Regular cannabis use shrinks the brain but increases the complexity of its wiring, a study has found.

To some extent the loss of brain volume is balanced by larger numbers of connections between neurons, scientists discovered.

But they warn that those who take the drug for too long are likely to suffer damaging effects.

The brain scan study of cannabis users is one of the first to investigate the drug’s long-term neurological impact in living people.

Dr Sina Aslan, from the University of Texas at Dallas, US, who co-led the research, said: “What’s unique about this work is that it combines three different MRI (magnetic resonance imaging) techniques to evaluate different brain characteristics.

“The results suggest increases in connectivity, both structural and functional that may be compensating for grey matter losses. Eventually, however, the structural connectivity or ‘wiring’ of the brain starts degrading with prolonged marijuana use.”

The team studied 48 adult cannabis users aged about 20 to 36 who were compared with a group of matched non-users.

On average, the cannabis users took the drug three times a day.

Although tests showed that regular users had lower IQs than non-users, this did not appear to be related to brain abnormalities.

The scans revealed that smoking cannabis every day was associated with shrinkage in the orbitofrontal cortex (OFC) region of the brain, which is involved in mental processing and decision making.

It also influences responses to rewards and adversity, and is strongly linked to empathy – the ability to sense other people’s feelings.

Neuroscientists believe damage to the orbitofrontal cortex may underpin some forms of psychopathy.

Earlier onset of cannabis use induced greater structural and functional connectivity, the research showed. The greatest connectivity increases occurred as an individual started taking the drug.

After six to eight years of continually taking cannabis the increases in structural wiring declined, but users continued to display higher connectivity than non-users.

This may explain why chronic, long-term cannabis users appeared to be “doing just fine” despite having smaller OFCs, said co-author Dr Francesca Filbey, also from the University of Texas.

She added: “To date, existing studies on the long-term effects of marijuana on brain structures have been largely inconclusive due to limitations in methodologies.

“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.”

The study, published in the journal Proceedings of the National Academy of Sciences, suggests that grey matter may be more vulnerable to the effects of THC, the main active ingredient in cannabis, than white matter.

Grey matter makes up the bodies of neurons, while white matter consists of the fibres, or axons, along which nerve signals pass.

Further work is needed to determine whether stopping cannabis use reverses the changes, and whether similar effects are seen in occasional users, say the scientists.