When it comes to telling stories, heavy marijuana users might be all fiction. Researchers have found that regular pot consumers have a harder time remembering details from the past and were more likely than non-marijuana users to report things as they imagined them, not as they actually were. The effects of heavy marijuana use on memory were similar to the memory issues that arise as humans age, according to a new study published this week in the journal Molecular Psychiatry.

The study builds on previous research that showed smoking marijuana can impair short- and long-term memory. “These findings indicate that cannabis users have an increased susceptibility to memory distortions even when abstinent and drug-free, suggesting a long-lasting compromise of memory and cognitive control mechanisms involved in reality monitoring,” researchers working in Spain’s Sant Pau and Bellvitge hospitals wrote. When in need of an accurate eyewitness – such as in a trial – don’t call a pot smoker to the stand, researchers implied.

“Avoiding memory distortions may be extremely relevant in certain contexts such as the courtroom and forensic examination, and in a more general context this ability provides us with an adequate sense of reality that guides future behavior based on past experiences,” they wrote. Researchers likened the memory distortions of regular pot users to those “found in neurological and psychiatric conditions.”

To reach their conclusions, scientists weeded out 16 “heavy cannabis users” – defined as people who had been smoking marijuana every day for at least two years – to participate in their experiments. The subjects were put through a series of tests involving word games. Participants were asked to identify which words on a list had already been shown to them  and which ones had not.

Those in the heavy marijuana use category were more likely to say they had already seen words that were not on the original list, however both groups were able to accurately recognize previously studied words. Through MRI scans, researchers found that cannabis consumers involved in the research had less active hippocampuses – the structure in the brain that stores memories – which could account for their false recollections.

Previous research has shown the effects of marijuana on memory to be just as distinct. A study published in March showed adults who smoked weed regularly as teens were more likely to have memory problems later on in life.

Tetrahydrocannabinol, or THC, the active ingredient in cannabis that produces the high, affects the brain’s frontal cortex – the region involved in decision-making and risk assessment, among other things. Some studies have suggested that smoking pot can actually shrink this area of the brain. MRI scans of heavy pot users revealed that they had smaller-than-average frontal cortexes, which could have resulted from the brain turning off some of its THC receptors as more THC was introduced into the system.

Some pro-marijuana advocates have taken umbrage with studies that report detrimental effects of marijuana on the brain but don’t show how such results affect day-to-day functioning. "Investigators in this study failed to assess whether any of these differences are positively associated with any measurable adverse performance outcomes, such as cognitive [mental] performance or quality of life," Paul Armentano of the National Organization for the Reform of Marijuana Laws , or NORML, told CBS News regarding the March study on teen marijuana use and adult memory.