Cannabis may encourage false memories

Cannabis not only induces forgetfulness, it also opens the door for false memories, according to new research.

And that’s serious, man. It means the evidence of witnesses, victims and even alleged perpetrators of crimes could be less reliable – and open to legal challenge – if they have recently been under the influence of the most widely used illicit substance in the world.

The main psychoactive ingredient in cannabis, delta-9-tetrahydrocannabinol (THC), already has been linked to short-term memory loss.

Researchers from Maastricht University, the Netherlands, and the University of Sydney, Australia, wanted to find out something slightly different, however: whether the drug also made consumers susceptible to false memories, particularly as a result of suggestion.

Lead researcher Lilian Kloft says this is the first study of its kind.

“With the growing global acceptance of cannabis and its widespread use by eyewitnesses and suspects in legal cases, understanding the popular drug’s ramifications for memory is a pressing need.”

The team enlisted 64 healthy, occasional cannabis users and gave some a single dose of vaporised THC, while others received a placebo. The psychoactive effects of THC are experienced immediately after smoking, with peak levels of intoxication after 15 to 30 minutes. Cognitive impairment is most prominent during the two hours after smoking, but may be detectable up to six hours later.

The participants performed memory tasks soon after inhaling, and a week later.

In one, the subjects were asked to recall a list of words on a common theme. The cannabis users had a higher tendency to believe other related words had been on the list, when in fact they had not. That’s a classic test for false memory.

The subjects also witnessed a virtual reality fight and theft, and were asked questions about what they had seen.

Critically, the participants were fed misinformation about these scenarios through suggestive questioning, and by the false testimony of a second virtual witness.

Writing in the journal PNAS, the researchers report that participants who had inhaled THC were more likely to say “yes” to questions, indicating suggestibility, and had more false memories, especially while they were still intoxicated.

Interestingly, the gap in accuracy between those who had inhaled THC and those who had not narrowed over the following week. This was not because the cannabis users regained their memory, but because the placebo group became increasingly likely to develop their own false memories over time.

Kloft says the findings have implications for the questioning of cannabis-intoxicated eyewitnesses and victims during investigative interviews.

For example, while crime suspects are routinely tested for drug use, this is not always the case for witnesses or victims.

“The most important message from this study is that cannabis exerted a general impact on memory by increasing various types of recollective errors.

“In terms of interviewing witnesses, victims, or suspects after the incidence of a crime, this means that interviewing while the individual is still intoxicated should be minimised due to elevated risk of false reporting.”