Cannabis and mental health: more evidence establishes clear link between use of cannabis and psychiatric illness.(Brief Article)

In the 1990s the use of cannabis increased much among young people so that it is now becoming more common than tobacco smoking in some countries. (1-2) The ready availability of the drug, the increasing social disapproval of cigarette smoking, stern drink driving laws, and perceptions that cannabis is safe or less harmful than cigarettes or alcohol may explain these changes. The increase in use is of concern because cannabis may be a gateway to other drugs, (3) and it may cause psychiatric illnesses. The link between cannabis and psychosis is well established, and recent studies have found a link between use of marijuana and depression. (4-7) Does cannabis cause these conditions, or do patients use cannabis to relieve their distress?

The explanation most accepted is that cannabis triggers the onset or relapse of schizophrenia in predisposed people and also exacerbates the symptoms generally. (4 5) Establishing direction of causality is difficult and is most appropriately assessed in non-clinical samples, but a low incidence of the illness and the fact that most drug users take other drugs in addition to cannabis create methodological problems and explain the dearth of reliable evidence.…