Chronic marijuana users have lower IQs than those who do not use the drug and MRI scans show degradation of the brain, researchers have discovered.

A team of scientists from the Centre for BrainHealth at the University of Texas at Dallas have published a study in the Proceedings of the National Academy of Sciences, investigating the impact of long-term marijuana on brain function and structure through MRI scans.

Researchers studied 48 long-term marijuana users and compared them with 62 gender and age-matched non users, taking into account alcohol and tobacco use.

Findings showed that on average, marijuana users had lower IQs compared to the non-user controls. Differences were not related to brain abnormalities, however.

Researchers found that chronic marijuana users had a smaller brain volume in the orbitofrontal cortex – normally associated with addiction – and increased brain connectivity.

They also found that earlier onset of regular use resulted in greater "structural and functional" connectivity.

Researcher Sina Aslan said: "The results suggest increases in connectivity, both structural and functional that may be compensating for gray matter losses. Eventually, however, the structural connectivity or 'wiring' of the brain starts degrading with prolonged marijuana use."

Structural wiring declines were found to start to decline after just six years of chronic use, but they continued to display more intense connectivity than non-users, which researchers say could help explain why marijuana users appear to function well.

Filbey said: "While our study does not conclusively address whether any or all of the brain changes are a direct consequence of marijuana use, these effects do suggest that these changes are related to age of onset and duration of use."

Francesca Filbey, Director of Cognitive Neuroscience Research in Addictive Disorders at the centre, said: "We have seen a steady increase in the incidence of marijuana use since 2007. However, research on its long-term effects remains scarce despite the changes in legislation surrounding marijuana and the continuing conversation surrounding this relevant public health topic."

Currently, cannabis is legal in some form in almost half of the states in the US, with more expected to follow next year.