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