Regular cannabis use shrinks the brain but increases the complexity of its wiring, a study has found.
The loss of brain volume is balanced to some extent by more connections between neurons, scientists discovered.
But those who take the drug for too long are likely to suffer damaging effects.
The study of cannabis users through brain scans is one of the first to investigate the drug’s long-term impact. Dr Sina Aslan, from the University of Texas, Dallas, who co-led the research, said scientists used three magnetic resonance imaging techniques to check the brain.
“The results suggest increases in connectivity … that may be compensating for grey matter losses,” said Dr Aslan.
“Eventually, however, the structural connectivity or ‘wiring’ of the brain starts degrading with prolonged marijuana use.”
The team studied 48 adult cannabis users aged 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.
Tests showed that regular users had lower IQs than non-users but this did not appear to be related to abnormalities of the brain.
The scans disclosed that smoking cannabis every day was associated with shrinkage in the region of the brain involved in mental processing and decision making.
The drug also influenced responses to rewards and adversity and was strongly linked to empathy or the ability to sense other people’s feelings. Neuroscientists believe damage to the region of the brain affected by marijuana use may underpin some forms of psychopathy.
Marijuana smokers who started taking the drug at a young age showed greater structural and functional connections between their brain neurons, the research showed.
The greatest increase in connections occurred as a person started taking the drug.
After six to eight years of continually smoking cannabis the increases in structural wiring in the brain declined. However, users continued to display higher connectivity than non-users.
This may explain why chronic cannabis users appeared to be “doing just fine” despite having smaller sections of the brain, said co-author Dr Francesca Filbey, also from the University of Texas.
She said: “To date, existing studies on the long-term effects of marijuana on brain structures have been largely inconclusive. 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 neurons may be more vulnerable to the effects of tetrahydrocannabinol, the main active ingredient in cannabis, than the fibres along which nerve signals pass.
The scientists say further work is needed to determine whether stopping cannabis use reverses the changes and if occasional users suffer similar effects.