An evening with friends, a joint spinning, and the next day everyone swears they didn’t hear the same thing. This discrepancy is not just an impression. A new study shows that carbon dioxide poisoning THCthe main active molecule in cannabis, doesn’t just make memories fuzzy.
Researchers at Washington State University followed 120 regular users to closely test the effect of cannabis on memory. Their work suggests that the THC
promotes the creation of false memories and disrupts several memory systems used continuously in daily life. The detail of the results is striking.
THC, false memories and memory: what the Washington State University study shows
In this randomized, double-blind, placebo-controlled trial, 120 users vaped either THC-free cannabis, 20 milligrams, or 40 milligrams. They then took about an hour of tests covering verbal memory, visuospatial memory, prospective memory, source memory, false memoriesepisodic content and temporal order. Significant differences appeared on 15 of the 21 measures. “This is the first study to comprehensively examine many memory systems at the same time, and what we found is that acute cannabis intoxication appears to largely disrupt most of them“, explained Carrie Cuttler, professor of psychology, quoted by Washington State University.
The most marked effects concern
false memories and source memory or “source monitoring” (i.e. all of the cognitive processes involved in recognizing the origin of information). Concretely, these source memory problems can make it more difficult to distinguish between information coming from a reliable source, a conversation or an online resource.
In one test, participants heard lists of words linked by a theme, but the key word was never spoken. Those who had used cannabis were more likely to remember words that had never been presented. “I realized that it was really common for people to come up with words that were never on the list“, said Carrie Cuttler. Sometimes these words were related to the theme, sometimes completely irrelevant.
When cannabis disrupts everyday life: forgotten appointments and distorted memories
The team also observed damage to the prospective memorythe one that helps you remember to do something later, like take medicine or go to an appointment. “These are things that we constantly rely on in our everyday lives. If you have something you need to remember later, you probably don’t want to be high by the time you need to remember it“, warned Carrie Cuttler. The memory of temporal order was also less reliable, making it more difficult to put events in the correct order.
Performance also declined in verbal and visuospatial memory, whether retaining information immediately or recalling it after a delay. On the other hand, so-called episodic content memory, that of personal events, did not show a clear difference in this study, which pushes researchers to remain cautious on this specific point.
Moderate doses, justice and decisions: why these results on THC are worrying
Another element surprised the scientists: no significant difference was found between the 20 and 40 milligram groups. In other words, a dose considered moderate is enough to significantly alter several forms of memory. These distortions can weigh heavily in sensitive contexts, such as hearings of witnesseswhere leading questions and fragile memories risk mixing.
“We live in a state where cannabis use is very common, but there is still a lot we don’t know about its acute effects. The goal is to help people make informed decisions about risks and benefits” concluded Carrie Cuttler.
In France where cannabis is by far the most consumed illicit substance, these results are striking. In conclusion, this drug can impair not only simple recall skills, such as remembering a list of words, but also essential everyday forms of memory, such as remembering appointments, following the thread of a conversation, or remembering the source of information.