Study Suggests Marijuana Use May Not Lower Teen IQ

Two people lighting jointsMarijuana use may not lower Intelligence Quotient (IQ) of teen users, according to a new twin study published in Proceedings of the National Academy of Sciences.

According to the study, marijuana is the most commonly used recreational drug, with roughly half of all Americans trying it at least once. Twenty-three states plus the District of Columbia currently allow some medicinal uses for marijuana, and a handful of states have legalized the drug for recreational use.

Marijuana remains illegal at the federal level, with proponents of the ban saying it is dangerous, particularly for young people whose brains are still developing. The latest study, which tracked twins in two data pools, suggests the drug may not have a direct effect on intellectual decline in teens.

Does Marijuana Affect Adolescent IQ?

Researchers pulled data from two separate studies. The Risk Factors for Antisocial Behaviour study assessed 789 twins’ IQs when they were between ages 9 and 10 in 2001. Over the next 10 years, researchers tested the twins’ IQs five more times.

A second study tested the IQs of 2,277 twins between 1990 and 1996, when the twins were between 9 and 11 years old. Researchers tested the twins’ IQs a second time between 1999 and 2006.

For both studies, participants also completed confidential surveys about their use of marijuana, binge drinking habits, and use of drugs such as painkillers and cocaine. Researchers then looked at pairs in which one twin had never used marijuana and the other used the drug on a daily basis for at least six months.

Both marijuana users and their non-user twins lost about four IQ points during the study. This suggests marijuana does not lower IQ. Instead, the researchers say something else might be at play, such as similar genetics or a change in the twins’ home environment. Marijuana users did have slightly lower scores on vocabulary and general knowledge tests, but the reduction was not correlated with usage frequency or amount.

Conflicting Research on Adolescent Marijuana Use

Marijuana use is a politically charged topic, and studies often show conflicting results. A 2012 study tied heavy marijuana use to lower IQ. However, a follow-up study that looked at the same data found researchers failed to control for other intelligence-lowering lifestyle factors, such as alcohol and tobacco use, mental health conditions, and low socioeconomic status. Though the latest study does not directly contradict the 2012 research, it may provide some comfort to parents concerned about the effects of adolescent marijuana experimentation.


  1. 23 legal medical marijuana states and DC. (n.d.). Retrieved from
  2. Crew, B. (2016, January 20). Extensive twin study finds no link between marijuana use and IQ decline in teens. Retrieved from
  3. Underwood, E. (2016, January 18). Twins study finds no evidence that marijuana lowers IQ in teens. Retrieved from

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  • Chase

    January 25th, 2016 at 2:56 PM

    This study suggests it, does not confirm it, so still won’t be taking any chances.
    Most of us need all the IQ points we can get

  • Josh

    January 25th, 2016 at 5:59 PM

    I have used cannabis from the time I was thirteen. I have chronic migraine headaches 29 out of 30 days in a month marijuana brings that down to 3 to 5 days in a month. My IQ is 175, maybe others are affected differently but the point is marijuana is not nearly as dangerous as you have been led to believe.

  • Chase

    January 26th, 2016 at 10:45 AM

    But how do you know that Josh? How do you know that years from now you will be confronted with all sorts of health issues that can immediately be traced back to marijuana use, no matter how casual?

  • Marianne

    January 27th, 2016 at 11:33 AM

    Let’s be truthful and honest here. The company that is sponsoring the research is always going to have their own agenda and the results will often be skewed to support their theories. This happens all the time, we know it, but we have to pay attention to where all of the information is coming from before jumping to any conclusions.

  • Jacqueline

    January 29th, 2016 at 10:58 AM

    So maybe we don’t want to confront the fact that anything that alters how you think in the moment is probably not such a good idea for you to introduce into your body. There is particular vulnerability with the adolescent and teenage mind. I wouldn’t want any of my own children taking this kind of risk and I am sure that most parents feel the same way that I do. I understand the need to have fun with friends but that should not mean that the only way to do this is to go out and smoke pot.

  • Josie

    January 30th, 2016 at 2:41 PM

    There are sure to be a lot of factors that will play into this. You have to look at things like the education of the family, whether they fall above or below the poverty line, and what other kinds of things do the kids have to witness in their homes. All of these things can play a role in the achievement or not of the child, not just one specific thing that you may be considering.

  • paul j

    January 31st, 2016 at 10:38 AM

    I still struggle with how something can be medicinal on the one hand but on the other it can be harmful.

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