Meanwhile, legalization advocates have squelched discussion of the serious mental health risks of marijuana and THC, the chemical responsible for the drug’s psychoactive effects. As I have seen firsthand in writing a book about cannabis, anyone who raises those concerns may be mocked as a modern-day believer in “Reefer Madness,” the notorious 1936 movie that portrays young people descending into insanity and violence after smoking marijuana.

A strange disconnect has resulted.

With large studies in peer-reviewed journals showing that marijuana increases the risk of psychosis and schizophrenia, the scientific literature around the drug is far more negative than it was 20 years ago. Comparing two major reports from the National Academy of Medicine , the nonprofit group that advises the federal government on health and medicine, makes the difference clear.

In a report in 1999, the academy (then called the Institute of Medicine) reported that “the association between marijuana and schizophrenia is not well understood.” It even suggested the drug might help some people with schizophrenia. But in its next major report on marijuana, released in 2017, the academy reached a very different conclusion: “ Cannabis use is likely to increase the risk of schizophrenia and other psychoses; the higher the use, the greater the risk.”

Yet the change in the scientific consensus has gone unnoticed. Americans in general are far more likely to believe the drug is safe, and even medically beneficial, than they once were. As a result, support for legalization has doubled since 1999.

Making matters worse, the ways Americans use cannabis are changing in ways that further increase its risks.