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Marijuana use reduces sense of reward over time, scientists find

'What we saw was that over time, marijuana use was associated with a lower response to a monetary reward,' says Neuroscientist Dr Mary Heitzeg

Thursday 07 July 2016 15:40 BST

A woman smokes marijuana during a rally in front of the Supreme Court of Justice in Mexico City (AFP/Getty)

Smoking marijuana dampens the brain's ability to react positively to rewards - monetary or otherwise - a new study has found.

Scientists from the University of Michigan discovered the dampening of reward responses in marijuana users can make them more susceptible to becoming addicted to cannabis or other drugs.

The results come from a long term study of 108 marijuana users in their early 20s monitoring their brain responses. They all had MRI scans of their brain at three points over a period of four years.

The study's senior author, neuroscientist Dr Mary Heitzeg said: "What we saw was that over time, marijuana use was associated with a lower response to a monetary reward.

"This means that something that would be rewarding to most people was no longer rewarding to them, suggesting but not proving that their reward system has been 'hijacked' by the drug, and that they need the drug to feel reward - or that their emotional response has been dampened."

When their brain was being scanned, the participants played a game which asked them to press a button when the saw a target on a screen.

Before each round they were told they could win 20 cents, $5 or nothing at all.

The researchers concentrated on the moment the subjects anticipated a potential monetary reward.

They found that the amount of dopamine, the "pleasure chemical", produced by the part of the brain responsible for reward, the nucleus accumbens, reduces over time due to marijuana usage.

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Crystal meth

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Krokodil

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Dr Heitzeg added: "Some people may believe that marijuana is not addictive or that it's 'better' than other drugs that can cause dependence

"But this study provides evidence that it's affecting the brain in a way that may make it more difficult to stop using it.

"It changes your brain in a way that may change your behavior, and where you get your sense of reward from."

The study was published in the journal JAMA Psychiatry.

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