Cannabis is used daily by some 200 million people worldwide, and its use and potency have increased in many countries; for example, my group has shown that high potency types have taken over 96% of the London “street” market. Furthermore, the spread of laws legalizing cannabis for medicinal or recreational use has been accompanied by more relaxed attitudes towards cannabis. Cross-sectional and prospective epidemiological studies, as well as biological investigations, demonstrate a causal link between cannabis use and psychotic disorder, especially of a paranoid type. Furthermore, our research has shown a) a dose-response association with the highest odds of psychotic disorder in the heaviest cannabis users, b) high potency types carry the greatest risk for psychotic disorder, c) patterns of cannabis use have a major impact on the incidence of psychotic disorder across Europe. High potency cannabis types have high concentrations of delta-9-tetrahydrocannabinol (THC) and virtually no cannabidiol (CBD), which, in experimental studies of healthy volunteers ameliorates the psychotogenic effect of THC. A minority of cannabis users develop clinical psychosis, but it is unclear what proportion of heavy users develop minor psychotic symptoms and paranoia in particular. Therefore, in this talk, I shall present the above evidence and discuss how they might inform how and if we can move to safely prescribe cannabis for medicinal use.