Cannabis derails train of thought, but may not affect long-term memory

The stereotype of a stoner – someone who uses copious amounts of cannabis – is that they are scatterbrained and absent-minded. But how cannabis affects memory is more complicated than this caricature lets on.

For starters, memory is a complex, multi-faceted process. There is long-term memory – the information we retain for months or years – and short-term memory, which lasts only a few minutes. Then there is working memory, which is where we hold information in mind and manipulate it. Working memory allows us to do mental maths and comprehend arguments. And it is clearly impaired by marijuana.

“There is no question that acute cannabis use affects working memory,” says Kristen Morie at Yale University. “If you talk to someone who is actively intoxicated, they will just, kind of, forget their train of thought.”

Researchers believe this is due to the main psychoactive compound in cannabis, tetrahydrocannabinol (THC), the ingredient that makes users feel high. THC binds to CB1 receptors, which are abundant in brain regions involved in memory including the hippocampus, cerebellum and neocortex. This binding process may disrupt our ability to store new information – and if we never store it, we can’t retrieve it later.

For example, a 2020 study had 15 people complete a verbal memory task before and after using THC. The task involved memorising a list of words and then later identifying them in a separate list. On average, participants were about half as likely to identify the words while intoxicated as they had been when sober.

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However, THC appears to interfere less with the ability to recall already established memories. A 2017 study found that people on THC were more accurate at recalling words they had previously memorised while sober than those they had memorised while under the drug’s influence.

Evidence also suggests that a compound in cannabis called cannabidiol, or CBD, may limit THC’s memory-impairing effects. In the same 2020 study, a separate group of 17 people completed the verbal memory test before and after using THC in conjunction with CBD. Participants’ accuracy was, on average, about the same for both tests. “So, if you give someone a [cannabis] strain with THC and a lot of CBD, they may not report as much deficit in working memory,” says Morie.

It is less clear, though, how prolonged cannabis use affects memory. Long-term THC exposure may degrade CB1 receptors, for instance. Some studies also show that people who frequently use cannabis have more problems with all forms of memory than those who don’t, especially if they started using in adolescence. “The idea is that if you’re intoxicated, more often than not, your working memory is so impaired that you’re eventually going to run into issues with short-term and long-term memory,” says Morie.

Yet, these deficits may not be permanent. A 2018 review of 69 studies involving adolescents and young adults found that when those who frequently used cannabis abstained from the drug for at least 72 hours, their memory significantly improved. In fact, they performed about as well on memory tests as people who rarely used the drug. Other research finds that when people who regularly use cannabis stop, the number of CB1 receptors in their brains begins to increase. After four weeks, the density of these receptors returns to normal and is linked with cognitive improvements.

But more research is needed to determine whether most people, especially those who began using cannabis in adolescence, can fully recover from the memory impairments of long-term cannabis use. A 2007 study of 34 people between 16 and 18 years old found that those who used cannabis performed as well on a working memory task after a month of abstinence as those who didn’t use the drug. Yet the study also revealed that the cannabis users showed increased activity in brain regions associated with attention, memory and planning.

“They’re putting more effort into doing the task correctly. It is harder for them,” says Morie. “If you’re using cannabis while your brain is still developing, you’re making changes that may not be reversible.”

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