Prospective studies in Australia, Germany, the Netherlands, New Zealand, and Sweden have found that regular use of cannabis is associated with an increased risk of psychotic symptoms and disorders in the general population.1 Some people have interpreted this as evidence that cannabis use is a contributory cause of psychoses.2 More sceptical researchers have argued that the association might be the result of residual confounding by—for example, the use of other drugs or genetic factors.3 Another possibility is that the association arises because people with psychosis use cannabis to self medicate their symptoms,2 even though evidence suggests that they use cannabis for much the same reasons as their peers without psychosis⇓.2

The results of the linked prospective study by Kuepper and colleagues (doi:10.1136/bmj.d738) argue against these two alternative explanations.4 The authors excluded anyone who reported cannabis use or psychotic symptoms in the baseline survey so that they could examine the relation between incident cannabis use and incident psychotic …