Smoking marijuana may not only blur your memory. It can actually change the way memories are formed and recalled.
A new study from Washington State University found that people who took THC were more likely to recall words that were never shown to them and had more difficulty completing everyday memory tasks, such as remembering to do something later.
Published in Journal of Psychopharmacologythis study provides one of the most detailed investigations to date into how cannabis affects memory. The results show that cannabis affects not only simple recall, such as remembering a list of words, but also several forms of memory that people rely on throughout the day. This includes remembering appointments, recording conversations, and identifying the source of information.
The researchers were also surprised to find no significant difference between participants who took 20 milligrams of THC and those who took 40 milligrams. This finding suggests that even moderate amounts of THC can have significant effects on memory.
Research investigating multiple memory systems
“Most previous studies have only looked at one or two types of memory, such as recalling lists of words,” said Carrie Cutler, lead author of the study and associate professor of psychology at WSU. “This is the first study to comprehensively examine many different memory systems at once, and what we found is that acute cannabis intoxication appears to broadly disrupt most of them.”
To investigate these effects, Cutler and co-author Ryan McLaughlin, an associate professor in WSU’s Department of Integrative Physiology and Neuroscience, recruited 120 regular cannabis users. Participants were randomly assigned to vape a placebo cannabis, 20 milligrams of THC, or 40 milligrams of THC in a double-blind experiment.
After ingesting their assigned substance, participants completed approximately 1 hour of memory testing. The assessment measured several types of memory, including verbal memory, visuospatial memory, prospective memory, source memory, false memory, episodic content memory, and temporal order memory.
Those who took cannabis performed significantly worse than those who took a placebo on most tests. Overall, cannabis affected the majority of memory measures, with significant differences emerging in 15 of 21 tests.
Cannabis is associated with false memory and source confusion
The most significant effects were seen in false memories and source memories, which help people recall information accurately and determine where that information came from.
In one test, participants heard a list of related words linked by theme, but the central keyword linking them was never spoken. Afterwards, those who ingested cannabis were more likely to say they remembered hearing words that were not presented.
“I’ve noticed that it’s really common for people to come up with words that aren’t on the list,” Cutler said. “Sometimes it was related to the theme of the list, and sometimes it was completely unrelated.”
Participants who used cannabis also had more difficulty identifying the source of previously learned information. Problems with source memory can make it difficult to determine whether information comes from a trusted source, a conversation, or something you encounter online.
These types of memory distortions can have significant effects in situations where accurate recall is essential. For example, memory errors and suggestive questions during eyewitness interviews can affect memory of events.
Effects on everyday memory tasks
Researchers also observed impairments in prospective memory, the ability to remember to perform tasks in the future. This type of memory is associated with everyday obligations such as taking medicine, 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 there’s something you have to remember to do later, you don’t want to get high at the point where you have to remember it.”
One form of memory, called episodic content memory, which involves remembering personal experiences, did not show a significant effect in this study. Cutler noted that additional research is needed before any firm conclusions can be drawn about that type of memory.
Cannabis use is on the rise, but research gaps remain
The study comes at a time when marijuana use is becoming increasingly common in states such as Washington. Even though legalization has expanded across North America, many of the short-term cognitive effects of cannabis remain unknown.
One reason for the limited research is that cannabis is still classified as a Schedule I substance under federal law, which has historically limited scientific research.
“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,” Cutler said. “The goal is to help people make informed decisions about risks and benefits.”
