PULLMAN, Wash. — Smoking cannabis can do more than blur memories. It can reshape them.
A new Washington State University study found that people who consumed THC were more likely to recall words that were never presented and struggled with everyday tasks such as remembering to do something later.
Published in the Journal of Psychopharmacology, the study is one of the most comprehensive looks yet at how cannabis affects memory. The findings suggest cannabis can impair not only simple recall, such as remembering a list of words, but also forms of memory people rely on in daily life, like remembering appointments, keeping track of conversations, or recalling where information came from.
Researchers were also surprised to find no meaningful differences between participants who consumed 20 milligrams of THC and those who consumed 40 milligrams, suggesting even moderate doses may cause substantial disruptions.
"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 explore those 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 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 about an hour of tests measuring verbal, visuospatial, prospective, source, false, episodic content, and temporal order memory.
Participants who consumed cannabis performed significantly worse than the placebo group on most measures. Overall, cannabis affected the majority of the memory tests, with significant differences appearing in 15 of the 21 measures.
The strongest impacts appeared in false memory and source memory, systems that help people accurately recall information and identify where it came from. 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 cannabis users had more difficulty remembering the source of information they had learned earlier. Problems with source memory can make it harder to distinguish whether information came from a reliable source, a conversation, or something seen online.
Such disruptions could matter in situations where accurate recall is important, such as eyewitness interviews where memory errors or suggestive questions can influence how events are remembered.
The study also found 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 want to be high at the time you need to remember to do it."
One type of memory, episodic content memory, or the ability to remember personally experienced events, did not show a significant effect. Cuttler said more research is needed before drawing firm conclusions in that area.
The research comes as cannabis use 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 know about its acute effects," Cuttler said. "The goal is to help people make informed decisions about the risks and benefits."
