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Casual marijuana use may damage your brain
April 16th, 2014
09:02 AM ET

Casual marijuana use may damage your brain

If you thought smoking a joint occasionally was OK, a new study released Tuesday suggests you might want to reconsider.

The study, published in the Journal of Neuroscience, is the first to link casual marijuana use to major changes in the brain. And according to the researchers, the degree of abnormalities is based on the number of joints you smoke in a week.

Using different types of neuroimaging, researchers examined the brains of 40 young adults between the ages of 18 and 25 who were enrolled in Boston-area colleges. Twenty of them smoked marijuana at least once a week. The other 20 did not use pot at all.

The marijuana smokers were asked to track their cannabis use for 90 days. All were given high-resolution MRIs, and users and non-users' results were compared.

Researchers examined regions of the brain involved in emotional processing, motivation and reward, called the nucleus accumbens and the amygdala.  They analyzed volume, shape and density of grey matter - where most cells in brain tissue are located.

"I think the findings that there are observable differences in brain structure with marijuana even in these young adult recreational users indicate that there are significant effects of marijuana on the brain," says Dr. Jodi Gilman, lead author and a researcher in the Massachusetts General Center for Addiction Medicine. "Those differences were exposure-dependent, meaning those who used more marijuana had greater abnormalities."

More than a third of the group - seven of the 20 - only used pot recreationally once or twice a week. The median use was six joints a week, but there were four people who said they smoked more than 20 joints a week. None of the users reported any problems with school, work, legal issues, parents or relationships, according to Dr. Hans Breiter, co-senior author of the study and a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine.

"There's a general idea out there that casual use of marijuana does not lead to bad effects, so we started out to investigate that very directly," Breiter said. "This research with the other studies we have done have led me to be extremely concerned about the effects of marijuana in adolescents and young adults and to consider that we may need to be very careful about legalization policies and possibly consider how to prevent anyone under age 25 to 30 from using marijuana at all."

Researchers have long been concerned about the effects of marijuana on the developing brain - teens and adolescents under the age of 25. Preliminary research has shown that early onset smokers are slower at tasks, have lower IQs later in life and even have a higher risk of stroke.

Dr. Staci Gruber, director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital in Boston and a professor of psychiatry at Harvard Medical School, has conducted numerous studies on marijuana use and brain function.

"This data certainly confirms what others have reported with regard to changes in brain structure," she said. "When we consider the findings of the Gilman ... study with our own and other investigations of marijuana use, it's clear that further investigation is warranted, specifically for individuals in emerging adulthood, as exposure during a period of developmental vulnerability may result in neurophysiologic changes which may have long-term implications."

Gruber says we need to take a closer look at all pot users whether they smoke once or twice a week or four or time times a week.

And she had this advice for adolescents: "Don't do it early–prior to age 16. That's what our data suggests, that regular use of marijuana prior to age 16 is associated with greater difficulty of tasks requiring judgment, planning and inhibitory function as well as changes in brain function and white matter microstructure relative to those who start later."

According to the National Survey on Drug Use and Health, in 2012 nearly 19 million Americans used marijuana. It's the most-used illegal drug in the country and use is increasing among teenagers and young adults.

Results of the new study match those of animal studies, authors say, showing that when rats are given tetrahydrocannabinol, or THC - the ingredient in marijuana the gets you high - their brains rewire and new connections are formed.

Gilman thinks when people start to become addicted to substances, their brains form these new connections too.

"The next important thing to investigate is how these structural abnormalities relate to functional outcomes," she said. "Currently we don't know how much marijuana is safe and I think this study shows that we should be cautious about marijuana use in adolescents and young adults whose developing brain may be even more susceptible to cannabis-induced changes."


soundoff (757 Responses)
  1. Julie Rambow

    To clarify a bit more, using marijuana moderately, (1 oz/mo), or heavily, (2-3 oz/mo), will cause serious structural and neurological changes. This is why they say only use it for medical purposes as needed. If used moderately or heavily it will leave large holds in the prefrontal cortex areas, (larger than any other drug). The prefrontal cortex controls: Attention span, judgement, impulses, organization/procrastination, forward thinking, internal supervision, and empathy/insight (needing left and right brain communication). A spect scan will show the "holes" that are not getting circulation. The brain will heal itself through therapy or meds, but not much. The worst symptoms, which greatly affects the social life, work, and driving skills, is the paranoia, ADD, and slow response times.

    January 12, 2015 at 06:05 | Report abuse | Reply
  2. Dmoney

    This lady is just trying to paint a negative picture to maintain the business she is in – "Addiction medicine"

    April 16, 2015 at 16:00 | Report abuse | Reply
  3. harold

    I been smoking since I was eight years old and I'm 46 years old and there is nothing wrong with me I'm still a healthy man and it's been helping me with my muscle spasm and all my chronic pain and I do not take pills anymore because all they did was make me sick and stayed in bed all day

    April 19, 2015 at 21:38 | Report abuse | Reply
    • Matt

      You also said "I been" and incorrectly used the singular form of "spasm," not to mention your incredibly long, unpunctuated run-on sentence.

      May 4, 2015 at 17:45 | Report abuse |
    • B

      Your post would indicate otherwise. Have you had your brain scanned lately, Harold?

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Get a behind-the-scenes look at the latest stories from CNN Chief Medical Correspondent, Dr. Sanjay Gupta, Senior Medical Correspondent Elizabeth Cohen and the CNN Medical Unit producers. They'll share news and views on health and medical trends - info that will help you take better care of yourself and the people you love.