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People suffering mental illnesses are more than seven times more likely to use marijuana on a weekly basis, according to a new study.

Researchers at Canada's Center for Addiction and Mental Health (CAMH) looked at U.S. data and found that people with mental illness were also 10 times more likely to have a marijuana use disorder.

While marijuana, used by an estimated 203 million people around the world, is the most widely used illegal substance globally, exact numbers of prevalence of problem marijuana use among the mentally ill has not been fully investigated.

The study published in the journal Comprehensive Psychiatry analyzed data from face-to-face interviews with over 43,000 respondents over the age of 18 from the National Epidemiologic Survey on Alcohol and Related Conditions.

Researchers assessed cannabis use as well as various mental illnesses including depression, anxiety, drug and alcohol use disorders and personality disorders, based on criteria from the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV).

Researchers found that rates of marijuana use were particularly high among those with bipolar disorder, personality disorder and other substance use disorders.

Overall, researchers found that 4.4 percent of people with mental illness reported using marijuana weekly compared to only 0.6 percent of people without any mental illness. What's more 4 percent of people with mental illness had marijuana use disorders compared to 0.4 percent of people without mental illness.

While marijuana use is generally higher among younger people, researchers found that the link between mental illness and marijuana use was pervasive across most age groups.

"We know that people with mental illness consume more cannabis, perhaps partially as a way to self- medicate psychiatric symptoms, but this data showed us the degree of the correlation between cannabis use, misuse, and mental illness," researcher Dr. Shaul Lev-ran, Adjunct Scientist at CAMH and Head of Addiction Medicine at the Sheba Medical Center, Israel, said in a statement.

"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," Lev-ran added.

Researchers said the latest finings emphasize the importance of screening for problem marijuana use among people with mental illness so that healthcare provider may implement targeted prevention and intervention.