Recreational marijuana is now legal in Canada, and as my colleague Tony Mecia writes, a Democratic takeover of the House could help facilitate legal pot in more places in the United States. And soon.

Legalizing cannabis has a number of upsides. It’s projected to help the farming economy while hurting the smuggling economy, and sparing small-time offenders from outsize sentences—and that’s not to mention its medicinal uses, for chronic pain, PTSD, and chemo patients’ nausea. But beyond these known and anticipated benefits looms one risk proponents ought to know but tend not to: Marijuana increases a user’s likelihood to develop psychotic symptoms.

A recent study placed marijuana among the five clearest triggers of schizophrenia, out of 98 catalysts under consideration. The other top triggers—a chemical predisposition, the experience of trauma as a child or adult, and obstetric complications (aka, trauma experienced in utero)—are less preventable than marijuana use. An earlier study, conducted over 15 years in Sweden, concluded that the risk of developing schizophrenia increased six times for high-level users. But even using moderately, just 11 to 50 times overall, doubles the risk according to a more recent study. And for those who start using younger, the risk is higher still: 15-year-olds who use marijuana are three-times likelier to develop psychotic symptoms than those who start at 18.

Psychosis usually rears its head during one’s late teens or early 20s, which happens to be within a couple years of a typical recreational user’s first experience with marijuana. It’s a complex set of mental disorders that covers symptoms like hallucinations, paranoid delusions, all the way to full-blown schizophrenia—which, while sometimes manageable, has no known cure.

Canadian brothers Joel and Ian Gold, a psychiatrist and a philosophy professor respectively, published a column the day that their homeland legalized cannabis, and threw open the doors to new state-run dispensaries. The Golds co-authored Suspicious Minds in 2015—a book that looks at the social and pop-cultural influences on psychosis. In the process of researching the book they were surprised to discover the depth of literature establishing a causal link between marijuana and psychosis, said Joel, a professor who also served as Bellevue Hospital’s ER director and outpatient director for a number of years.

The link, Gold is certain, will come to bear as legalization gains. “I certainly expect there will be an uptick,” he said, citing a study showing an overall increase in use after legalization. “Let's presuppose that there will be an increase in the number of young people using cannabis. Then by extension there will be an uptick in the number of people developing psychosis a year down the road,” Gold said. As legalization takes root, the perception of marijuana’s harmfulness and knowledge of its risks have diminished.

Just as Gold and his brother found the link something of a surprise in the midst of their research, most patients find it quite a shock. Even men and women who take their health seriously, he said, consider marijuana—especially when it’s not smoked—more or less harmless. Though the research they cite is robust, “It hasn’t really crossed over into the public consciousness and I’m not sure why.”

The Schizophrenia Society of Canada published an awareness campaign: An ominous and obviously teen-targeted public service announcement intones, “It’s not always easy to tell whether you’re experiencing the negative effects of being high, or something more serious. What sucks is that’s it’s just the luck of the draw.”

And, as is often the case, lawmakers will be left to regulate in reverse. One imagines further adjustments to existing age limits, dose restrictions, and purchasing cut-offs would follow in the wake of what mental health crisis—or what growing fear of one—may yet come. The aforementioned age-level study, which found 15-year-old first-time users had a higher risk than 18-year-olds, suggests the strict enforcement of legal age restrictions could have a hand in regulating around so tender a problem as the link.

But, Gold also said, users will be more likely to learn of their heightened risk and heed warnings about it once cannabis is more widely legal stateside: Legalizing something has a way of making it seem suddenly very prevalent and therefore seem, paradoxically, more dangerous.

There’s reason to hope, meanwhile, that widening cultural acceptance of marijuana as a consumer product might also lead to productive exploration of the plants’ other, beneficial uses—even as a groundbreaking anti-psychotic, in fact. Cannabidiol (CBD), a separate chemical from THC, derives from cannabis but doesn’t get you high. For now, it’s mostly popular among naturopaths. But psychiatric researchers hope it will someday help combat schizophrenia. Though still in the early stages, CBD’s development as anti-psychotic suggests the possibility of perfect chemical counterbalance within the plant itself: It’s THC-potency that increases a frequent marijuana user’s risk of psychosis, according to a study from the U.K.

Responses to their co-authored op-ed have poured in since Canada’s legalization day, but primarily from people who were not surprised to learn the statistics they laid out. Men and women who’ve seen family members or loved ones use cannabis heavily and later develop psychosis—many of them the parents of psychotic children—sent the brothers messages of thanks. Some added words of regret. If only we’d known, they’d say.

Widespread legalization is politically—and practically—inevitable. But it may also exact an insidious burden on a sensitive segment of the population. A possibly preventable burden, the psychiatrists would stress, and one that may not manifest for several years. For the moment, it’s an epic experiment, in other words, with the potential to do a lot of damage.