Literature pertaining to the effects of cannabis use and health which has been published during the past 11 years has been reviewed. Many older concerns about adverse effects on health (chromosomal damage, 'cannabinol psychosis', endocrine abnormalities, cardiac events, impaired immunity) no longer seem to elicit much interest. Continuing concerns about the adverse cognitive effects of chronic use indicate that these can be demonstrated by proper testing; some studies suggest that they may be long-lasting. Although cannabis does not produce a specific psychosis, the possibility exists that it may exacerbate schizophrenia in persons predisposed to that disorder. However, evidence from retrospective surveys must always be questioned. Tolerance and dependence have occurred in man, confirming previous findings in many other species. Addiction tends to be mild and is probably less severe than with other social drugs. Driving under the influence of cannabis is impaired acutely; how long such impairments last is still unknown. More exacting tasks, such as flying an airplane, may be impaired for as long as 24 hours. While there is no doubt that marijuana smoke contains carcinogens, an increase in cancer among users has thus far been anecdotal. Because of the long latent period between cancer induction and initiation of cigarette smoking, the full story is yet to be told. Marijuana use during pregnancy is not advised although the consequences are usually not greater than those of smoking cigarettes, and far less than those from alcohol use. Whether smoked marijuana should become a therapeutic agent requires a cost-benefit analysis of the potential benefits versus the adverse effects of such use as we now know them.