Cannabis users are more likely to have distorted memories and can even end up imagining situations that differ from reality, scientists have discovered.

Researchers used neuroimaging to look at the brains of heavy cannabis users compared with healthy people and found the former group have a less active hippocampus – a structure related to the storage of memories.

The findings were published in the journal Molecular Psychiatry and sheds light on why one of the side effects of consuming the drug is memory problems.

Scientists from the Biomedical Research Institute of Hospital de Sant Pau and the Universitat Autònoma de Barcelona also found chronic cannabis users had more difficulties than the general population in storing and recovering memories, as well as having a greater tendency towards imaginary or false memories.

Memory creation is a flexible progress that can be subject to distortions – our brain can remember things that never happened. These false memories are seen more often in people with neurological and psychiatric disorders but it is also apparent in healthy people and is more common the older we get.

In the study, researchers compared a group of chronic cannabis consumers to a healthy control group while they learned a series of words. After a short break, they were shown the original words with new ones that were either related or unrelated.

All participants were asked if the words belonged to the original list. Cannabis consumers tended to say they had already seen the related words compared with the control group.

With MRI scans, the researchers found a lower activation in the areas of the brain related to memory processes and cognitive resources. There was also a correlation between how much the participant had used throughout their life and the level of activity in the hippocampus.

Findings suggest cannabis has a long-term effect on brain mechanisms that allow us to distinguish between real and imaginary events.

"The present results indicate that long-term heavy cannabis users are at an increased risk of experiencing memory errors even when abstinent and drug-free," the authors wrote.

"These deficits show a neural basis and suggest a subtle compromise of brain mechanisms involved in reality monitoring... This lingering diminished ability to tell true from false may have medical and legal implications.

"Future studies should address these issues and assess whether the deficits found here extend to other forms of memory distortion and reality monitoring beyond the false memory phenomenon."

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