Cannabis use and psychiatric and cogitive disorders: the chicken or the egg?

Cannabis is the world's most commonly used illicit drug. In this review, we consider the recent literature on the effects of cannabis on mental health and on cognition.

Cannabis use in adolescence increases the risk of later schizophrenia-like psychoses, especially in genetically vulnerable individuals. Not surprisingly, patients already suffering from psychosis who use cannabis have a worse outcome than those who do not. These effects of cannabis may be consequent on its impact on the dopamine system. There is less evidence of cannabis playing an aetiological role in other mental disorders including depression, but there have been far fewer studies. Heavy cannabis use has also been shown to affect memory and learning performance, both in healthy individuals and in patients suffering from psychosis. Combined cognitive-behavioural therapy and motivational interviewing seems a promising psychological intervention to achieve a cessation of cannabis use in patients suffering from schizophrenia.

Further research is needed to understand the biological mechanisms underlying the effects of cannabis on mental health, but intervention strategies to help patients abstain should currently be implemented in psychiatric services, and public education campaigns should be directed at increasing awareness of the health risks of cannabis.