Recent research published in Journal of Psychopharmacology Cannabis use has been shown to have a wide range of negative effects on different types of memory. The findings provide evidence that getting high impairs everyday cognitive functions, such as remembering to perform future tasks and remembering the exact order of past events.
“It is well established in the empirical literature that acute cannabis intoxication negatively impacts memory,” said study author Carrie Cutler, associate professor at Washington State University and co-director of the WSU Cannabis Research Center.
“However, most previous studies investigating the acute effects of cannabis on human memory have focused on a relatively narrow set of memory tests, primarily verbal memory tasks that involve recalling lists of words, and working memory tasks that require temporarily holding and manipulating information in consciousness.
“Far fewer studies have investigated how cannabis affects other types of memory related to everyday life. These include temporal order memory (remembering the order in which events happened), prospective memory (remembering to perform future tasks), source memory (remembering where information came from), false memory (remembering information that was never presented), and episodic content memory (remembering 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,” Cutler explained.
Researchers recruited 120 adults who regularly used cannabis at least once a week. Participants were divided evenly into three groups of 40 people each. One group received a vaporized placebo flower that did not contain active tetrahydrocannabinol. The second group vaporized a medium dose of the drug, 20 milligrams, and the third group vaporized a high dose of 40 milligrams.
Participants were randomly assigned to different groups, and neither the participants nor the scientists knew who received the real drug or the fake substitute until the end of the experiment.
Participants waited approximately seven minutes after inhaling steam from a tabletop vaporizer. They then completed a series of specific cognitive tests over an hour. These tests used established psychological tools to measure various memory domains.
To measure verbal memory, participants heard a list of words and recalled them either immediately or after a delay. The researchers also tested working memory, which involves retaining and manipulating information, by asking participants to repeat a series of numbers in reverse order. Visual-spatial memory, which involves remembering shapes and their physical locations, was tested by having participants study a geometric figure and draw it from memory.
Prospective memory was measured by checking whether participants remembered to rate the difficulty of a task immediately after finishing it or if they remembered to ring a bell every 10 minutes. The researchers also tested source memory, the ability to remember the origin of information. This was done by checking whether participants remembered whether they saw a particular item as a picture or as a letter earlier in the session.
Susceptibility to false memories was tested by asking participants to listen to a list of related words. For example, after hearing words such as “bed” or “tired,” you might be asked whether you heard the unsaid target word “sleep.” Temporal order memory was assessed by asking participants to organize physical cards into the exact order in which they completed the cognitive test.
Finally, the researchers assessed memory for episodic content. This concept refers to an individual’s ability to recall specific events they have experienced. To test this, participants were asked to freely describe all the different cognitive tasks they had just completed during the session.
Data provide evidence that acute cannabis intoxication has widespread negative effects on most memory types. Compared to the placebo group, participants who inhaled medium and high doses performed worse on immediate and delayed verbal memory tasks. They also suffered from immediate and delayed visual-spatial memory and had trouble drawing shapes they had seen previously.
The drug increased the creation of false memories. Drunk participants were more likely to confidently remember seeing words that were not actually presented during the listening task. They also have trouble remembering sources, forgetting whether they saw a picture or a word first.
Researchers detected direct negative effects of cannabis on prospective memory and temporal order memory. Inebriated participants often forgot to perform tasks at the right time or had trouble remembering the correct order of the test they had just taken. These findings suggest that this drug has a significant impact on practical memory abilities needed for daily life.
Not all types of memory were significantly affected by drugs. The researchers found that there was no significant effect on memory for episodic content. This means that intoxicated users can still broadly recall which events they attended during a session. Performance on specific short-term memory tasks was also relatively stable across groups.
“The key takeaway is that acute cannabis intoxication appears to negatively impact a wide range of memory processes, rather than selectively affecting only some types of memory,” Cutler told SciPost. “We found significant negative 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 the negative effects of acute cannabis intoxication on prospective memory and temporal order memory.”
“Because our study involved recalling tasks completed by participants during the session, we did not detect significant effects on memory for episodic content. However, this is the first study to investigate the effects of acute cannabis on this specific aspect of memory.”
Interestingly, this data suggests that taking higher doses does not necessarily make these memory deficits worse. The researchers found that there were no significant differences in test scores between the middle and high dose groups.
“We were surprised that there was no significant difference between the medium-dose (20 mg THC) and high-dose (40 mg THC) groups,” Cutler said. “This may be because participants in both groups experienced significant levels of intoxication.”
Although this study was comprehensive, it also has some limitations. For example, some memory tests were relatively easy and many participants achieved perfect scores regardless of what they smoked. This phenomenon may have masked some of the subtle effects of the drug on time-based tasks. Scientists also acknowledged that artificial laboratory settings may not fully capture how people remember their own personal events.
“Future research should investigate autobiographical memory using more complex tasks that involve recalling real-life experiences outside of the laboratory,” Cutler said.
Additionally, some participants in the group actively using cannabis found the effects too strong and chose not to inhale the full dose. The researchers noted this, but found that removing these individuals from the data did not dramatically change the results. Participants were also experienced cannabis users with an average age of 28 years.
“It is also important to note that although these effects were statistically significant, they did not reveal large-scale memory impairment or amnesia that would lead to functional impairment,” Cutler noted. “Our effects show a slight reduction in performance on a wide range of memory tests. Nevertheless, our results suggest that people should avoid being under the influence of cannabis when they need to rely on their memory, whether it’s remembering past information or forgetting to perform future tasks.”
Looking ahead, the researchers hope to investigate how other compounds in the cannabis plant interact with human cognition, and how addiction affects other complex brain functions.
“One of our previous clinical trials found that cannabigerol (CBG) may enhance memory, so we ultimately want to find out whether cannabinoids like CBG may reduce or offset the negative effects of THC on memory,” Cutler said. “Additionally, we plan to map which aspects of executive function (such as cognitive flexibility, inhibitory control, and planning) remain intact and which are impaired in the context 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 Cutler and Ryan J. McLaughlin.
