Smoking cannabis can do more than blur memories. It can reshape them.
This is according to a study – “Mapping the acute effects of cannabis on multiple memory domains: A randomized, double-blind, placebo-controlled study” by Carrie Cuttler and Ryan J. McLaughlin – that was published in the Journal of Psychopharmacology.
For this study, the researchers 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.
Researchers also found 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.
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 similarly 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. Yet, despite growing legalization, 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.”
