New research published in Neuropsychopharmacology found evidence that cannabis weakens working memory by increasing off-task thinking and by disrupting one’s ability to accurately monitor task performance.

Tetrahydrocannabinol (THC), the main psychoactive ingredient in cannabis, is commonly believed to disrupt working memory. A research team led by Kirsten C. S. Adam notes that little scientific evidence actually supports this theory. Prompted by a lack of consistency in existing data, the researchers conducted their own controlled study, involving a lengthier working memory task and a greater number of subjects than previous studies.

Two separate experiments were conducted, each involving 24 healthy, non-daily cannabis users with an average age of 23. In Study 1, subjects were separated into two groups and either received a 15 mg dose of THC or a placebo pill. In Study 2, subjects were separated into three groups and either received a 7.5 mg dose of THC, a 15 mg dose of THC, or a placebo pill.

Between 2 and 3.7 hours after ingesting the capsules, subjects in both experiments took part in a working memory task that involved recalling a pattern of colored squares on a grid.

The two experiments were double-blind — which meant that neither the researchers nor the participants knew who received the placebo and who received the THC.

Researchers found that, in both experiments, those who received the 15 mg THC doses performed significantly worse on the working memory tests, recalling fewer of the colored squares correctly than those who received the inactive pills. Notably, those who received the smaller dose of 7.5 mg of THC did not differ significantly from the placebo group.

The first study additionally found that those who received the THC reported greater “mind-wandering”, which was described as being inwardly focused on something unrelated to the task. They also reported more frequent “zoning out”, defined as a lack of attention towards anything in particular.

The second study found that those receiving the 15 mg of THC were less able to accurately monitor their performance throughout the task. That is, they were less accurate in predicting which of their responses were correct.

The authors discuss that this memory impairment may be related to a disruption in the dorsolateral prefrontal cortex, which plays a critical role in working memory and contains a wealth of cannabinoid receptors. The authors further suggest that this impairment manifests itself by pulling cannabis users’ attention away from relevant information.

“Similar to the effects of nicotine cravings and alcohol,” the authors say, “THC appears to increase mind wandering and other off-task mental states (e.g., “zoning out” or “mind blanking”), and decrease awareness of task performance. These broad effects on conscious experience are likely to drive performance decrements in a broad range of cognitive tasks.”

Adam and colleagues emphasize that the two studies presented an overall robust effect for THC on working memory, indicating that a single 15 mg dose of THC can diminish working memory in healthy adults. Future studies, they say, will need to examine the effects of the drug at different points in time after consumption, at higher doses, and through other routes of administration, such as inhalation.

The study, “Tetrahydrocannabinol (THC) impairs visual working memory performance: a randomized crossover trial”, was authored by Kirsten C. S. Adam, Manoj K. Doss, Elisa Pabon, Edward K. Vogel, and Harriet de Wit.