Researchers found that people who consumed THC were more likely to recall words that were never shown to them and had more trouble completing everyday memory tasks, such as remembering to do something later.

Published in the Journal of Psychopharmacology, the results from the double-blind placebo-controlled trial indicate that cannabis affects not only simple recall, like remembering a list of words, but also several forms of memory that people rely on throughout the day. 

These include remembering appointments, keeping track of conversations, and identifying where information originally came from.

The research team also found no meaningful differences between participants who consumed 20 milligrams of THC and those who consumed 40 milligrams. 

This finding suggests that even moderate amounts of THC may significantly interfere with memory.

The study provides one of the most detailed examinations to date of how cannabis influences memory, according to the researchers. 

“Most previous studies have only looked at one or two types of memory, like recalling lists of words,” said Carrie Cuttler, senior author of the study and an 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, Cuttler and co-author Ryan McLaughlin, an associate professor in the Department of Integrative Physiology and Neuroscience at WSU, recruited 120 regular cannabis users. 

Participants were randomly assigned to vaporise placebo cannabis, 20 milligrams of THC, or 40 milligrams of THC in a double-blind experiment.

Participants then completed a series of tests to measure several types of memory, including verbal, visuospatial, prospective, source, false, episodic content, and temporal order memory.

The results showed that people who consumed cannabis performed significantly worse than those who received the placebo on most of the tests. 

Overall, cannabis was found to affect the majority of the memory measures, with significant differences appearing in 15 of the 21 tests.

Cannabis linked to false memories and source confusion

The most pronounced effects were seen in false memory and source memory, which help people accurately recall information and determine where that information came from.

In one test, participants listened to lists of related words that were connected by a theme, but the central keyword tying them together was never spoken. 

Later, individuals who had consumed cannabis were more likely to say they remembered hearing words that had not been presented.

“I found it was really common for people to come up with words that were never on the list,” Cuttler said. 

“Sometimes they were related to the theme of the list, and sometimes they were completely unrelated.”

Participants who used cannabis also had more difficulty identifying where previously learned information originated. 

Problems with source memory can make it harder to determine whether information came from a trusted source, a conversation, or something encountered online.

The researchers also observed impairments in prospective memory, which is the ability to remember to perform tasks in the future. 

“These are things we rely on constantly in our day-to-day lives,” Cuttler said. “If you have something you need to remember to do later, you probably don’t want to be high at the time you need to remember to do it.”

One form of memory called episodic content memory, which involves recalling personal experiences, did not show a significant effect in this study. 

Cuttler noted that additional research is needed before drawing firm conclusions about this type of memory.