Does Marijuana Lower IQ?

Who was ever harmed by smoking a little weed? Right? Wrong. Good research shows that people who start smoking marijuana, or cannabis, regularly when they are in their teens and continue into their twenties have a later permanent decrease in IQ. The decrease does not seem to be reversible even after marijuana use is stopped. http://www.pnas.org/content/109/40/E2657 Of course, people make decisions every day which are trade-offs between benefits and risks, but here is a brief, accurate summary of current medical and scientific opinion on this important issue.

First, there are a number of health risks from marijuana use: potential for addiction, increased risk of the serious psychiatric disorder schizophrenia and others which I will have to discuss in a later article. Even so, there seems to be a widespread opinion that marijuana use is a harmless pastime which should be legal or only minimally regulated. Canada just this month legalized the recreational use of marijuana. https://www.medscape.com/viewarticle/895197 A number of US states have legalized recreational marijuana use. Twenty-nine states, with little supporting medical evidence, allow the use of “medical marijuana” with a physician’s prescription.

Concerns that regular marijuana use by adolescents can decrease IQ in later life are based on a New Zealand study, called the Dunedin study, which followed a group of people born in 1972. IQ testing was done in 1985 and 1986, before the onset of marijuana use, and again in 2010 – 2012 at an average age of 38. Compared to nonusers, individuals who started regular marijuana use during adolescence had an approximately six-point decrease in IQ at the end of the study. This change is significant, alarming, and the IQ decrease was not reversed even after study subjects stopped marijuana use.

Is it possible that this study did not really prove this association between cannabis and lower IQ? That seems unlikely, although the study is what is called an epidemiologic study, so there might have been unknown characteristics of study subjects which could have caused both cannabis use and the later IQ decrease. Such incorrect conclusions are minimized in drug trials which randomly assign people to drug vs no drug (placebo) groups. In medical research, however, it is unethical to assign patients to an intervention which might be harmful, in this case, marijuana use. This is why such a study will never be done. Researchers in the Dunedin study controlled for a number of important variables and their conclusions seem valid.

There are, of course, a few legitimate medical uses for drugs derived from cannabis. A cannabis derivative was recently approved by the FDA for the treatment of two rare forms of epilepsy usually seen in children – Dravet syndrome and Lenox-Gastaut syndrome. https://www.medscape.com/viewarticle/898510 The FDA has also approved two cannabis derivatives for nausea and vomiting due to chemotherapy and for stimulation of appetite. Aside from the two types of childhood epilepsy, cannabis drugs are less effective than existing drugs and are rarely used. https://www.medscape.com/viewarticle/895197

In conclusion, a potential permanent decrease in IQ due to regular use marijuana which begins in adolescence should be widely known, publicly discussed and should receive widespread attention from the media. While few people support increasing legal penalties for marijuana use, decreasing the penalties and condoning widespread marijuana use does not seem prudent.