A new study published in Biological Psychiatry indicates that THC, the main psychoactive constituent of cannabis, can increase false recollections when trying to remember words and images.

“Because patients with PTSD are haunted by emotionally negative experiences and also use cannabis at a high rate, I wanted to see whether THC would suppress the ability to retrieve emotional memories that were encoded under sober conditions,” said study author Manoj K. Doss, a doctoral student at the University of Chicago.

“Previous work from our group had found that THC can impair the encoding (i.e., the acquisition or learning) of new emotional experiences, and I had previously found some evidence that MDMA can dampen the encoding and retrieval of negative emotional memories, so it made sense to see whether THC could impact emotional memory retrieval.”

“In contrast, other work from our group had found that dextroamphetamine during memory retrieval increased false memories, and there were trends for a similar effect with MDMA at retrieval. Although these effects may have been limited to stimulants, it was also possible that a general effect of drugs of abuse may be distorting memory retrieval.”

The double blind, placebo-controlled study of 23 volunteers found that THC intoxication increased the rate of false recollections. The participants had used cannabis 4 to 100 times in their lives.

“In this study, we showed healthy participants images of neutral and emotional scenes while they were sober,” Doss explained to PsyPost. “Two days later, we tested their memory for these pictures by cuing them with short verbal labels describing those scenes, as well as labels describing never before viewed scenes. On this second day, participants could have been given THC (15 mg) or a placebo prior to this memory test.”

“In contrast to our first hypothesis, when participants were on THC, they were able to correctly identify pictures that they had previously seen, suggesting that memory for emotional memories was not necessarily suppressed. What we did find was that when participants were on THC, they claimed with high confidence to have seen both neutral and emotional pictures that had never been previously presented, suggesting that they had formed false memories.”

The study should not be misinterpreted as suggesting that cannabis has no role in PTSD treatment.

“Although this could potentially be a negative effect of acute THC intoxication, I don’t necessarily think that THC has no medical potential for the treatment of PTSD. Our findings may just limit when it should be used,” Doss explained.

“For example, during psychotherapy, memory can be especially susceptible to distortion, and patients with PTSD tend to already have distorted memories. Therefore, psychotherapy may be a situation in which acute THC intoxication would not be warranted in order to not exasperate potential memory distortions. In the past, other drugs of abuse, specifically sedatives, were also observed to increase false memory formation during psychotherapy, as well as interrogations.”

“More work is needed to see under what conditions THC may especially increase false memory formation, as well as those conditions that may reduce false memory formation. Furthermore, it would be interesting to see whether the memory distorting effects of different classes of drugs occur via similar mechanisms,” Doss added.

“For example, the ventral striatum is a structure that all drugs of abuse appear to modulate, and this structure is also involved in memory retrieval. Although speculative, it may be that this is a common pathway to drug-induced memory distortion.”

“For more research on drugs of abuse and memory distortion, follow me on Twitter: @ManojDoss

The study, “Δ9-Tetrahydrocannabinol at Retrieval Drives False Recollection of Neutral and Emotional Memories“, was authored by Manoj K. Doss, Jessica Weafer, David A. Gallo, and Harriet de Wit.