Does Weed Kill Brain Cells? And 5 Other Things to Know
We don’t know for sure if using marijuana can kill your brain cells.
More research is also needed to determine whether each form of use — including smoking, vaping, and ingesting edibles — has a different effect on your brain’s overall health.
Studies evaluating the cognitive effects of long-term marijuana use are ongoing.
Here’s what we currently know about how weed affects the brain.
A well-known 2012 study from New Zealand evaluated marijuana use and cognitive ability in more than 1,000 individuals over a 38-year period.
The researchers reported an association between ongoing marijuana use and cognitive decline.
In particular, they found that:
- People who started using marijuana heavily as adolescents and continued as adults lost an average of six to eight IQ points by the time they reached midlife.
- Among the group above, people who stopped using marijuana as adults didn’t regain lost IQ points.
- People who started using marijuana heavily as adults didn’t experience any IQ loss.
This study had a significant impact for a few reasons.
First, it was among the first large, longitudinal (long-term) studies to assess marijuana use and cognitive functioning.
Next, the results suggest that marijuana use during adolescence may have an irreversible effect on adolescent brain development. Some additional research supports this conclusion.
However, the New Zealand study also has significant limitations.
For one, it isn’t possible to conclude that marijuana use causes lower intelligence based on this study alone.
While the researchers controlled for differences in participant education levels, they didn’t rule out additional factors that may have contributed to cognitive decline.
A 2013 reply to the New Zealand study suggests that personality factors may play a role in both marijuana use and cognitive decline.
The author cited conscientiousness as an example. Low conscientiousness might explain both drug use and poor performance on tests of cognition.
Genetic factors may also contribute to cognitive decline, as suggested by a longitudinal twin study from 2016.
In this case, the researchers compared changes in IQ between twins who used marijuana and their abstinent siblings. They didn’t find any significant differences in IQ decline between the two groups.
The key takeaway? More research needs to be done to understand how marijuana use affects intelligence over time.
Researchers haven't determined whether smoking weed kills brain cells, but that hasn't stopped some groups from comparing the substance to nicotine and alcohol, two established brain cell killers. Here's what the science actually says about marijuana's effects on the brain, which users are most at risk, and more.
Does Marijuana Kill Brain Cells?
I have heard that marijuana use can lead to lower IQ, but I have also seen marijuana use help children with seizure disorders. I know that alcohol kills brain cells, is it true that marijuana also kills brain cells?
Thanks for your question.
First, let’s take a moment to address the notion that marijuana use leads to lower IQ. This idea came from a research paper which looked at data from a longitudinal study done on 1000 people from Dunedin, New Zealand. The study followed subjects from age 13 to 38 and conducted IQ tests a both these ages. The findings revealed that people who were chronic users of marijuana, which are those that had a physical dependence to the drug, before the age of 18 had a drop in IQ of 8 points by the age of 38.
There are three important things to note about this study. First, the people who were shown to have a decline in IQ are significantly small – 38 out of 1000, or 3.8 percent. Second, these individuals were using significantly more marijuana (four days per week) than the average marijuana user. Finally, they were using more marijuana consistently for much longer (20 years) than the average marijuana user.
Moreover, follow up studies of this data have shown that differences in socioeconomic status may account for the differences in IQ found in the study, not to mention the discussion surrounding IQ as a valid measurement of brain function and intelligence which are beyond the scope of this response.
In terms of marijuana and its effects on brain cells, or neurons, there is little evidence to suggest that any of the active ingredients in the marijuana plant administered at doses appropriate for human consumption have neurotoxic effects.
This is in direct contrast to alcohol, where the body’s digestive process creates metabolites such as acetaldehyde and other “reactive oxygen species” which are toxic to the brain and other cells in the body. This is why you have a hangover, after drinking large quantities of alcohol.
The active ingredients in the marijuana plant, called phytocannabinoids, affect specific receptors within the body. In fact, the body produces its own set of cannabinoids called endocannabinoids and has an endocannabinoid system which regulates the activity of all cannabinoids in the body.
There are two types of receptors within the endocannabinoid system CB1 receptors and CB2 receptors. CB1 receptors are located primarily in the brain and nervous system, while CB2 receptors are located primarily in the immune system.
The fact that these receptors are located all over the body is part of the reason why marijuana has been found to be useful for so many different medical conditions.
Specifically in the case of seizures, there is preliminary research which shows that the cannabinoid, cannabidiol or CBD, raises the threshold for seizure activity within the brain making it overall more difficult to have seizures, and thus providing hope to many parents of children with intractable seizure disorders. There is also some preliminary research which show that other cannabinoids within the plant actually protect brain cells from damage associated with Alzheimer’s disease. However, much more research needs to be done in order to make definitive claims on the medical benefits.
The evidence at this point indicates that marijuana does more good than harm when it comes to its effects on the adult brain.
Malik Burnett, MD
Dr. Malik Burnett is a former surgeon and physician advocate. He also served as executive director of a medical marijuana nonprofit organization. Amanda Reiman, PhD, holds a doctorate in Social Welfare and teaches classes on drug policy at the University of California-Berkeley.
This infographic has more information on how the brain processes cannabis.
UPDATE: This piece was originally posted on July 31, 2014 and was updated to include the infographic.
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Dear Docs: I have heard that marijuana use can lead to lower IQ, but I have also seen marijuana use help children with seizure disorders. I know that alcohol kills brain cells, is it true that marijuana also kills brain cells? Sincerely, Head-scratcher See the full infographic