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Causal marijuana smokers risk altering the shape of their brains, according to a US study Reuters

Casual cannabis smokers risk damaging the parts of the brain that control motivation, emotion and reward.

A US study found that recreational marijuana use changed to the shape and density of regions of the users' brains involved in reward and addiction and parts that help process emotion and form long-term memories.

Researchers from Northwestern University Feinberg School of Medicine concluded the changes caused by just a low-level intake of marijuana could potentially make a person more vulnerable to drug addiction or change their thought processes and emotions in unknown ways.

The research studied 40 college students aged between 18 and 25 who were casual users of cannabis and analysed MRI scans of their brains.

They focused on the amygdala and nucleus accumbens - the parts of the brain that process emotion and addiction respectively - looking at the volume, shape and density of gray matter to see how each region was affected.

The scientists found that the density of gray matter significantly increased in cannabis smokers compared with non-users, abnormally enlargening the amygdala and nucleus accumbens.

Dr. Hans Breiter, a professor of psychiatry and behavioral science at Northwestern University Feinberg School of Medicine who contributed to the study said: "Anytime you find there's a relationship to the amount of marijuana consumed, and you see differences of core brain regions involved in processing of rewards, the making of decisions, the ability to assess emotions, that is a serious issue.

"There was a direct, consistent relationship between how much marijuana they used and the abnormalities we saw."

The professor added that cannabis smokers today were at more risk from potential side effects than users in the 60s as strains today contained a higher concentration of THC, the chemical that gives the drug its high.

Researchers will now look at how the abnormalities could effect users' behaviour.