Given that cannabis is the most commonly used recreational drug after alcohol and tobacco, it is no surprise that scientists are keen to investigate how it affects our bodies, in particular the mind. Much research has focused on whether it could have an impact on our memory, and the general consensus is that it can.

Now, lending further support to this notion, a new investigation has found that cannabis users are more prone to false memories. Additionally, after scanning participants’ brains, they found that pot smokers also showed reduced activity in areas involved in memory processing, further suggesting the drug can compromise memory. The work has been published in Molecular Psychiatry.

We all know that forgetting is frustratingly commonplace, but did you know that our brain can also do the opposite, adding in memories that never actually took place? These incorrect recollections are called false memories, and they can vary from minor fabrications, like believing you left your shoes by the front door, to major fallacies such as believing you were abused during childhood. These fake or altered memories are the result of the fact that memory formation is a progressive, malleable process that is therefore subject to distortion.

Although these are actually fairly common, especially of childhood events as people often repeatedly describe them in slightly different ways, they are observed more frequently in certain neurological or psychiatric disorders. And given the fact that some studies have shown that cannabis use is associated with alterations in the region of our brain that is involved in memory formation, the hippocampus, scientists wondered whether false memories may also be more common in cannabis users.

To investigate this further, researchers from the Universitat Autònoma de Barcelona and the Bellvitge Institute for Biomedical Research enrolled 16 heavy cannabis users, defined as using it daily for at least two years, and matched them with a control group who had used the drug less than 50 times in their life and were free of psychiatric and neurological conditions. They then subjected the participants to a memory test in which participants were asked to learn a series of words. After a few minutes, the volunteers were presented with the words again, but the researchers had also added in extras, some of which were related in meaning, whereas others were completely unrelated. Participants were then asked to recall which words were on the original list.

The team found that, compared with the controls, cannabis users were much more likely to falsely identify new, related words as ones that had appeared on the original list, suggesting that they have a greater susceptibility to false memories. Furthermore, MRI brain scans revealed that cannabis users had reduced activity in areas known to be involved in memory processing, attention and performance monitoring. Interestingly, the participants had actually abstained from smoking cannabis for one month before the study began, suggesting a long-lasting compromise of both memory and the control mechanisms involved in reality monitoring, the researchers write.

Although the study was small, it does add to the growing body of evidence that smoking cannabis could be affecting our memory. However, the investigation should be followed up with a larger number of participants, and possibly also occasional users, to make the findings more watertight.