Teenagers who smoke cannabis weekly or more are twice as likely as non-users to have an anxiety disorder in their late 20s, even if they stop using, a study of 2000 Victorian teenagers has found.

Those who used frequently in their teens and continued to use on a daily basis at the age of 29 were three times as likely to have an anxiety disorder compared with non- or infrequent users. Those who used minimally in their teens but became daily users in their late 20s were two and a half times as likely to have an anxiety disorder. But the really striking finding say the authors is the persistent association between frequent teenage cannabis use and adult anxiety disorders up to a decade after cannabis use has ceased. The relationship between cannabis use and anxiety disorders was present even after the researchers took into account other possible explanations such as mental health problems in their teens or other drug use in their twenties.

The findings, published online in Addiction are based on secondary analyses of a landmark study of nearly 2000 Victorian secondary school students - the 2000 stories cohort, led by Professor George Patton of the Centre for Adolescent Health at the Murdoch Childrens Research Institute in Melbourne. The students have been followed up and interviewed over 13 years, starting in 1992. They were interviewed at six six-monthly intervals during their teens and then again when they were aged 20-21, 24-25, and 29.

Lead author of the analysis, Professor Louisa Degenhardt from the National Drug and Alcohol Research Centre at the University of New South Wales, said that most studies looking at cannabis use and mental health outcomes focus on adolescence and early adulthood. “What we are seeing is a persistent association with anxiety disorders over a much longer period. “Given that anxiety is the most prevalent mental health disorder in the Australian population, affecting over 14 per cent of adults in any 12 month period, we need to investigate the findings further because it is highly possible that early cannabis use causes enduring mental health risks.”

Professor Patton, lead investigator of the 2000 stories cohort, said that the findings could be explained by lasting changes to brain function caused by introducing cannabis at a time when the brain is developing rapidly. Equally it could be that the very factors which predispose people to use cannabis early also predispose them to common mental health problems. “We know from animal studies that introducing cannabis during puberty brings about long lasting changes in behaviour which persist even after administration of cannabis is stopped. These findings suggest that a similar thing may be happening,” said Professor Patton. “During the teen years the parts of the brain that are involved in managing emotions are still developing rapidly and it is highly possible that heavy cannabis use at this sensitive point could have long lasting effects.”

However the authors write that they cannot rule out the possibility that the factors that predispose people to use cannabis early also put them at risk for common mental disorders.

“These common factors might include biological, personality, social and environmental factors, or a combination of these factors. This is a plausible hypothesis because social disadvantage is more common among persons who are problematic substance users and who meet criteria for common mental disorders,” they write.

Citation: Degenhardt et al (2012). The persistence of the association between adolescent cannabis use and common mental disorders into young adulthood. Addiction (early online view). http://onlinelibrary.wiley.com/doi/10.1111/j.1360-0443.2012.04015.x/pdf

Media inquiries: Marion Downey (National Drug and Alcohol Research Centre) 02 9385 0333 or Simone Myers (Murdoch Childrens Research Institute) (03) 8341 6433