File: The effects of cannabis use on the brain remain disputed / Sean Gallup/Getty Images

Difference could be caused by substance abuse – or simply make people more likely to do it

A new study into the effects of cannabis has discovered that regular users have smaller brains than those who never smoke the substance.

The research into “chronic” marijuana use confirms in humans a phenomenon that has previously been observed in laboratory mice – the existence of reduced grey matter.

According to a team of scientists from the University of Texas and the Mind Research Network, whose study was published in the PNAS journal on Monday, marijuana users had “significantly less volume” to their orbitofrontal cortex, a region of the brain that is critical to how a person processes reward, motivation and addictive decisions.

But the study, which compared 48 users with 62 “control” subjects who never smoked, found that the brains of the former had “higher functional and structural connectivity”.

Though the research did not observe users over time, its results did suggest that those who had used cannabis regularly over a “protracted” period had better-connected brains, which they speculated could be the result of the body compensating for the damage caused, the LA Times reported.

And while in rodents the chronic marijuana use has been shown to result in reduced brain capacity, the results in humans remain much more mixed. Because the studied region of the brain is responsible for governing addictive behaviour, the researchers said it remained possible that cannabis users simply shared a pre-existing trait that left them more likely to start smoking in the first place.

The study concluded that, despite the possible observations to be made from the ages of the different subjects, more long term experiments were needed “to determine causality of these effects”.