Cannabis can not only make it harder to recall memories, but also alter the processes involved in forming and retrieving them.

A study conducted at the University of Washington found that the use of THC (tetrahydrocannabinol) is associated with more frequent memory errors, such as recalling words that were never presented, as well as difficulties with everyday tasks that require remembering, like needing to do something later. The research, published in the Journal of Psychopharmacology, is considered one of the most comprehensive to date on the effects of cannabis on memory.

Impact on Different Types of Memory

The results showed that cannabis affects not only simple information recall, such as remembering a list of words, but also several types of memory essential for daily life. These include remembering appointments, following the flow of a conversation, and identifying the source of information. The study found no significant differences between participants who consumed 20 mg of THC and those who received 40 mg, suggesting that even moderate doses of THC can have a substantial impact on memory.

Research Methodology

The study involved 120 regular cannabis users. Participants were randomly assigned to groups that inhaled either a placebo, 20 mg, or 40 mg of THC in a double-blind experiment. After consumption, participants completed about an hour of memory tests assessing various types of memory: verbal, visual-spatial, prospective, source, false, episodic, and event order memory.

Main Findings

Those who used cannabis performed worse than the placebo group on most tests. Significant differences were found in 15 out of 21 tests. The most pronounced effects were observed in tests of false memory and source memory, which help accurately recall information and determine its origin. In one test, participants were read lists of related words grouped by a common theme, but the key word was omitted. Later, people who had used cannabis were more likely to claim they had heard words that were not on the list—sometimes related to the theme, sometimes not.

These participants also had trouble identifying the source of previously received information, making it harder to know whether information came from a reliable source, a conversation, or was found online. Such memory distortions can have serious consequences in situations where accuracy is crucial, such as during witness testimony.

Effect on Prospective Memory

A decline in prospective memory—the ability to remember to perform a task in the future—was also noted. This type of memory is important for daily responsibilities, such as taking medication, attending appointments, or remembering to stop by the store on the way home. If you need to remember to do something later, cannabis use may interfere with that ability.

Episodic Memory

Episodic memory, which relates to recalling personal events, did not show significant changes in this study. However, further research is needed for definitive conclusions on this aspect.

Study Context

This research comes amid rising cannabis popularity in several states, including Washington. Despite legalization, many short-term cognitive effects of cannabis remain insufficiently studied. One reason for the limited research is that, under U.S. federal law, cannabis is still classified as a Schedule I substance, which has historically made scientific work more difficult. The goal of such studies is to provide people with information to make informed decisions about the risks and benefits of cannabis use.