If pot caused schizophrenia, we’d see schizophrenia incidence go up and down with pot use. We don’t. So it doesn’t. Any questions?

There are renewed efforts, both in this country and in Britain, to re-ignite the moral panic around cannabis use. There’s much less there than meets the eye.

The falling age of initiation (with the median now in the early-to-mid teens) is indeed a legitimate source of worry. Rising potency (as measured by the concentration of delta-9 THC) probably isn’t a problem because users learn to “titrate” their doses, but the increasing ratio of THC to cannabidiol may significantly raise the risk of panic attacks.

But the most frightening claim out there is that cannabis causes schizophrenia. That claim is without any adequate scientific basis.

The purported evidence is from on uncontrolled retrospective correlational studies that wouldn’t pass the laugh test if submitted to the FDA to show a benefit. The studies, by their very design, are incapable of distinguishing between evidence that cannabis causes schizophrenia or precipitates its onset and evidence that sufferers from the undiagnosed early stages of schizophrenia may find cannabis more attractive than do healthy people.

If cannabis caused schizophrenia, we ought to see rates of schizophrenia correlated with rates of cannabis use, both across countries and over time. In the U.S., for example, birth cohort 1953, which came to cannabis-using age at the peak of use, ought to have higher rates of diagnosed schizophrenia than birth cohort 1963, whose years of cannabis initiation coincided with the trough in cannabis use.

So far, there is exactly zero cross-section or cohort evidence showing a link between cannabis and schizophrenia. In Australia, very large increases over time in cannabis use appear not to be correlated with any corresponding increase in schizophrenia.

But the null hypothesis doesn’t make headlines, so I doubt we’ll hear much about cohort effects in the press.

Update Maia Szalavitz reports on more junk science from the drug warriors, this time about cannabis and lung cancer. What Timothy Leary said about LSD is true of cannabis as well: it’s so potent that it causes persistent delusions in people who don’t even take it.

Author: Mark Kleiman

Professor of Public Policy at the NYU Marron Institute for Urban Management and editor of the Journal of Drug Policy Analysis. Teaches about the methods of policy analysis about drug abuse control and crime control policy, working out the implications of two principles: that swift and certain sanctions don't have to be severe to be effective, and that well-designed threats usually don't have to be carried out. Books: Drugs and Drug Policy: What Everyone Needs to Know (with Jonathan Caulkins and Angela Hawken) When Brute Force Fails: How to Have Less Crime and Less Punishment (Princeton, 2009; named one of the "books of the year" by The Economist Against Excess: Drug Policy for Results (Basic, 1993) Marijuana: Costs of Abuse, Costs of Control (Greenwood, 1989) UCLA Homepage Curriculum Vitae Contact: Markarkleiman-at-gmail.com