Why does marijuana make it hard to remember stuff?

Along with Bob Marley and velvet blacklight 420 posters, cannabis is strongly associated with memory loss. Stoner flicks like the 2000 movie “Dude, Where’s My Car?” plays this trope to absurd lengths, but a good deal of scientific evidence backs up the idea that if you toke up, you might struggle with total recall.

Prohibitionists have been hand-wringing about the alleged damaging effects of cannabis on the brain for almost a century. But if you’re anxious about using marijuana and your memory, note that their claims are overblown. Describing cannabis as neurotoxic (that is, poisonous to the brain) is an oversimplification that overlooks its potential for treating Alzheimer’s, Huntington’s disease, Parkinson’s and epilepsy, to name a few.

Yet when it comes to memory, how serious of an issue is it, really? As cannabis prohibition is rolled back in more countries and U.S. states, it’s worth clarifying this question for the millions of active American users.

However, the evidence surrounding marijuana use and memory can be murky, so even though scientists are sure that cannabis does influence memory to some degree, it’s not clear how detrimental or permanent this effect is.

First, we need to be specific about what kind of memory we’re discussing. There are three major classifications of memory: sensory memory, short-term memory (which includes working memory) and long-term memory, which is subdivided into explicit and implicit.

Cannabis only seems to impact short-term memory, so taking a bong rip won’t delete memories of your childhood or something. (Sorry to disappoint anyone hoping weed works like “Eternal Sunshine of the Spotless Mind.”) Furthermore, these distortions seem to be temporary and reversible with abstinence.

“It doesn’t cause any long-term or retroactive harm to your memory. It just affects memories while you’re under the influence,” Peter Grinspoon, a primary care physician at Harvard Medical School who specializes in medical marijuana, told Salon. Grinspoon is also a board member of the advocacy group Doctors For Cannabis Regulation, which seeks to provide patients with evidence about the benefits and limitations of medical cannabis.

Grinspoon describes it as a trade-off. There are many drugs that also impact short and long-term memory, from the anti-anxiety drug Xanax to the allergy pill diphenhydramine, commonly sold over-the-counter as Benadryl. Many sleep aids, antihistamines and even some antidepressants have outsized effects on memory. Of course, the biggest one is alcohol, notorious for causing blackouts with excessive use that can lead to severe memory impairments later in life.

“No drug or medicine is without any side effects or harms,” Grinspoon said. “And cannabis has so many other things it does to cognition that people find useful, such as increased creativity, spirituality, helps with the writing, helps them connect with other people, helps them with art and music.”

The question then becomes how serious and long-lasting are these memory effects? And can they be mitigated?

It’s really hard to say definitively because there are so many variables. It depends on the dose, the type of cannabis, the frequency of use, age, tolerance, sex and much more. It also gets confusing considering that some cannabis users also use alcohol, tobacco or caffeine, not to mention prescription medications. “How can you ascribe the harm or the benefit to cannabis if there are all these other things in the mix?” Grinspoon said.

Even though it’s been demonstrated to affect short-term memory, we’re not really sure how cannabis accomplishes this. The main culprit seems to be THC, a chemical in cannabis responsible for the “stoned” feeling marijuana lovers seek. The cells in our body are flush with cannabinoid receptors and THC loves to snap into the receptors called CB1 and CB2.

However, THC is just a partial agonist, meaning it doesn’t fully activate the receptor. This fuzziness is what gives that signature psychoactive effect. To make matters more complicated, along comes CBD, another chemical in cannabis that doesn’t like to bind to CB1 and CB2 receptors, unless THC is present.

If CBD meets THC at a receptor in the brain, it will kick the THC out. That antagonism means that the THC will be actually less potent. So if you’re ingesting a strain of cannabis that is a balance of THC and CBD, the memory problems are likely to be less of an issue. On the other hand, high-THC, low-CBD products—such as dabs, wax or vapes — can potentially worsen memory issues.

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Scientists aren’t entirely sure why this is the case, but one prevailing theory about THC and memory involves a region of the brain called the hippocampus, which is one of the most important components for learning and memory. It’s also filled with CB1 and CB2 receptors.

When THC enters receptors in the hippocampus, it may trigger the release of more glutamate, a neurotransmitter important for forming memories. More glutamate sounds good, right? But the brain seems to recognize there’s too much glutamate, so it downregulates — or turns off — the receptors glutamate needs to stick to form memories. Suddenly, it’s like the brain is writing with an ink pen that has run dry.

Keep in mind this is just one theory, and experts aren’t one hundred percent sure this is what’s happening. THC may also interact with other receptors involved in memory as well, such as acetylcholine and serotonin. It’s likely a combination of some of these receptors, a 2020 review of animal studies concluded, but with so many complicated relationships, it’s hard to say for sure.

Part of the reason we don’t know exactly how cannabis influences memory is because research into the drug is difficult in the United States. Moreover, many studies are also in animals, like rats and mice, so the results may not translate to humans.

In January 2022, Canadian researchers in the journal Addiction reviewed 10 meta-analyses on cannabis and brain function. Even with a combined sample size of almost 44,000 people, the evidence only ranged from low to moderate quality. Only “small-to-moderate deficits were reported for working memory and decision-making.”

It isn’t clear how long these memory dampening effects linger. “Some studies say the effect can last a couple of days, some say it lasts a couple of weeks,” Grinspoon said. “There’s no good evidence that it causes a long-term or permanent decrement in your memory, but it definitely interferes with people’s ability to sort of encode new information.”

Again, all of this is based on variables like tolerance, age and the potency of cannabis being consumed. If you’re using cannabis and experiencing memory problems, there are steps you can take to prevent getting too stoned. For example, start with a small amount and go slowly, Grinspoon recommended.

“I would say keep the doses of THC on the lower side, if you can. Add CBD,” Grinspoon said. “And then you do all the other things that are really good for your memory, like eat healthy, exercise, get enough sleep. It’s all within the context of how well you’re taking care of your brain anyways.”