Cannabis Intoxication Disrupts Multiple Types of Memory

Smoking cannabis can do more than blur memories; it can reshape them. A recent Washington State University study found that individuals who consumed THC were more likely to recall words that were never presented and experienced difficulties with everyday tasks, such as remembering future intentions.

Published in the Journal of Psychopharmacology, the study is one of the most comprehensive investigations to date on how cannabis affects memory. The findings suggest that cannabis can impair not only simple recall, like remembering a list of words, but also forms of memory crucial for daily life, such as remembering appointments, tracking conversations, or recalling the source of information.

Researchers were 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 substantial disruptions.

Broad Disruption of Memory Systems

“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 Washington State University. Cuttler explained, “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 explore these effects, Cuttler and co-author Ryan McLaughlin, an associate professor in the Department of Integrative Physiology and Neuroscience at Washington State University, recruited 120 regular cannabis users and randomly assigned them to vaporize placebo cannabis, 20 milligrams of THC, or 40 milligrams of THC in a double-blind experiment. Participants then completed approximately one hour of tests measuring verbal, visuospatial, prospective, source, false, episodic content, and temporal order memory.

Key Findings

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

The most pronounced impacts were observed in false memory and source memory, systems that support people accurately recall information and identify its origin. In one test, participants heard lists of related words but not the key word tying 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 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.”

Researchers also found that cannabis users had more difficulty remembering the source of information they had learned earlier. Problems with source memory can hinder the ability to distinguish whether information came from a reliable source, a conversation, or something encountered online.

Such disruptions could be significant in situations where accurate recall is critical, such as eyewitness interviews where memory errors or suggestive questioning can influence how events are remembered.

Impact on Prospective Memory

The study also revealed impairments in prospective memory—the ability to remember to carry out 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 constantly in our day-to-day lives,” Cuttler said. “If you have something you need to remember to do later, you probably don’t desire to be high at the time you need to remember to do it.”

One type of memory, episodic content memory—the ability to remember personally experienced events—did not show a significant effect. Cuttler noted that further research is needed before drawing firm conclusions in this area.

Growing Relevance Amidst Legalization

The research comes as cannabis employ becomes increasingly common in states like Washington. Despite growing 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’re living in a state where cannabis use is very common, but there’s still a lot we don’t grasp about its acute effects,” Cuttler said. “The goal is to help people develop informed decisions about the risks and benefits.”