People with mental illnesses are significantly more likely to use cannabis on a weekly basis compared to those without mental illnesses, a new study has found.

According to researchers from the Centre for Addiction and Mental Health in Canada, cannabis continues to be the most commonly used illicit substance in the world, including in Ireland. An estimated 203 million people globally use the drug.

While previous research has found links between mental health and the drug, findings have been unclear. The researchers decided to investigate this issue further. They analysed data from a US study involving over 43,000 people over the age of 18.

Cannabis use and mental health problems, such as depression, addiction and personality disorders, were assessed.

The study found that people with mental illnesses were at least seven times more likely to use cannabis on a weekly basis compared to people without mental illnesses.

Rates of use were particularly high among people with bipolar disorder, personality disorders and addictions to other substances.

"Based on the number of individuals reporting weekly use, we see that people with mental illness use cannabis at high rates. This can be of concern because it could worsen the symptoms of their mental illness," the researchers said.

They noted that while cannabis use in general tends to be higher among younger people, the link between cannabis use and mental illness was the same throughout almost all age groups.

The researchers said that people with mental illnesses should be screened for ‘frequent and problem' cannabis use, in order to prevent or deal early with this problem.

Details of these findings are published in the journal, Comprehensive Psychiatry.