Long-term users of cannabis may suffer from low levels of the brain chemical dopamine, researchers have reported.
The findings were unexpected as it was thought that users who suffered psychotic symptoms might be suffering from increased levels of the chemical.
They come from a study of 19 users who had suffered from these symptoms.
All the users had been taking the drug since their teenage years.
Researchers at Imperial College, London, UK, said the lowest levels of dopamine were found in those who had taken the drug for longest – or had used high levels of it.
They have reported their findings in the journal Biological Psychiatry.
Researcher Dr Michael Bloomfield said the findings might explain claims that regular cannabis users lose "motivation."
He said: “The results weren’t what we expected, but they tie in with previous research on addiction, which has found that substance abusers – people who are dependent on cocaine or amphetamine, for example – have altered dopamine systems.
“Although we only looked at cannabis users who have had psychotic-like experiences while using the drug, we think the findings would apply to cannabis users in general, since we didn’t see a stronger effect in the subjects who have more psychotic-like symptoms.
"This needs to be tested though."
M Bloomfield et al. Dopaminergic Function in Cannabis Users and its Relationship to Cannabis-Induced Psychotic Symptoms. Biological Psychiatry 2013; doi.org/10.1016/j.biopsych.2013.05.027
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