The stronger marijuana available nowadays means teenagers who smoke it are now 11 times likelier to develop mental health problems than those who abstain, according to a study published in the journal Psychological Medicine.

“The association between cannabis and psychotic disorders may be stronger than indicated by previous research, which has relied largely on older data when cannabis was less potent than today,” the researchers said, warning that teens are especially vulnerable to the effects of cannabis.

From the University of Toronto, The Centre for Addiction and Mental Health and ICES – formerly known as the Institute for Clinical Evaluative Science – the researchers looked at medical records covering 11,000 “youths” in Ontario, taking in hospitalizations, emergency department and outpatient visits.

They said theirs is the first research to show “age-dependent association between self-reported cannabis use and subsequent psychotic disorder diagnosis.”

“We found a very strong association between cannabis use and risk of psychotic disorder in adolescence. Surprisingly, we didn’t find evidence of association in young adulthood,” said researcher André McDonald.

Teenagers’ developing brains are likely worse affected by the much stronger joints being passed around nowadays compared to decades ago. The “average THC potency of cannabis” has gone up from “roughly 1% in 1980 to 20% in 2018,” the researchers said.