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A rational cannabis policy would arguably be one that minimized the harms of both cannabis use and the legal policies adopted to control its use. We, therefore, review recent epidemiological evidence on the harmful effects of cannabis use and social research on the costs and benefits of cannabis prohibition.Epidemiological evidence suggests that cannabis increases the risk of road crash injury if users drive while intoxicated. When used chronically, cannabis can produce dependence, respiratory disease and psychotic symptoms, especially in vulnerable young adults. It probably also increases poor educational outcomes and possibly increases the use of other illicit drugs, although it is debated whether these relationships are causal. Proponents of a relaxation of cannabis prohibition argue that prohibition has failed to deter cannabis use, incurs substantial economic costs, has generated a large black market, has increased the potency of cannabis and users' access to other drugs and involves foregone tax revenue from the legal sale of cannabis.Development of a more rational cannabis policy requires better evaluations of both the health consequences of regular cannabis use and of the costs and benefits of enforcing the existing prohibition on its use. It also requires the liberalization of the international control system to allow member states to experiment with different methods of regulating and controlling cannabis use.