Cannabis intoxication broadly impairs multiple memory types, new study shows

A recent study published in the Journal of Psychopharmacology suggests that using cannabis has widespread negative effects on many different types of memory. The findings provide evidence that getting high impairs everyday cognitive functions, like remembering to perform future tasks or recalling the exact sequence of past events.

“It is well established in the empirical literature that acute cannabis intoxication is detrimental to memory,” said study author Carrie Cuttler, an associate professor at Washington State University and a co-director of WSU’s Cannabis Research Center.

“However, the bulk of prior research examining the acute effects of cannabis on memory in humans has focused on a relatively narrow set of memory tests primarily verbal memory tasks, which involve recalling lists of words, and working memory tasks, which require temporarily holding and manipulating information in consciousness.

“Far fewer studies have examined how cannabis affects other types of memory that are more relevant to everyday life. These include temporal order memory (remembering the order in which events occurred), prospective memory (remembering to perform tasks in the future), source memory (remembering where information came from), false memory (recalling information that was never presented), and episodic content memory (recalling personally experienced events).”

“To address this gap, we conducted a randomized, double-blind, placebo-controlled study examining the acute effects of cannabis across multiple memory domains,” Cuttler explained.

The researchers recruited 120 adults who regularly used cannabis at least once a week. The participants were divided evenly into three groups of 40 people. One group vaporized a placebo flower containing zero active tetrahydrocannabinol. A second group vaporized a moderate dose of 20 milligrams of the drug, while the third group vaporized a high dose of 40 milligrams.

The participants were randomly assigned to different groups, and neither the participants nor the scientists knew who received the actual drug or a fake substitute until after the experiment ended.

After inhaling the vapor from a tabletop vaporizer, the participants waited about seven minutes. They then completed an hour-long series of specific cognitive tests. These tests measured a wide variety of memory domains using established psychological tools.

To measure verbal memory, participants listened to and recalled lists of words immediately and after a delay. The researchers also tested working memory, which involves holding and manipulating information, by asking participants to repeat sequences of numbers in reverse order. Visuospatial memory, which involves recalling shapes and their physical locations, was tested by having participants study geometric figures and draw them from memory.

Prospective memory was measured by seeing if participants remembered to rate the difficulty of a task immediately after finishing it or ring a bell every ten minutes. The researchers also tested source memory, which is the ability to remember the origin of information. This was done by checking if participants could remember whether they had seen a specific item as a picture or as a written word earlier in the session.

Susceptibility to false memories was tested by having participants listen to lists of related words. For instance, they might hear words like “bed” and “tired” and later be asked if they heard the unsaid target word “sleep.” Temporal order memory was evaluated by asking participants to organize physical cards into the exact sequence in which they completed the cognitive tests.

Finally, the researchers evaluated episodic content memory. This concept refers to a person’s ability to recall specific, personally experienced events. To test this, participants were simply asked to freely describe all the different cognitive tasks they had just completed during the session.

The data provides evidence that acute cannabis intoxication broadly harms most memory types. Compared to the placebo group, participants who inhaled the moderate and high doses performed worse on immediate and delayed verbal memory tasks. They also struggled with immediate and delayed visuospatial memory, having a harder time drawing the shapes they had seen earlier.

The drug increased the creation of false memories. Intoxicated participants were more likely to confidently remember seeing words that were never actually presented to them during the listening tasks. They also had a harder time with source memory, meaning they forgot whether they originally saw a picture or a word.

The researchers detected immediate negative effects of cannabis on prospective memory and temporal order memory. Intoxicated participants frequently forgot to execute tasks at the right moment and struggled to remember the correct order of the tests they had just taken. These findings suggest that the drug heavily impacts the practical memory skills needed for daily functioning.

Not all types of memory were significantly impacted by the drug. The researchers found no significant effect on episodic content memory, meaning intoxicated users could still broadly recall which events they had participated in during the session. Performance on certain short-term memory tasks also remained relatively stable across the groups.

“The key takeaway is that acute cannabis intoxication appears to negatively impact a broad range of memory processes, rather than selectively affecting only a few types of memory,” Cuttler told PsyPost. “We found significant detrimental effects of acute cannabis intoxication on verbal memory, visuospatial memory, working memory, prospective memory, source memory, false memory, and temporal order memory. To our knowledge, this is the first study to detect detrimental effects of acute cannabis intoxication on prospective memory and temporal order memory.”

“We did not detect a significant effect on episodic content memory, which in our study involved recalling the tasks participants completed during the session. However, this is the first study to examine acute cannabis effects on this particular aspect of memory.”

Interestingly, the data suggests that taking a higher dose does not necessarily worsen these memory impairments. The researchers found no significant differences in test scores between the moderate dose group and the high dose group.

“We were surprised that there were no meaningful differences between the moderate-dose (20 mg THC) and high-dose (40 mg THC) groups,” Cuttler said. “This is likely because participants in both groups experienced substantial intoxication.”

While the study was comprehensive, it does have some limitations. For instance, some of the memory tests were relatively easy, causing many participants to achieve perfect scores regardless of what they inhaled. This phenomenon might have masked some of the subtle impacts of the drug on time-based tasks. The scientists also acknowledge that the artificial laboratory setting might not perfectly capture how people remember their own personal events.

“Future research should examine autobiographical memory using more complex tasks that involve recalling real-life experiences outside of the laboratory,” Cuttler said.

Additionally, some participants in the active cannabis groups felt the effects were too strong and chose not to inhale the entire dose. The researchers noted this but found that excluding these individuals from the data did not drastically change the results. The participants were also young, experienced cannabis users with an average age of 28.

“It is also important to note that while these effects were statistically significant, they are not revealing massive memory impairments or amnesias that would lead to functional impairments,” Cuttler noted. “Our effects represent small reductions in performance on a broad array of memory tests. Nevertheless, our results suggest that people should avoid being under the influence of cannabis when they need to rely on memory, whether that involves remembering past information or remembering to carry out future tasks.”

Looking ahead, the research team hopes to explore how other compounds in the cannabis plant interact with human cognition, as well as how intoxication affects other complex brain functions.

“One of my previous clinical trials found that cannabigerol (CBG) may enhance memory performance so eventually I’d like to examine whether cannabinoids like CBG might potentially mitigate or offset the detrimental effects of THC on memory,” Cuttler said. “Further, we plan to map out which aspects of executive functioning (e.g., cognitive flexibility, inhibitory control, planning) are spared and impaired under conditions of acute cannabis intoxication.”

The study, “Mapping the acute effects of cannabis on multiple memory domains: A randomized, double-blind, placebo-controlled study,” was authored by Carrie Cuttler and Ryan J. McLaughlin.