The study, published by Joyce Cheung and colleagues in the American Journal of Drug and Alcohol Abuse, surveyed over 14,000 Ontarians and found that anxiety and mood disorders were most common in people who used cannabis almost everyday (heavy users) and lowest in those who did not use cannabis at all (abstainers).
Eighteen percent of heavy users reported the presence of anxiety and mood disorders, which is about double the Canadian average. Respondents who reported occasional use between once a month to up to a few times a week did not show elevated rates of mental health problems. But surprisingly, infrequent users, (those who used cannabis less than once a month), also reported elevated rates of anxiety and mood disorders – an increase of 43% compared to abstainers.
"This result was something we didn’t expect to see," said Dr. Robert Mann, CAMH Senior Scientist and investigator on the study. "We thought that the data would likely show a positive correlation between use and psychiatric disorders – with rates of anxiety and mood disorder increasing with use – but these results show that the frequency of use is not necessarily the only factor affecting the rates of these disorders."
The reason for this result may lie in a person’s genes. "We know that with a mental illness such as schizophrenia, there is no threshold for use. If a person is genetically predisposed to developing schizophrenia, any level of cannabis use can hasten the onset – this may be the same case here," added, Dr. Mann.
Those who treat people seeking to stop or reduce their cannabis use should also be aware of the increased likelihood that their clients may be experiencing psychiatric disorders as well.
The Centre for Addiction and Mental Health (CAMH) is Canada’s largest mental health and addiction teaching hospital, as well as one of the world’s leading research centres in the area of addiction and mental health. CAMH combines clinical care, research, education, policy development and health promotion to help transform the lives of people affected by mental health and addiction issues.
CAMH is fully affiliated with the University of Toronto, and is a Pan American Health Organization/World Health Organization Collaborating Centre.