Cannabis is the most widely used illegal recreational drug in Europe; up to 5% of the total population are regular users in some countries. Smoking cannabis can damage the lungs, and some users may become psychologically dependent on the drug. Heavy cannabis use may also be associated with an increased risk of psychiatric illness — although no cause and effect relationship has been established. Nevertheless, there is a general consensus among medical and scientific experts that the health hazards of cannabis have been exaggerated. European countries differ widely in their attitude to enforcing the 1961 UN Convention on Narcotic Drugs, which declared cannabis to be an illegal narcotic. In the Netherlands the so-called ‘Dutch Experiment’ has decriminalized cannabis use for almost 30 years without any serious adverse social or public health consequences. On the other hand, most Scandinavian countries maintain a fiercely punitive legal regime, while other countries in Europe are moving towards a relaxation of the criminal penalties for possession and use. There is an urgent need for more debate about the need to reform the cannabis laws and for more consistency across Europe.