Cannabis and anxiety and depression in young adults: a large prospective study.

Objective: To examine whether age of first use or frequency of use of cannabis is associated with anxiety and depression (AD) in young adults, independent of known potential confounders, including the use of other illicit drugs.
Method: A cohort of 3,239 Australian young adults was followed from birth to the age of 21 when data on AD were obtained from sample members along with information on their use of cannabis at 21 years. Potential confounding factors were prospectively measured when the child was born and at 14 years.
Results: After controlling for confounding factors, those who started using cannabis before age 15 years and used it frequently at 21 years were more likely to report symptoms of AD in early adulthood (odds ratio 3.4; 95% CI 1.9-6.1). This association was of similar magnitude for those who had only used cannabis and those who reported having used cannabis and other illicit drugs.
Conclusion: The relationship between early-onset and frequent use of cannabis and symptoms of AD is independent of individual and family backgrounds. Frequent cannabis use is associated with increased AD in young adults independently of whether the person also uses other illicit drugs.