Does Marijuana Really Trigger Psychosis?

Many in the media have speculated that Jared Loughner’s heavy use of marijuana could have been in some way behind his recent rampage in Arizona that cost six people their lives, and left Congresswoman Gabrielle Giffords critically wounded. Is this claim just media hype or based on real science? 

Big Think expert James Watson, co-finder of the structure of DNA, linked marijuana and schizophrenia last year in his interview:

“It’s clear that if you’re pre-disposed to schizophrenia, smoking marijuana will tip youover. But marijuana won’t tip over someone into schizophrenia that is probably notpredisposed toit. So, you know, most people smoke it and do not end up inmental hospitals.But some do.”

Big Think expert Michael Stone, a forensic psychiatrist at Columbia University, tells us that Loughner may indeed have induced his illusory preoccupations and paranoid thoughts from the marijuana. He says it’s true that “among an unknowable subgroup of adolescents, heavy abuse of marijuana will cause a psychotic episode.” But this apparent psychosis could be indicative of one of two different things: drug-induced psychosis or drug-aggravated schizophrenia. 

“If you have 100 adolescent pot smokers, there’s going to be several of them (probably half a dozen) who will have a temporary drug-induced psychotic episode, including auditory or visual hallucinations. But this psychosis will generally subsist after the marijuana use is stopped. Then there is another group (probably just one or two people), who are destined to become schizophrenic but have not yet begun to exhibit symptoms.” In these cases, the marijuana could aggravate latent schizophrenia, causing its symptoms to manifest before they would otherwise. Where Loughner falls among these categories is unknown, but it does seem clear that he was a heavy marijuana smoker and also displayed psychotic behavior. Can marijuana really be blamed for this tragedy? And does this linkage, which occurs in very small minority of cases, really justify making marijuana illegal? 

Or, as his haunting mugshot suggests, is Loughner just plain “evil”? Below is a clip from Stone’s Big Think interview in which discusses the neurobiology of what we widely perceive to be “evil” behavior.

More Resources:

—”High-potency cannabis and the risk of psychosis,” (2009) by Marta Di Forti, et al., in The British Journal of Psychiatry