated with driving when cannabis is combined with alcohol or other illicit substances.