Marijuana could put users at risk for addiction, researchers find

Contrary to the belief that marijuana is a “safe drug,” a researcher at FIU’s Center for Children and Families has found that the use of this drug actually has long-term effects on the brain.

A recent study suggests participants who smoked more marijuana had less activity in the brain in response to reward compared to those that reported less use. This means that individuals who commonly smoke marijuana are likely to seek out drugs more frequently to counteract their weaker response to natural rewards from daily life.

The brain’s reward center controls and regulates a person’s ability to feel pleasure. Feeling pleasure motivates us to repeat behaviors that are critical to our existence.

“We are all born with an innate drive to engage in behaviors that feel rewarding and give us pleasure,” said FIU Psychologist  Elisa Trucco, one of the authors of the study . “We now have convincing evidence that regular marijuana use impacts the brain’s natural response to these rewards. In the long run, this increase in more compulsive marijuana use is likely to put these individuals at risk for addiction.”

Currently, recreational marijuana use is legal in four states and 23 states support medical marijuana use. At least 11 more states are likely to legalize recreational marijuana use in the near future with a growing misconception that marijuana has no long-term impact on the brain. Participants in the longitudinal study included 108 20-year-olds that were asked several questions regarding their marijuana use and their brains were scanned using functional magnetic resonance imaging (fMRI) at approximately two-year intervals. While in the scanner, participants played a game that asked them to click a button at the moment they saw a target appear on a screen in front of them. Before each round, they were told they might win 20 cents or $5, that they might lose that amount, or have no reward or loss.

Non-marijuana users showed lots of activity in the part of the brain that responds to rewards at the thought that they may win some money. But for marijuana users, the response was blunted which researchers say may actually open them to more risk of becoming addicted to that drug or others.

“What we saw was that over time, marijuana use was associated with a lower response to a monetary reward,” said lead author and University of Michigan Neuroscientist Mary Heitzeg. “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.”

The findings were published in the Journal of the American Medical Association Psychiatry. The study was conducted in collaboration with researchers from the University of Michigan and funded by the National Institutes of Health (NIH) including the National Institute on Drug Abuse and the National Institute on Alcohol Abuse and Alcoholism.

Trucco is among the neuroscientists and psychologists leading a NIH landmark study on Adolescent Brain Cognitive Development. That study will track thousands of today’s pre-teens — including marijuana users — nationwide over 10 years, looking at many aspects of their health and functioning, including brain development via brain scans, providing a better picture of what happens to users over time.