Does Cannabis Lower Your IQ?

byeearlenbaugh4 minutes

Cannabis doesn’t have the best reputation when it comes to affecting intelligence. The media, politicians, and even doctors and scientific researchers repeat the same line year after year: “Don’t use marijuana. It will make you stupid.” I have to admit that this is what I believed growing up. I fully thought that while cannabis might be enjoyable, I should avoid it because it would lower my IQ.

The story that cannabis will make you stupid is repeated so often that it’s accepted as fact by many. But is this even true?

Common wisdom would say yes. But despite the common wisdom, the best scientists disagree. More and more research is now coming out that myth-busts this tired old theory. And if there’s one thing these new studies show, it’s that marijuana’s negative effects on intelligence have been vastly overestimated.

The Case Against Cannabis: Evidence for Lowered IQ

It’s not that surprising that people think cannabis consumption lowers IQ. After all, if you’ve tried cannabis or been around people consuming the plant, you might have noticed some intelligence-altering effects. With some types of cannabis, people may experience side effects like:

  • Mental confusion
  • Slowed reflexes
  • Difficulty focusing
  • Poor memory

While these effects occur while cannabis is consumed and usually a few hours afterward, many wonder if these effects might be longer-lasting with regular use.

Scientists jumped to answer this question—and at first, it seemed like the research confirmed our worst fears. Studies reported that adolescents who consumed cannabis would permanently lower their IQ. One major study in 2012 led by Duke University’s Madeline Meier looked at a group of adolesents in New Zealand. The research concluded with a chilling statement: “Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning.” It seemed clear that cannabis was no good for the developing mind.

Confounding Factors: The Cannabis IQ Evidence Reviewed

This study seemed to confirm that cannabis lowered IQ. But as more scientists looked over Meier’s research, it became clear the picture wasn’t as simple as it first seemed. Other researchers began to point out that certain factors hadn’t been taken into consideration during the study. In 2013, Ole Rogeberg, a researcher from Norway, pointed out that the New Zealand study hadn’t accounted for socioeconomic status in the analysis. Since those with lower socioeconomic status are both more likely to have lower IQs and use cannabis as adolescents, it needed to be accounted for in the statistical analysis.

When Rogeberg simulated what kind of IQ differences to expect when taking that into account, his analysis reproduced the reported associations from the Meier study. This suggested that the Meier study may have just been picking up on the IQ differences in socioeconomic groupings, rather than discovering a lowered IQ for cannabis users in particular.

Other researchers believed there might be confounding factors in other areas as well. Some ran tests to see if there were any IQ differences between cannabis consumers and non-consumers when taking into account factors like cigarette use, childhood mental-health symptoms, and behavioural problems. Interestingly, when these factors were accounted for, there was no association between cannabis consumption and lowered IQ.

Most recently, a large meta-review was published on the science that looked at cannabis and IQ so far. The authors concluded that the reports of cannabis’s permanent effects on IQ are vastly overestimated. Previous studies didn’t wait long enough after cannabis cessation to test cognitive function. Tests conducted at least 72 hours after last cannabis use showed that the cognitive differences dropped to a number below statistical significance.

All of these later studies report the same thing: When the important factors are accounted for, there’s no IQ drop or cognitive difference amongst cannabis users and non-users.

Acute Cannabis Effects on IQ

Based on this research, it seems like you only have to worry about cannabis affecting your cognitive function while you’re consuming it. So does this mean that whenever someone is using cannabis, they’re less cognitively functional? While some types of marijuana can be temporarily impairing for some people, cannabis doesn’t always affect cognitive function negatively.

For one thing, strain matters. There are many active components in cannabis, and while some give users a hazy confused high, others produce a clear-minded state. Finding strains that are functional for you might take some experimenting, but there are plenty of marijuana options that can result in a clear and focused mind.


A second factor is the person consuming the cannabis. Research shows that certain people may find cannabis gives them more focus. Patients with ADHD, for example, are often able to focus better under the influence of cannabis than those who don’t have trouble focusing and might therefore find cannabis more disorienting. If you suffer from a lack of focus, marijuana might actually help, not hurt you.

A final factor to consider is that heavy chronic cannabis consumers don’t usually suffer from the disorienting effects of cannabis. While you’d think a chronic daily consumer would be feeling hazy and unfocused all day, every day, the research shows it’s just the opposite. When tested, chronic marijuana users have been indistinguishable from non-users on cognitive functioning tests. While those who occasionally consume cannabis might find its effects disorienting or impairing, those who consume it every day seem to be able to adjust to its effects. The science shows that these heavy users perform normally even when they’ve just consumed marijuana.

We’re far from a definitive answer when we ask if cannabis can lower IQ or negatively affect cognitive performance. Even if your cannabis experience thus far has included slower reactions and poor memory, there are strains out there that could give you focus and energy. The beauty of the plant is that it affects everyone differently. So, if you want to consume marijuana but you’re concerned about how it affects your reflexes or focus, try a different strain to see if it makes you feel more alert and better able to concentrate.

Photo credit: Marco Verch