Reason suggests the more widely and socially acceptable a behavior becomes — even something illegal in most states — the more likely those who adapt the behavior are to think said behavior is harmless. Certainly those who use marijuana recreationally are inclined to hold to that opinion.

But according to a recent study by researchers at Columbia University Medical Center (CUMC), chronic use of marijuana is not harmless. In fact, it can be quite harmful and permanently so. The study scrutinized 11 adults between the ages of 21 and 40. All participants were dubbed “severely dependent” on marijuana. Participants had, on average, started using marijuana at age 16 and had been “dependent” for at least seven years before the study began.1

Research shows heavy use negatively impacts learning and behavior, much like dopamine, heroin and cocaine.

Nearly all participants smoked daily during the month prior to the study. The participants were then required to adhere to a week of abstinence, qualified by a seven-day stay in the hospital where the study was carried out. At the end of the seven days, all eleven participants were given an oral amphetamine to trigger a release of the reward hormone dopamine. Before they took the medication, researchers scanned their brains using positron emission tomography (PET). A second test was done after ingesting the amphetamine and was then compared to the previous scan.2

The results revealed that chronic cannabis users experienced significantly lower dopamine release in the striatum, the brain’s engaged working memory, impulsive behavior and attention center. The same was true for sub-regions involved in associative and sensory-motor learning.3

“The bottom line, said Dr. Anissa Abi-Dargham, professor of psychiatry and a lead author of the paper, “is that long-term, heavy cannabis use may impair the dopaminergic system, which could have a variety of negative effects on learning and behavior.” 4

The Evidence Continues to Grow

Woman smoking jointThis study is far from the first to suggest that chronic marijuana use may result in changes in brain function. As early as 1993, case-control studies suggested that regular users suffered from decreased cognitive performance.5 One analysis of the case studies that span over 20 years explains: “It was unclear whether this was because cannabis use impaired cognitive performance, people with poorer cognitive functioning were more likely to become regular cannabis users, or some combination of the two.”

Recently, a greater number of better-controlled studies have continued to point toward deficits in verbal learning, memory and attention with repeated marijuana use. One study of more than 1,000 New Zealanders born in the early 1970s indicated an average IQ decline of 8 points among early and persistent marijuana users compared to non-users or occasional users.6

As a later analysis emphasized, this 8-point decline “was not trivial: it was half a standard deviation lower than their peers. This means that the average IQ of these heavy users was below 70 percent of their peer group.”

Studies like these worry Nora Volkow, the director of the National Institute on Drug Abuse. In a 2014 blog post directed at states looking to change their marijuana laws, she summed up the research this way:

“The science of marijuana’s long-term effects is increasingly clear. Besides being addictive, marijuana is cognitively impairing even beyond the phase of acute intoxication, and regular use during adolescence may cause a significant, possibly permanent IQ loss. Brain scans in users who started when they were young show impaired neural development, probably because cannabis interferes with normal brain maturation.”7

Of course, this is not to say that policy should remain stagnant on the issue of marijuana, or any other type of drug. Volkow concludes that changes in marijuana laws must take their cue from the research.

Finding Help for Marijuana Addiction

Marijuana abuse can cause damage to the developing brain and lead to the need for stronger and more powerful drugs. If you or a loved one struggles with substance abuse, we are here for you. Call our toll-free helpline 24 hours a day to speak to an admissions coordinator about available treatment options.

Written by Tamarra Kemsley


1 Yarmolinsky, Rachel. “Heavy Cannabis Use Associated with Reduced Dopamine Release in Brain, Similar to Other Addictions.” Columbia Psychiatry. N.p., 30 Mar. 2017. Web. 13 June 2017.

2 “Heavy Cannabis Use.” Columbia Psychiatry. N.p., 30 Mar. 2017. Web. 13 June 2017.

3 “Heavy Cannabis Use.” Columbia Psychiatry. N.p., 30 Mar. 2017. Web. 13 June 2017.

4 “Heavy Cannabis Use.” Columbia Psychiatry. N.p., 30 Mar. 2017. Web. 13 June 2017.

5 Volkow, Nora D. “Research Report Series: Marijuana.” The National Institute on Drug Abuse. December 2014. Web, 13 June, 2017.

6 Weir, Kirsten. “Marijuana and the Developing Brain.” Monitor on Psychology. American Psychological Association, November 2015, Vol 46, No. 10. Web. 13 June 2017.

7 Volkow, Nora D. “Science Should Guide Marijuana Policy.” NIDA. 18 Aug. 2014. Web. 13 June 2017.