In the first study of 1,600 students from 44 secondary schools in Australia, frequent cannabis use predicted later depression and anxiety, particularly in teenage girls.
Some 60% of participants had used cannabis by the age of 20 and 7% were daily users. After adjusting for use of other substances, daily use in young women was associated with a more than fivefold increase in the odds of later depression and anxiety. Weekly or more frequent use as a teenager predicted a twofold increase in later risk.
Given recent increasing levels of cannabis use, measures to reduce frequent and heavy recreational use seem warranted, suggest the authors.
The second study clarifies earlier findings that cannabis is associated with later schizophrenia and that this is not explained by use of other psychoactive drugs or personality traits. The results show that use of cannabis increases the risk of schizophrenia by 30%.
The weight of evidence is that occasional use of cannabis has few harmful effects overall, say the authors. Nevertheless, these results indicate a potentially serious risk to the mental health of people who use cannabis particularly in the presence of other risk factors for schizophrenia. Such risks need to be considered in the current move to liberalise and possibly legalise the use of cannabis in the United Kingdom and other countries, they conclude.
In the third study, researchers found that using cannabis in adolescence increases the likelihood of experiencing symptoms of schizophrenia in adulthood, with the youngest cannabis users (by age 15) at greatest risk. These findings suggest that cannabis use among psychologically vulnerable adolescents should be strongly discouraged, while policy and law makers should concentrate on delaying onset of cannabis use, say the authors.
The shown dose-response relation for both schizophrenia and depression highlights the importance of reducing the use of cannabis in people who use it, write two psychiatry experts in an accompanying editorial.