Whether cannabis, or marijuana, kills brain cells remains unknown, and current research studies have yielded conflicting results.
Keep reading to learn more about what current studies have to say about marijuana’s short- and long-term effects on the brain.
Please note that the studies covered in this article mainly consider the effects of tetrahydrocannabinol (THC) on the brain. THC is the psychoactive compound in marijuana, or cannabis, that creates the ‘high’ effect. THC is just one of over 100 cannabinoids found in marijuana.
The body naturally produces endocannabinoids, which are similar to cannabinoids. Both cannabinoids, such as THC, and these natural endocannabinoids, bind to the same receptors in the brain.
Researchers have identified two types of these receptors:
- CB1 receptors located in the central nervous system
- CB2 receptors, which develop in the peripheral nervous system
When endocannabinoids bind to CB1 and CB2 receptors, they affect the following body functions:
- appetite and metabolism
- pain regulation
- cardiovascular functions
- reproductive functions
- immune system functions
- muscle and bone formation
- coordination and motor control
- reward and addiction behaviors
Cannabinoids, such as THC, and naturally occurring endocannabinoids may have significant effects on brain function and development. This is because regions of the brain that control memory, learning, motor control, and sensory perception contain high concentrations of CB1 receptors.
Current research on this topic has yielded conflicting results. Some studies suggest that THC has potentially permanent neurotoxic effects that impair people’s verbal learning, memory, and focus. Other studies indicate the opposite.
In one 2017 animal study, researchers compared differences in working memory between adult rats exposed to a synthetic cannabinoid and those exposed to glucose during adolescence.
The researchers found that the rats they exposed to cannabinoids had a significantly better working memory in adulthood than the control rats.
In another 2018 animal study, researchers found that exposure to THC induced brain tissue growth and improved learning and memory function in rats.
In a 2016 study, researchers assessed the cognitive function of 3,385 people aged 18–30 by looking at the data from the 25-year-long Coronary Artery Risk Development in Young Adults (CARDIA) Study that began in 1986. At year 25, 84% of these participants reported previous marijuana use, but only 9% reported using marijuana into middle age.
After adjusting for demographic factors, psychiatric conditions, and other drug use, the researchers found long-term exposure to marijuana was associated with impaired verbal memory.
However, they found no evidence to suggest an association between marijuana use and cognitive processing or executive function.
In one 2016 study, researchers compared changes in the IQ scores of adolescent twins when one used marijuana, and the other did not. Those who used marijuana had an average reduction of 4 IQ points by early adulthood.
However, the researchers found that individuals who started using marijuana as adolescents had lower baseline IQ scores anyway, which suggests that marijuana does not necessarily have a direct effect on IQ.
It is also worth noting that a subset of the study participants who used marijuana in adolescence had slightly better working memory scores than those who did not use marijuana.
In a 2019 study, researchers took MRI scans of 781 teenagers and young adults aged 14 to 22. 147 of the participants reported either occasional or frequent marijuana use, while 634 participants identified as non-users.
The MRI scans of individuals who reported occasional or frequent marijuana use showed no significant differences in the brain volume, cortical thickness, or gray matter density compared to their abstinent peers.
Gray matter refers to regions of brain tissue that contain nerve cell bodies. These regions control muscle movement, sensory perception, and executive functions, including self-control and decision making.
These findings directly oppose research on alcohol use in young adults, such as this 2017 study, which shows heavy drinking can significantly reduce the amount of gray matter in the brain.
Potential short-term side effects of heavy marijuana use include:
Researchers do not fully understand the long-term risks and side effects associated with cannabis use. Research in this area is still ongoing.
However, long-term marijuana may increase a person’s risk of substance use disorders, as well as problems with memory and concentration.
Available research offers some insight into the short-term effects of drug use. However, it is difficult to draw definitive conclusions regarding the long-term effects of drug use. This because many studies track small samples of people over short periods. This is likely to change in the near future.
In 2015, the National Institutes of Health (NIH) began the Adolescent Brain Cognitive Development (ABCD) study that aims to understand how tobacco, alcohol, marijuana, and other drug use impacts adolescent health and brain development.
The NIH has now enrolled 11,878 children between the ages of 9 and 10 and their families onto the ABCD study. Over the next decade, researchers will use neuroimaging technology to track brain development from late childhood to early adulthood.
There are still a lot of unknowns surrounding how marijuana affects adolescent and adult brains.
Research in this area is ongoing.
Long-term, large-scale studies should provide useful insights within the next decade.