High potency cannabis

A recent study by Di Forti and colleagues suggests that daily use of “skunk”— cannabis with high levels of Δ9-tetrahydrocannabinol (Δ9-THC) and low levels of cannabidiol—is a contributory cause of schizophrenia.1 The study compared patterns of cannabis use in first episode cases of psychosis and matched controls recruited from areas in south London. It found equally high rates of cannabis use in cases and controls, but cases were three to five times more likely to report daily use of skunk than controls, and this association persisted after statistical adjustment for confounders. The researchers estimated that one in four new cases of schizophrenia in this area of London were attributable to skunk, if the association is causal.

Their finding fits with prospective studies of the association between cannabis and schizophrenia. For example, a 27 year follow-up of 50 000 Swedish conscripts found a dose response relation between the number of times cannabis had been used by age 18 and the risk of receiving a diagnosis of schizophrenia.2 This relation persisted …