Heavy marijuana use has been linked with impaired motivation, attention, learning and memory, but common beliefs maintain that casual use of the drug does not result in any negative outcomes. Now, a new study suggests young adults who smoke marijuana at least once a week have altered areas of the brain involved in emotion and motivation.
The researchers, from Northwestern University and Massachusetts General Hospital/Harvard Medical School, publish their results in the Journal of Neuroscience.
According to the most current National Survey on Drug Use and Mental Health, marijuana is the most commonly used illegal drug in the US, with nearly 19 million people having used it recently.
In previous studies, animals repeatedly exposed to tetrahydrocannabinol (THC), the main psychoactive element of marijuana, have exhibited structural changes in brain regions involved with motivation, learning, attention and memory.
But how low or moderate marijuana use affects brain structure in humans has been unclear.
To investigate further, the researchers, including Jodi Gilman, PhD, used magnetic resonance imaging (MRI) to analyze the brains of 40 subjects between the ages of 18 and 25 years old who smoked marijuana at least once per week. They then compared these brain scans with the scans of individuals with little or no history of marijuana use.
Marijuana use prompted changes in brain region involved in reward processing
Study participants who smoked a moderate amount of marijuana regularly showed changes in brain regions involved in reward processing and emotion.
Each marijuana user was asked to estimate his or her drug intake over a 3-month period, providing the number of days and amount they smoked.
Psychiatric evaluation ruled out the chance that the marijuana users were dependent on the drug, however, the MRI images showed "significant" brain differences.
Compared with the non-smokers, the marijuana users had a larger nucleus accumbens - the brain region involved in reward processing. Additionally, it had an altered shape and structure in the brains of the marijuana users.
And the more marijuana the smokers used, the greater the abnormalities in both the nucleus accumbens and the amygdala, a region involved in emotion.
Dr. Hans Breiter, a co-author of the study, says their findings raise "a strong challenge to the idea that casual marijuana use isn't associated with bad consequences."
Speaking with Medical News Today, Jodi Gilman explained the impact these brain changes could have:
"Abnormal neuronal growth in the nucleus accumbens could be an indication that the brain is forming new connections that may encourage further use of marijuana.
The study results fit with animal studies that show when rats are given THC, their brains rewire and form many new connections. It may be that we're seeing a type of drug learning in the brain."
She said she and her team believe that when people are becoming addicted to a drug, their brains may create these new connections.
Commenting on the study, Carl Lupica, PhD, who studies drug addiction at the National Institute on Drug Abuse, and who was not involved in the study, says the findings "are particularly interesting because previous studies have focused primarily on the brains of heavy marijuana smokers, and have largely ignored the brains of casual users."
Gilman told us that future research from the team will investigate "how these structural abnormalities relate to functional outcomes."
"For example, how changes in the shape of the nucleus accumbens relate to behaviors such as memory, reward sensitivity and motivation is unknown," she added.
In 2013, Medical News Today reported on a study that suggested marijuana use is linked to schizophrenia-related brain changes.