Heavy marijuana use may cause changes in users’ dopamine systems similar to the changes caused by heroin and cocaine, a new study suggests.1
A New Approach
Carried out by researchers at Columbia University Medical Center (CUMC), the study looked at 11 adults between the ages of 21 and 40. Dubbed “severely dependent” by the researchers, the participants had, on average, begun using marijuana at 16 years old and had been “dependent” for the seven years leading up to the study.
Nearly all had smoked daily during the month prior to the study. As part of the experiment, however, the participants were required to adhere to a week of abstinence, ensured by a seven-day stay in the hospital where the study was carried out.
Once this period was over, all 11 participants were given an oral amphetamine in order to trigger a release of the reward hormone dopamine. Before they did, however, researchers scanned their brains using positron emission tomography (PET), comparing the results to a scan done after.
The results, published online in Molecular Psychiatry, revealed that compared to 12 control group subjects, the cannabis users experienced “significantly lower” dopamine release in the striatum, a part of the brain engaged in working memory, impulsive behavior and attention. The same was true for sub-regions involved in associative and sensorimotor learning.
“Previous studies have shown that addiction to other drugs of abuse, such as cocaine and heroin, have similar effects on dopamine release, but such evidence for cannabis was missing until now,” a press release accompanying the study stated.2
That said, there is still much that remains unclear, including which came first: lower dopamine levels or heavy marijuana use.
“But the bottom line is that long-term, heavy cannabis use may impair the dopaminergic system, which could have a variety of negative effects on learning and behavior,” said Dr. Anissa Abi-Dargham, professor of psychiatry and a lead author of the paper.3
But the bottom line is that long-term, heavy cannabis use may impair the dopaminergic system, which could have a variety of negative effects on learning and behavior
The findings come at a time of ever-increasing acceptance, both socially and legally, of marijuana use. According to a Gallup poll, 2013 marked the first year “a clear majority” of Americans favored the legalization of marijuana.4 And while the number of teens reporting use of marijuana has remained steady since the mid-1990s, perceived risk has dropped markedly over the last 25 years, particularly among 12th graders.5
According to Abi-Dargham, it is precisely this shift that drove the researchers to the lab. “In light of the more widespread acceptance and use of marijuana, especially by young people, we believe it is important to look more closely at the potentially addictive effects of cannabis on key regions of the brain,” she explained.6
A Tale Worth Remembering
R. was in her late teens when she began smoking to escape the chronic pain leftover from a series of back surgeries. For three years she smoked, as she put it, “all day, every day.”
“All I could think about was that next bowl. How I could sneak it during work? Where I was going to get my weed from? I felt like crap when I was ‘coming down,’ and it was all I thought about.”
On days she couldn’t get high, she spiraled into a dark depression.
Despite all this, R. quit when, at the age of 20, she discovered she was pregnant. Three years later, she returned after going through detox for painkillers.
These days, R. limits her smoking to those days when the pain is the worst. Though, she says, she can’t help but notice some shifts in her cognitive abilities since her teens.
“I used to be able to read a book and even after a couple years I would still know most of the book,” she said. “Now I have to read something four or five times to remember.”
The latest 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.
She admits she has very few memories of the period before her daughter was born, or many memories at all for that matter. And then there’s the daily struggle of constantly forgetting why she walked into a room, or just having a hard time remembering something simple like what she ate for breakfast.
“It’s hard to know whether what I experienced was from the marijuana or just life in general, but for sure my memory capacity has decreased since high school.”
The Evidence Continues to Grow
The latest 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. But as one analysis of the literature 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.”7
Since then, a greater number of better-controlled studies have continued to point toward deficits in verbal learning, memory and attention.8 Included in this group is a longitudinal look at more than 1,000 New Zealanders born in the early 1970s. The results pointed to an average decline in IQ of 8 points among early (beginning in adolescence) and persistent marijuana users compared to non-users or occasional users.9 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.”10
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.11
Of course, this is not to say that policy should remain stagnant on the issue of marijuana, or any other type of drug. Only that, Volkow concludes, those changes that do take place must take their cue from the research.12
And that research? Well, it can be a bit of a buzzkill.
Written by Tamarra Kemsley