I'm no scientist, but this pot-makes-you-crazy news seems, well, crazy to me. Missing from most news reports ("Pot Smoking Linked to Psychotic Disorders," Los Angeles Times, July 27) is this fact: Only a teeny proportion of humans go psycho overall, and smoking the reefer only increases one's chances by a minuscule amount. And, as with the family-and-friends-make-you-fat study, isn't there a presumption of cause and effect here that's completely flawed? What about the utterly logical explanation that those who eventually become schizophrenic or psychotic are more inclined to smoke pot as an attempt at self-medication along the way? Um, Science—antidrug agenda much?


Not That I Would Know


Before we beat up on the offensive Lancet article—Science has been looking forward to this all week—let's look at how this study was conducted. No new data was collected; the authors mixed together data from papers where early pot smoking could be correlated to later rates of psychosis. Seven cohort studies made the cut. Psychosis is defined pretty loosely here as "the presence of psychotic symptoms," like hallucination, disorganized thoughts, agitation, or aggression. For most of the studies used, the symptoms could be so mild as to not impair functioning. By this standard, most of us are crazy; the way Science drives during rush hour would qualify as "psychotic" under this definition.

Blending the data from the seven studies, the authors found that people who had ever smoked pot had an overall 40 percent increased chance of having psychotic symptoms later; for heavy pot users—daily use or dependence—the risk was closer to being doubled. But half of the underlying observations made no effort to correct for the use of other drugs or alcohol. (Pot users, particularly heavy pot users, also using alcohol or other drugs? Never!) Could the subjects just be high when tested for psychotic symptoms? It "can be a difficult judgment in people who use cannabis frequently," the authors note dryly. Pot is illegal; it can be mixed with other drugs or toxins and doesn't exactly come in a standard strength. Most importantly, the authors admit "an association seen in an observational study does not necessarily reflect a causal relation." Exactly. Hume would be proud.

So, should we believe that pot now causes psychosis later? Maybe. Only armed with this meta-analysis, we can't say. Other studies show that pot changes the brain in ways that resemble the brains of people with psychotic disorders, giving the pot-causes-psychos idea a bit more credence. Without an interventional study—where the participants are randomized to either use or not use pot—we can't rule out that having a brain prone to psychotic symptoms later encourages you to smoke pot now.

For the sake of argument, let's assume that smoking pot does increase your chances of developing psychosis later. About five percent of the population has psychotic symptoms sometime during life, whether from Alzheimer's, a high fever, extreme stress, mania, or schizophrenia. Based on this study, it would take 20 people with a lifetime of heavy pot usage to have just one extra person with a (mild) psychotic episode. And, of course, they might just be high at the time. Science is unconvinced.

Empirically Yours,


Send your questions to dearscience@thestranger.com.