Segra International | #ASKDRJOHN – Does Smoking Marijuana Make You Dumber
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#ASKDRJOHN – Does Smoking Marijuana Make You Dumber

By John Brunstein
13 Apr 2016

#AskDrJohn has received a question:

I smoked a lot of marijuana in high school and felt like it dumbed me down. I became quite forgetful. What are the side effects of Cannabis and is killing brain cells one of them or is that just a myth? GS

GS, all drugs have side effects. Partly that’s a semantic argument, because “side effect” really just means “a consequence you didn’t want”. That doesn’t mean it wasn’t a consequence someone else didn’t want, so it’s a matter of personal interpretation. Someone taking Cannabis to control nausea and assist appetite while on chemotherapy thinks the appetite increase is a good thing; to someone taking Cannabis as a sleep aid, “getting the munchies” is a side effect. Based on your question, I’ll focus this reply on cognitive and memory related side effects.

Much of that question has already been addressed in detail by our QA Manager, Dr. She, so I’ll direct you to his article Cannabis Part 3: Cannabinoids and Mental Health available in Segrapedia. In addition to what he’s said there, I’ll add a few other comments below. Basically, they can be summarized in one word.


Not a very helpful answer, is it? The lack of certainty in this answer isn’t because the topic hasn’t been studied heavily; it’s because many studies have come up with statistically weak or contradictory results, or ones that can’t clearly establish a causal relationship (did the Cannabis use lead to the observed brain changes, or did the brain changes predispose the person to be a Cannabis user?). I’ll try to summarize what some of these studies have suggested, though, and what the “best guess” is for an overall answer in light of the collected present data.

Figure 1: Does smoking marijuana make you more forgetful?

During adolescence, the human brain is still in the process of maturing which includes ‘fine tuning’ of connections between neurons, and possibly even killing off some neurons as cognition and memory functions get fully established. In doing this, neurons are receptive to various neurotransmitters including the endocannabinoids which operate via CB1 and CB2 receptors in the brain. It’s not irrational to think that changing the neurochemical environment by taking Cannabis may influence this maturation; in effect the brain will try to ‘optimize’ against a set of conditions which aren’t really the correct, no-Cannabis-intake, baseline. How big of an impact this is, from none at all to quite significant, may however depend on a number of things including personal genetic factors (sensitivity), frequency and dose of Cannabis exposure, and whether there is concomitant use of alcohol.

Short term (acute) effects of Cannabis use on memory and cognition are generally pretty well supported – there’s lowered test scores on a number of memory function and other cognition tests in users immediately after Cannabis use. However, after a period of non-use, data generally suggests a recovery of these functions. That period of non-use needs to be quite long though, on the order of several weeks or more. If you were using Cannabis as an adolescent once every few weeks, then noting loss of memory wouldn’t be too surprising compared to other data; although your term “quite forgetful” would seem rather severe. Since you didn’t comment on your amount or duration of use, though, I can’t really comment.

With regard to ‘killing brain cells’, the data is again very mixed for adolescent use of Cannabis. Some studies have detected differences in grey and/or white matter between heavy users and non-users, but some other studies have not detected these. Again, part of the reason for differing and unclear outcomes is probably because studies are “lumping together” people with different as-yet unknown genetic susceptibilities or confounding factors as if they’re the same. That leads to data sets where some people may show an effect and some don’t, and as a group we can’t tell what’s going on.

On the whole though these studies do suggest that there is at least some risk that adolescent use of Cannabis may have unwanted long term effects on mental function and structure, and that these risks are made worse by earlier onset of Cannabis use, and heavier use. While we can’t be absolutely sure there is a long-term risk, we can be quite sure the data doesn’t unequivocally show no risk. (You may want to re-read that sentence, since it’s the punch line). With these sorts of odds, many jurisdictions which permit medical Cannabis chose to be safe and restrict use to adults only. A particular exception is sometimes made for young children with intractable epileptic seizures but in these cases, use of CBD as opposed to whole THC-containing Cannabis would appear to be the safer option.

If you’d really like to dig into this topic more, I’d suggest one reference in particular as a starting point. Jacobus and Tapert’s review paper on this exact topic [1] is available freely online, is written with less technical jargon than many other papers in the field, and has a very balanced coverage including sections on changes to grey and white matter macrostructure in adolescent Cannabis users compared to various controls. The conclusions still end up as “maybe”, but based on more depth of material and in more detail than I can address in this brief reply.


1: Jacobus J, Tapert SF. Effects of Cannabis on the adolescent brain. Curr Pharm Des. 2014;20(13):2186-93.