Could legalizing cannabis impact IQ? : The debate surrounding the most popular recreational drug

Due to recent legislation concerning legalizing cannabis in the United States there has been a need to better understand what the drug is actually doing to the people who use it. Cannabis, also known as marijuana, is a drug that is commonly used both medicinally, as a treatment for muscle spasms from disorders like multiple sclerosis, seizure disorders, chronic pain or illnesses, weight loss or poor appetite, nausea from cancer therapy, or Crohn’s disease, and recreationally.

Historically, cannabis was not legal in the U.S. and many other nations, especially since the U.S. passed the Marijuana Tax Act in 1937 and essentially prohibited its use. In the 1970s a few states voted to decriminalized the drug and since then more and more states have been following similar trends. In recent years, states have been moving to legalize its use either for medicinal purposes or for recreational use because of its medicinal benefits and because it is believed to have fewer serious and lasting medical consequences than some of the other popularly used recreational drugs.

This really began in 2012 when voters in Washington and Colorado moved to legalize recreational use of marijuana for adults who were 21 or over. This initiative allowed companies to produce and distribute the drug. There have been many points made about increasing tax rates or regulating the drug in the same way as alcohol. People argue that this could be a huge benefit for the economy and would be a low risk endeavor. Legalizing the drug would get the dealers who would be illegally selling products that may or may not be completely safe due to the possibility of unknown substances being cut into it. This would also benefit people who use the drug for medicinal purposes because they would be able to access it more easily and get the relief they need.

Other states are more against the liberal move and argue that it could be more harmful than people may think. There is still debate over the effects of cannabis on the brain and body of users and there have been contrasting results in many of the research that has been conducted so far.  One study found evidence that current cannabis users showed lower IQ scores, however this trend was not seen in former users (Fried et al., 2005). Another study found evidence of a decline in IQ in adults who were frequent users of cannabis in their adolescence and these effects were seen even if the adult was no longer using cannabis (Meier et al., 2012). Both of these studies show seemingly condemning results for cannabis even if they are not in complete agreement; however, the directionality of the results is questionable. Does the use of cannabis result in the lower IQ or could it be that the cannabis use was a result of is a confounding variable in play?

A third, recent study showed that when controlling for several outside variables, like cigarette or alcohol use, childhood mental-health symptoms, and behavioral problems, the IQ of adolescents who frequently used cannabis did not differ significantly from those who had never used the drug (Mokrysz et al., 2016).  This study seems to contradict the precious ones completely!  Does this mean the cannabis does not actually affect IQ?

This debate then raises questions about what then caused the lower IQ or the cannabis use. Was there some other factor from the environment or a genetic aspect? Many similar questions could be asked of alcohol use and abuse. People in lower socioeconomic status, who had a parent who abused alcohol, or who were abused themselves as a child show a greater risk for alcohol abuse as an adult (Widom & Hiller-Sturmhöfel, 2001; Caleyachetty et al., 2016). This lends itself to the idea that the childhood home environment or something in the genetics was responsible for the alcohol abuse in adulthood, so could this same conclusion be made for cannabis use?

The questions of directionality and causality has yet to be concretely decided upon. Did the drug abuse result from a lower IQ or something in the environment of the individual or did the drug abuse cause a decrease in IQ? This is a prominent question in the debate of legalizing cannabis in the United States and it, and others like it, need to be addressed before a distinctive decision can be made for everyone to agree upon.


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Caleyachetty R., Khaw K.T., Surtees P.G., Wainwright N.W., Wareham N., & Griffin S.J. (2016). Cumulative social risk exposure in childhood and smoking and excessive alcohol use in adulthood. The European Journal of Public Health, 26(1), pp 1-6. DOI:

Fried P., Watkinson B., & Gray R. (2005). Neurocognitive consequences of marihuana – a comparison with pre-drug performance. Neurotoxicol Teratol, 27, pp 231–239.

Meier M.H., Caspi A., Ambler A., et al. (2012). Persistent cannabis users show neuropsychological decline from childhood to midlife. Proceedings of the National Academy of Science of the United States of America, 109(40), pp 2657–2664. doi: 10.1073/pnas.1206820109

Mokrysz, C., Landy, R., Gage, S.H., Munafò, M.R., Roiser, J.P., & Curran, H.V. (2016) Are IQ and educational outcomes in teenagers related to their cannabis use? A prospective cohort study. Journal of Psychopharmacology, 30(2), pp. 159-168. DOI: 10.1177/0269881115622241

Widom, C.S. & Hiller-Sturmhöfel, S. (2001). Alcohol abuse as a risk factor for and consequence of child abuse. Alcohol Research and Health, 25(1), pp 52-57.




2 thoughts on “Could legalizing cannabis impact IQ? : The debate surrounding the most popular recreational drug

  1. Marijuana does not retard IQ. I haven’t read one of those studies in a while, refresh my memory for one. But what was the age of the subjects in the cohort? If they were young teens or teenagers, IQ would definitely be lower during a retake at adulthood, as at adulthood any environmental effects dissappear and your genetics mediates your IQ.


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