How Casual Marijuana Can Cause Brain Abnormalities

man's hand holding marijuana joint

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Even casual, recreational use of marijuana by young people can affect the regions of the brain involved in emotion, motivation, and decision-making. Scientists say that they now have the evidence to prove it.

A review of studies on the functional and morphological impact of marijuana use on the brain shows changes in three areas. These include the prefrontal cortex, amygdala and hippocampus. The differences in brain abnormalities compared to non-smokers is directly related to how much marijuana is consumed, the researchers found.

Studies of Heavy Marijuana Smokers

There have been many previous studies that have linked marijuana use to impairment in motivation, attention, learning, and memory. Studies have found that long-time marijuana use can hamper motivation. Other studies have linked marijuana use to impaired learning and social skills.

Other research has found that smoking marijuana can impair the ability to maintain attention and another study found that early marijuana use can cause cognitive impairment not seen in those who begin smoking marijuana later in life.

Research on the Effect of Occasional Marijuana Use

The difficulty is that most, if not all, of the previous studies on the subject involved chronic, heavy marijuana smokers.

A study published in The Journal of Neuroscience was the first to link casual, occasional marijuana use with negative effects on the brain.

Although the sample size of the study was small (only 40 total subjects, including 20 non-marijuana using controls) the differences in the brains of the two groups were remarkable, the authors reported.

Researchers at Northwestern University and Massachusetts General Hospital/Harvard Medical School used magnetic resonance imaging to compare the brains of 20 young people who reported smoking marijuana at least once per week with 20 others age 18-25 who reported little to no history of marijuana use.

The scientists measured the volume, shape, and density of grey matter in the nucleus accumbens and the amygdala. The nucleus accumbens is involved in reward processing and decision making, while the amygdala is associated with emotion.

The participants were screened to determine that none were dependent upon marijuana or any other drugs and none of the participants had ever abused any other drugs.

Those who used marijuana were asked to estimate their marijuana consumption over a three-month period, including the number of days they smoked and the amount of the drug consumed each day.

Effects of Marijuana on the Size, Shape, and Density of the Brain

The researchers found that the more the marijuana users reported consuming, the greater the abnormalities in the nucleus accumbens and the amygdala on the MRI images. The shape and density of both regions of the brain were also different between marijuana users and non-users.

The study revealed that the brains of those who smoked only one joint a day or those who smoked only once a week were changed.

"This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences," said Hans Breiter, M.D. one of the study's authors. "Some of these people only used marijuana to get high once or twice a week,"

Why Occasional Marijuana Use Can Cause Problems

"People think a little recreational use shouldn't cause a problem if someone is doing OK with work or school. Our data directly says this is not the case," Breiter said in a news release.

Other researchers, not involved in the Massachusetts General study, agreed that finding changes in the brains of casual marijuana users was surprising.

"This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy," said Carl Lupica, Ph.D., of the National Institute on Drug Abuse. "These observations 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."

The research was funded by the National Institute on Drug Abuse, the Office of National Drug Control Policy, Counterdrug Technology Assessment Center, and the National Institute of Neurological Disorders and Stroke.

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4 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Buddy T
Buddy T is an anonymous writer and founding member of the Online Al-Anon Outreach Committee with decades of experience writing about alcoholism.