Prevalence of recreational cannabis use has risen in many countries. Users are starting younger, continuing for longer and preferring more potent plant preparations. Such use is likely to increase cannabis-associated risks, which, on present evidence, include vehicle and other accidents, dependence, exacerbation of psychosis, impairment of adolescent development and school performance, respiratory disease and perhaps progression to other illicit drugs. Devising effective control policies is difficult, but perhaps greater efforts should be directed towards reducing prevalence and providing treatment for dependent users. Increased knowledge of the way in which plant cannabinoids interact with endogenous cannabinoid systems has helped to explain some of the adverse effects of cannabis and has also opened the way to therapeutic uses. Pure plant cannabinoids and synthetic analogues have a potential role in the treatment of pain conditions, spastic disorders and in palliative care, and give promise of future benefits in a range of illnesses.