There was a new smell to New York City on my first visit since the pandemic. The New York I remember from 2018 was scented with subway fumes, car exhaust and pretzels. Now the air was a heady blend of forest fire, car exhaust and cannabis.

Recreational marijuana was legalized in the state of New York in 2021. But even if cannabis is easy and legal to buy in 23 states and all of Canada, the risks of chronic use aren’t talked about much.

Several studies have shown that chronic cannabis use is linked to a higher incidence of schizophrenia among men in their early 20s, the age when the disease is usually diagnosed. The first paper on the topic, a Swedish study published in 1997, found that heavy cannabis use was associated with a sixfold increase in schizophrenia risk. In the decades since, social scientists have unearthed a strong link between heavy cannabis use and other severe psychological illnesses, including clinical depression and bipolar disorder.

Now a new longitudinal study has examined the medical records of all citizens of Denmark over the age of 16, some 6.5 million people in all, for patterns of diagnosis, hospitalization and treatment for substance use between 1995 and 2021. In the paper, published in the journal JAMA Psychiatry in May, Dr. Oskar Hougaard Jefsen of Aarhus University and colleagues showed that people who had previously been diagnosed with cannabis use disorder were almost twice as likely to be diagnosed later with clinical depression. According to the Centers for Disease Control and Prevention, cannabis use disorder is characterized by craving marijuana, using it more often than intended, spending a lot of time using it, and having it interfere with friends, family and work.

Even more dramatically, the paper also found that people with cannabis use disorder were up to four times as likely to be diagnosed later with bipolar disorder with psychotic symptoms. As is true of many psychological disorders, the increased risk was higher in men than in women, and the more a person consumed, the greater the risk. The study did not distinguish between different forms and concentrations of cannabis.

Though the association was strong, the authors note that they can’t say for certain whether chronic and heavy cannabis use induces psychosis, or whether people prone to mental illness are more likely to be heavy users. It makes sense that people who feel the symptoms of incapacitating depression or mania, or who sense apparitions or voices only they can hear, might try to self-medicate with cannabis. Without a randomized controlled trial, which would be unethical in the extreme, it’s hard to untangle these strands definitively.

But the study is still eye-opening due to its sheer magnitude. With so many people over so many years, there is very little statistical “noise.” And because the information was gathered from the national Danish Health Registry, there were few dropouts—often a big problem in longitudinal studies. As much as possible, the researchers confirmed that the symptoms of a person’s psychiatric disorder emerged after their chronic cannabis use and diagnosis, not before, and that they compared people who were alike in all ways except the frequency of their use.

Like cigarettes decades ago, cannabis is now widely considered a harmless habit: easy and legal to buy in most places, socially acceptable, and pleasurable in the moment. Over the long term, it may be safer than drinking alcohol. But is it really safe for you and your teenage kids? Only time—and more research—will tell.