Does cannabis really lower your IQ?

My recent research has shown that differences other than cannabis use might be causing the much-discussed disparities in cognitive function

Will smoking that joint really make you less intelligent? Photograph: Alamy Stock Photo

Claire Mokrysz

Whether or not using cannabis can lead to cognitive impairment is a hot topic of research and public interest. Given the extensive media attention granted to findings that suggest detrimental effects of cannabis on cognition, brain function and mental health, you would be forgiven for thinking smoking a spliff was akin to repeatedly bashing yourself over the head with a giant bong. However, since much of the work to date is cross-sectional (that is, measurements are taken only at one time in a person’s life), we cannot know whether cannabis users would have performed any differently before they started using cannabis. In short, we’re faced with a classic “chicken or egg” problem.

Cannabis use does not occur in a vacuum. And teenagers who start using cannabis from a young age will almost certainly differ from those who will never try, it or who delay until they are older. The evidence suggests that those who start using cannabis from a young age often have less stable backgrounds and more behavioural problems than their non-using peers. Teenage cannabis use also typically goes hand in hand with other drug use and risky lifestyle choices in general. The poorer cognitive performance of cannabis users may therefore result from other factors associated with cannabis use, rather than cannabis use itself. However it is of course very difficult to control for all these other factors.

To attempt to tackle these issues, along with other researchers from University College London and University of Bristol (including Suzi Gage who hosts this blog), I have been involved in a new study, with potentially surprising findings. Using data on 2235 teenagers collected as part of the “Children of the 90’s” cohort from South West England, we looked at the relationship between how many times someone reported having used cannabis by the age of 15, and their performance on an IQ test completed at the same age.

Importantly the teenagers had also taken an IQ test when they were 8 years old (before any of them had used cannabis), so we could tackle the “chicken or egg” problem.

At first look our results suggested that those teenagers who had used cannabis performed worse on their teenage IQ tests, after accounting for their ‘baseline’ IQ at 8 years old. Even those who had only used cannabis a handful of times scored roughly 2 IQ points lower than those who had never tried cannabis.

However, we also noted that the teenagers who had used cannabis were much more likely to have used cigarettes, alcohol and other illicit drugs- and all these factors also predicted lower teenage IQ scores. Most strikingly we saw that cannabis users were also much more likely to be tobacco cigarette smokers- 84% of those in our heaviest cannabis use group (who reported having used cannabis at least 50 times by age 15) had smoked cigarettes more than 20 times in their life, compared to just 5% of those who had never used cannabis.

When we statistically adjusted for these differences in rates of other substance use, along with other factors including childhood behavioural problems and mental health symptoms, cannabis use no longer predicted lower IQ scores. After this adjustment even our heaviest group of cannabis users had predicted IQ scores no different to those who had never tried cannabis. We also ran a similar analysis to look at the same teenager’s school GCSE grades, which they sat at age 15/16. The findings were similar to our IQ findings- while cannabis users achieved lower grades at GCSE (the equivalent of 2 grades lower on one subject), once we took account of these other related factors cannabis use no longer predicted worse school performance.

It seems therefore that there is something else about these two groups of teenagers (those who had used cannabis by age 15 and those who had not) that is responsible for the differences in IQ and school grades, rather than their cannabis use, though it’s not clear what from our study. Although cigarette smoking was identified as a potentially important factor, we clearly can’t know from this type of study whether it actually causes lower IQ and school performance, and there is little evidence elsewhere to suggest this is the case.

While this may sound like great news for those 15% of 15-24 year old Europeans who have used cannabis in the past year, the take home message is sadly not so clear cut. This is just one study from one cohort in one area of England, and as authors of the paper we are the first to acknowledge the limitations of this work, including the young age of the participants when we measured IQ, and the relatively moderate levels of cannabis use.

A well-publicised study from 2012 suggested that cannabis use starting in adolescence and persisting into mid-life “is” related to IQ decline. So how do these potentially opposing findings fit together? The key difference between the 2012 study and ours is the type of cannabis users included in the study. Our heaviest using teenagers had been using cannabis for approximately 2 years, and had used cannabis at least 50 times each (although 57% of this group reported having used cannabis at least 100 times).

In the 2012 study those who showed the most dramatic IQ decline had been persistent cannabis users from adolescence until their late 30’s, and had been diagnosed with cannabis addiction at numerous points in their life. So it’s possible that cannabis addiction, rather than cannabis use per se, is related to lower IQ, or that persistent heavy cannabis use throughout your lifetime can to have these negative effects.

Our study is by no means definitive, but it does highlight that we should all be more cautious when jumping to conclusions about the harms of a drug before we have strong evidence either way. Overly forceful conclusions about the potential negative effects of cannabis are unscientific and based on an incomplete evidence base. This can lead to the unfair marginalisation of teenagers who use cannabis, which is the last thing we would want, given that this group is likely to include some of the most vulnerable in society. 

Claire Mokrysz is a PhD student at University College London investigating whether teenagers are particularly susceptible to harm from cannabis and alcohol use.