Acute cannabis intoxication appears to greatly disrupt most of the memory systems

Smoking weed can do more than just blur memories. It can reshape them.

A new Washington State University study found that people who consumed THC were more likely to remember words that were never presented and had difficulty with everyday tasks, such as remembering to do something later.

Posted on Journal of Psychopharmacologythe study is one of the most comprehensive examinations to date of how cannabis affects memory. The findings show that cannabis can impair not only simple recall, such as memorizing a list of words, but also forms of memory that people rely on in everyday life, such as remembering appointments, following conversations, or recalling where information came from.

The researchers were also surprised to find no significant differences between participants who consumed 20 milligrams of THC and those who consumed 40 milligrams, suggesting that even moderate doses can cause significant disturbances.

Most previous studies have examined only one or two types of memory, such as word list recall. This is the first study to comprehensively examine several different memory systems simultaneously, and what we found is that acute cannabis intoxication appears to greatly disrupt most of them.”

Carrie Cuttler, senior author of the study and associate professor of psychology at WSU

To explore these effects, Cuttler and co-author Ryan McLaughlin, an associate professor in the Department of Integrative Physiology and Neuroscience at WSU, recruited 120 regular cannabis users and randomly assigned them to vape a cannabis placebo, 20 milligrams of THC, or twice as much THC in blind experiments. Participants then completed approximately one hour of tests measuring verbal, visuospatial, perspective, source, false, episodic content, and temporal sequence memory.

Participants who used cannabis performed significantly worse than the placebo group on most measures. Overall, cannabis affected the majority of memory tests, with significant differences appearing in 15 of the 21 measures.

The strongest effects were seen in false memory and source memory, systems that help people accurately recall information and determine where it came from. In one test, participants heard lists of related words but not the keyword linking them together. Later, those who had consumed cannabis were more likely to “remember” related and unrelated words even though they had never been presented.

“I found that it was very common for people to find words that were never on the list,” Cuttler said. “Sometimes they were relevant to the topic of the list and sometimes they were completely unrelated.”

The researchers also found that cannabis users had more difficulty remembering the source of information they had learned earlier. Problems with source memory can make it more difficult to distinguish whether the information comes from a trusted source, a conversation, or something found online.

Such disruptions could matter in situations where accurate recall is important, such as eyewitness interviews where memory errors or suggestive questions can affect how events are remembered.

The study also found impairments in prospective memory, the ability to remember to perform tasks in the future, such as taking medication, attending a meeting or stopping at the store on the way home.

“These are things we rely on all the time in our daily lives,” Cutler said. “If you have something you have to remember to do later, you probably don’t want to be high when you have to remember to do it.”

One type of memory, episodic content memory or the ability to remember personally experienced events, did not show a significant effect. Cuttler said more research is needed before firm conclusions can be drawn in this area.

The research comes as cannabis use is becoming more common in states like Washington. Despite increasing legalization across North America, many of the drug’s short-term cognitive effects remain poorly understood because cannabis is still classified as a Schedule I substance under federal law.

“We live in a state where cannabis use is very common, but there’s still a lot we don’t know about its acute effects,” Cuttler said. “The goal is to help people make informed decisions about risks and benefits.”