SOME RESEARCH HAS FOUND THAT MARIJUANA USERS HAVE AN INCREASED RISK of developing schizophrenia and other forms of psychosis. Could rising concentrations of tetrahydrocannabinol, or THC—the marijuana plant’s major psychoactive component—intensify that link? Last March, a study from England that was published in Lancet Psychiatry found that daily users of high-potency cannabis (known in the United Kingdom as “skunk”) were five times more likely than people who don’t use cannabis to be diagnosed with a first psychotic episode. A brain imaging study published later last year involving some of the same patients found evidence that skunk users may be at risk for damage to the corpus callosum, a thick bundle of fibers connecting the brain’s hemispheres.

These and other studies helped spark a debate in Nature magazine. If high-potency pot induces schizophrenia, then rates of the disease should be on the rise, but that doesn’t appear to be true, wrote Matthew Hill, a cannabinoid neuropharmacologist at the University of Calgary’s Hotchkiss Brain Institute. Hill concedes that people with schizophrenia may be more likely to use pot, but argued that is likely because they’re self-medicating their anxiety and other symptoms. Several researchers investigating the pot-psychosis link responded to Hill’s assertions, arguing that there’s too little data to say whether or not schizophrenia rates are rising among users of potent marijuana, though they noted that one study reported the number of people in southeastern London using pot in the year prior to receiving an initial diagnosis of schizophrenia rose between 1965 and 1999.

A number of investigators have reported an association between pot and psychosis, says McLean Hospital neuroscientist Staci Gruber. “But I think it’s more likely that using marijuana exacerbates or unmasks” an individual’s underlying predisposition toward developing a psychotic disorder, Gruber says. Indeed, several genes have been identified that appear to predispose pot users to develop schizophrenia, though the role of those genes remains controversial. “Whether or not marijuana causes psychosis,” says Gruber, “remains to be seen.”