With state marijuana legalization sweeping the country, adults are using it in record numbers, and scientists, like Carrie Cuttler and Ryan McLaughlin of Washington State University, now have freer rein to study its effects. 

“We’re living in a state where cannabis use is very common, but there’s still a lot we don’t know about its acute effects,” Cuttler said in a statement. “The goal is to help people make informed decisions about the risks and benefits.”

Along those lines, they recently conducted one of the comprehensive reviews of how cannabis use affects memory, and published their findings in the Journal of Psychopharmacology. 

After recruiting 120 cannabis users, lab technicians fired up the Volcano Vaporizer, and let them inhale either 40 milligrams of THC, 20 milligrams of THC, or a placebo. After the vape session, the subjects completed a battery of tests on their memory. Participants who consumed cannabis—even a moderate 20 milligrams—performed worse than the placebo group on most measures while they were high. 

“Most previous studies have only looked at one or two types of memory, like recalling lists of words,” Cuttler said. “This is the first study to comprehensively examine many different memory systems at once, and what we found is that acute cannabis intoxication appears to broadly disrupt most of them.”

Read more: “The Ecology of Good Weed”

Cannabis use showed some of the most dramatic effects on false memory. In one test, participants were read a list of words that were related but didn’t include a key word linking them. For example, the list might include “nurse,” “hospital,” and “medicine,” but not “doctor” (kind of like a reverse version of the game Taboo). When they were asked to recall the list later on, cannabis users were more likely to falsely remember words that hadn’t been read. 

In a finding with troubling implications given our fractured media ecosystem, cannabis users also had difficulty recalling the source of information. Researchers showed participants a series of written words as well as images representing words, asking them later to recall the words and whether they were written or depicted. Identifying the source proved to be a struggle for cannabis users. Problems with source memory, researchers say, could make it more difficult to discern whether information comes from a reliable outlet, like a news report, or something seen online, like a meme.

They also found cannabis use impaired prospective memory, or the ability to remember to carry out tasks in the future. At the beginning of the testing period, participants were asked to rate the difficulty of each subsequent test and were also instructed to ring a bell every 15 minutes (with the help of a clock). Members of the test groups didn’t always remember to complete these basic tasks, indicating cannabis use could hamper the ability to remember to take care of everyday things like taking medication.

“These are things we rely on constantly in our day-to-day lives,” Cuttler said. “If you have something you need to remember to do later, you probably don’t want to be high at the time you need to remember to do it.”

Wise words for cannabis users everywhere.

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