Smoking cannabis even casually can damage core structures of the brain, an authoritative study has found.
Just using the drug once or twice a week affects the size and shape of two key brain regions involved in emotion and motivation, the research shows.
Previous studies have focused on heavy users of cannabis – revealing that the active compound in the drug effectively ‘rewires’ the brain.
But this is the first study that has revealed the impact of casual marijuana use.
The scientists, from Harvard Medical School and Northwestern University in Chicago, analysed MRI brain scans of 20 young cannabis users aged 18 to 25.
They compared them to the brain scans 20 young people who never smoked the drug.
Professor Hans Breiter, one of the researchers from Northwestern University said: ‘This study raises a strong challenge to the idea that casual marijuana use is not associated with bad consequences.’
The scientists found major differences in two brain areas, the nucleus accumbens and amygdala. Both are linked to emotions and motivation, and also associated with addiction.
In each case changes were seen that were directly related to how much cannabis was smoked.
The nucleus accumbens of cannabis users was unusually large, while the amygdala was deformed.
Professor Breiter said: ‘Some of these people only used marijuana to get high once or twice a week.
‘People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case.’
Co-author Dr Anne Blood, from Harvard, said: ‘These are core, fundamental structures of the brain. They form the basis for how you assess positive and negative features about things in the environment and make decisions about them.’
The drug users in the study smoked cannabis at least once a week but were not psychologically dependent on their habit. The scientists, whose findings are reported in the Journal of Neuroscience, believe the effects are the result of users’ brains adapting to low-level exposure to cannabis.
Previous research has shown that rats given the psychoactive compound in cannabis, THC, effectively have their brains rewired. Psychologist Dr Jodie Gilman, who led the latest study, said: ‘It may be that we’re seeing a type of drug learning in the brain.
‘We think when people are in the process of becoming addicted, their brains form these new connections.’
Drug addiction expert Dr Carl Lupica, from the United State National Institute on Drug Abuse, added: ‘This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy.
‘These observations are particularly interesting because previous studies have focused primarily on the brains of heavy marijuana smokers.’
Dr Michael Bloomfield, of the UK Medical Research Council, said: ‘It’s been known for some time that heavy cannabis use can affect the brain.
‘This new, well-conducted study has some really interesting and important results because it suggests that even moderate use of cannabis in young adults may be associated with changes in the brain.’
He said the study would inform separate scientific research that looks at the links between mental illness and cannabis use.
He added: ‘Taken together, these studies therefore have implications for understanding some of the mental health problems that are associated with cannabis use including schizophrenia, particularly as the younger people are when they use start using cannabis, the higher the risk of mental illnesses down the line.’
Professor Peter Jones, professor of psychiatry at the University of Cambridge, said: ‘This is an interesting piece of research. However, it is limited as it is only a small study. The main point is that, as usual, more research is needed.’