Casual cannabis use damages brain, new study finds

Smoking cannabis recreationally can alter the size and shape of the brain, a new study has revealed.

Young people who smoke the drug at least once a week were found to have changes in parts of the brain involved in emotion and motivation.

Scientists from Northwestern University and Harvard Medical School in the USA compared 3D brain scans of 20 cannabis smokers with 20 non-users. All participants were aged 18 to 25.


They asked the study’s participants to estimate their drug consumption over a three-month period and concluded that the higher the cannabis intake, the greater the brain abnormalities.

Dr Hans Breiter, professor of psychiatry and behavioural sciences at Northwestern University Feinberg School of Medicine, said: “This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences.

“Some 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.”


In Britain, some 10 million people have used illegal drugs of which cannabis is the most popular.

Susan Bedack, whose son began smoking cannabis at the age of 16, says the drug “wrecked” her son’s life.

“We were on a skiing holiday when he first became noticeably unwell. I found some cannabis in his room.”

She said: “His mood became erratic, laughing one minute, crying the next he spent all day in bed, had no energy and no motivation.”

Aaron was 17 when he was first sectioned under the Mental Health Act and has been in and out of hospital since.


Dr Carl Lupica who studies drug addiction at the National Institute on Drug Abuse explained why the new observations are “particularly interesting”.

He said “previous studies have focused primarily on the brains of heavy marijuana smokers, and have largely ignored the brains of casual users”.

“This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy”, he also commented.

Mental health

Dr Michael Bloomfield, clinical research fellow at the UK’s Medical Research Council, cited earlier studies which show that heavy cannabis use in adolescence also produces changes in the brain.

He said: “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”.

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