A study from Washington State University found that cannabis might do more than make people forget things. It may actually cause people to remember things that never happened.

Researchers looked at how THC, the main chemical in cannabis that causes the “high,” affects different types of memory. The study was published in the Journal of Psychopharmacology.

“Most previous studies have only looked at one or two types of memory, like recalling lists of words,” said Carrie Cuttler, a professor at Washington State University. 

“This is the first study to comprehensively examine many different memory systems at once.” 

To run the experiment, researchers asked 120 regular cannabis users to take part. Some participants used cannabis with THC while others received a placebo. Afterward, everyone completed about an hour of memory tests.

The results showed that people who used cannabis had a harder time remembering things correctly. In many cases, they remembered things that were never actually said or shown.

In one test, participants listened to a list of related words. Later, some people who had used cannabis said they remembered hearing words that were never on the list.

“I found it was really common for people to come up with words that were never on the list,” Cuttler said. 

“Sometimes they were related to the theme of the list, and sometimes they were completely unrelated.”

The study also found that cannabis made it harder for people to recall where information came from. Known as “source memory,” it helps people remember whether something came from a conversation, a book or the internet, for example.

Researchers also noticed problems with prospective memory. This type of memory helps people remember to do things later, such as attend a meeting, take medicine or stop by the store.

“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.”

The study found that moderate and higher doses of THC caused similar memory problems, suggesting even moderate cannabis use can affect memory in the short term.

As cannabis becomes more common and more places legalize it, researchers say it is important to understand how it affects the brain.“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. “The goal is to help people make informed decisions about the risks and benefits.