Marijuana and Psychosis: Correlation or Causation?

The Denver Post republished an article by Nicole Ostrow of Bloomberg News that begins, “Young adults who used marijuana as teens were more likely to develop schizophrenia and psychotic symptoms…”

Ostrow claims, “The authors said the study was the first to look at sibling pairs to discount genetic or environmental influence and still find marijuana linked to later psychosis.”

No, the authors did not “discount genetic or environmental influence,” nor did they discount other nongenetic differences between siblings.

Here’s what the study (John McGrath etc., Archives of General Psychiatry) says instead: “Prospective cohort studies have identified an association between cannabis use and later psychosis-related outcomes, but concerns remain about unmeasured confounding variables. The use of sibling pair analysis reduces the influence of unmeasured residual confounding.”

Reducing “the influence of unmeasured residual confounding” is hardly demonstrating the direction of causal flow.

Quite obviously, siblings are quite different from each other, not only genetically but according to their environmental interactions and, of critical importance, in their choices. (Does anyone doubt that “psychosis-related outcomes” are at least in many cases significantly the result of a person’s poor choices?) The most obvious explanation for the study’s findings is that the the siblings with the most problems tended to abuse drugs more. In other words, the drug abuse was a symptom of a person’s problems, not a cause of them.

Notably, the study uses an extremely wide definition of “psychosis-related outcomes” that includes “nonaffective psychosis, hallucinations, and Peters et al Delusions Inventory score.” But marijuana is a hallucinogenic drug. So, in part, the study is claiming, “People who take hallucinogenic drugs tend to suffer hallucinations.” (And it cost how much money to figure that out?)

“Nonaffective psychosis” includes things like poor concentration and mood disorders, which are obvious short-term effects of using the drug as well as reflections of personalities with deeper problems.

And what, you may wonder, is the “Peters et al Delusions Inventory score?”It asks questions like the following:

“Do you ever feel as if people seem to drop hints about you or say things with a double meaning?”

“Do you ever feel as if some people are not what they seem to be?”

Besides the fact that some of these questions are ridiculous and need not indicate psychosis, again, people with more problems tend to abuse drugs more. Big insight, there.

Moreover, the differences associated with marijuana use are relatively small. For example, whereas 26 of 1246 people (two percent) who never used marijuana showed signs of “nonaffective psychosis,” 12 of 310 people (3.9 percent) who had used marijuana for six years or more showed signs. Nintety of 1182 people (7.6 percent) who had never used marijuana showed signs of hallucinations, while 54 or 268 (20 percent) of those who had used marijuana for six years or more showed signs (again, not surprising given that marijuana is a hallucinogenic drug).

The upshot is that a small minority of people who didn’t use marijuana showed “psychosis-related outcomes,” while a somewhat larger minority of people who did use marijuana showed such signs. Again, this is consistent with the idea that people with more problems tend to abuse drugs more.

Now, I do not doubt that abusing marijuana (or any drug) can also contribute to a person’s mental and emotional problems. Certainly drug abuse can reinforce a person’s negative tendencies; I don’t need a costly study to convince me of that. However, it is equally obvious that far and away the major problem is something other than the drug abuse. Mostly, the drug abuse is a symptom of deeper problems, not a cause of them. (Regardless, there are many other good reasons not to use marijuana except perhaps medicinally.)

I’m sure that won’t stop politicians and bureaucrats from citing nonsensical news reports of meaningless studies to stir up more Reefer Madness. (Say, wouldn’t paranoia about the impacts of smoking marijuana count as a “psychosis-related outcome?”)

One thought on “Marijuana and Psychosis: Correlation or Causation?”

  1. Comment by Patrick Sperry March 4, 2010 at 9:11 AM

    Have to just love those “ever” questions…
    I have a friend that was involved in a rather difficult divorce, and during it all she accused him of sexually abusing their children, as well as her. Well, he ended up having to go through a screening process via court order. Virtually all of the questions he had to respond to were presented in the “ever” format. The results were being used against him, until the Judge asked him if he had ever acted out any of the things that he was being accused of. Well, he said no, but the tests had said “ever” as in thought about. Since he was a Paramedic, he had been taught about them, and so had thought about them. Go figure!

    Comment by Drew March 9, 2010 at 6:36 PM

    Marijuana is most certainly NOT a hallucinogenic drug. It may alter your senses/perceptions (e.g. music may sound different, food may taste different, sex may feel different while under the influence — and many users will claim that these sensory changes are an improvement) but it will absolutely not cause you to see, hear, or feel things that don’t actually exist in reality — cannabis is just not capable of producing these kinds of effects.

    Comment by Ari March 9, 2010 at 10:09 PM

    Note to “Doctor Drew:” Next time try including some sort of plausible citation. A simple internet search reveals tons of sources claiming that marijuana is hallucinogenic.

    Comment by Drew March 10, 2010 at 1:50 PM

    Citation is not necessary. Sometimes doing a “simple internet search” will reveal all sorts non-sense, such as your claim that cannabis is hallucinogenic. Google is not the Gospel. If you yourself have experience with cannabis (which seems unlikely considering your claim) then you’d know first-hand that it is not a hallucinogen. Of course if you’re lacking in personal experience, rather than taking your talking points from drug prohibition dinosaurs, you can ask folks who actually use cannabis (I know, radical concept here) what they experience when under the influence — shouldn’t be hard to find a few, they’re estimated to be about 10% of the US population, give or take. 1%-2% are daily users, the so-called “wake-n-bake” variety.
    Do you really believe that 2% of the US population is walking around every day hallucinating from sun up to sun down? This should immediately strike you as being absurd! I believe it is estimated that in Colorado by the end of 2010 there will be 100,000 patients on the medical marijuana registry — about 60,000 currently on the registry or awaiting the state to process their application. So 60,000 people in Colorado right now experiencing hallucinations… really? Your claim is just non-sense — and you want to stand by and defend this naiveté on the basis that an internet search tells you so? It’s no wonder we’ve been flushing our tax dollars down the toilet for 70+ years fighting a never ending war on drugs. God help us all. Nobody under normal circumstances is hallucinating from cannabis. Nobody.

    Comment by Ari March 10, 2010 at 2:00 PM

    Drew, your comments are off base, and I’m not going to publish any additional unsubstantiated comments from you.

    First, I have in fact smoked marijuana, and I have in fact experienced a (very minor) hallucination after doing so. (This was many years ago.)

    Second, nobody has claimed that marijuana causes severe hallucinations “from sun up to sun down.” You’re attacking a straw man.

    Third, nobody has claimed that most or even many marijuana users suffer hallucinations. Indeed, the very study I review claims that only a minority of marijuana users experienced a hallucination, and in any given case that may or may not have had anything to do with the marijuana use. (A smaller minority of those who didn’t use marijuana also experienced a hallucination.) You are again attacking a straw man.

    If you wish to submit additional comments, please include some sort of plausible citation, and please refrain from Making Stuff Up about what I’ve written. I don’t have time for nonsense.

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