Study says marijuana addicts prefer drugs over money

Marijuana use can reduce the brain's response to rewards over time, exposing users to the risk of becoming addicted to the drug, a new study has found.

The first long-term study of young marijuana users that tracked brain responses to rewards over time shows measurable changes in the brain's reward system with marijuana use - even when other factors like alcohol use and cigarette smoking were taken into account.

"What we saw was that over time, marijuana use was associated with a lower response to a monetary reward," said Mary Heitzeg, from the University of Michigan Medical School.

"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," Heitzeg said.

The study involved 108 people in their early 20s - the prime age for marijuana use.

All were taking part in a larger study of substance use, and all had brain scans at three points over four years.

Three-quarters were men, and nearly all were white.

While their brain was being scanned in a functional MRI scanner, they played a game that asked them to click a button when they saw a target on a screen in front of them.

Before each round, they were told they might win 20 cents, or USD 5 - or that they might lose that amount, have no reward or loss.

The researchers were most interested in what happened in the reward centres of the volunteers' brains - the area called the nucleus accumbens - at the moment when the volunteers knew they might win some money, and were anticipating performing the simple task that it would take to win.

In that moment of anticipating a reward, the cells of the nucleus accumbens usually swing into action, pumping out a 'pleasure chemical' called dopamine.

The bigger the response, the more pleasure or thrill a person feels - and the more likely they'll be to repeat the behaviour later.

However, the more marijuana use a volunteer reported, the smaller the response in their nucleus accumbens over time, the researchers found.

While the researchers did not look at the volunteers' responses to marijuana-related cues, previous research has shown that the brains of people who use a high-inducing drug repeatedly often respond more strongly when they're shown cues related to that drug.

The increased response means the drug has become associated in their brains with positive, rewarding feelings, which makes it harder to stop using then drug.

The study was published in the journal JAMA Psychiatry.