This system that marijuana is able to exert influence on is known as the endocannabinoid (EC) system. It affects virtually every part of the brain, including the amygdala (emotion regulation), brain stem (pain sensitivity), and hypothalamus (hunger and sexual impulses), all of which are responsible for those hallmark signs of a marijuana high.
While weed doesn’t directly result in the death of the neurons the way that stress, head trauma, or other types of substances can, it can still go on to cause significant—and long-lasting—damage. This brain damage can lead to permanent side effects such as impaired memory, mental illness, and in the case of adolescents and their still-developing brains, a lower IQ.
How Marijuana Works
What are the side effects that come to mind when you think of marijuana use? Most likely giggling, slowed speech and movements, and a sudden craving for snacks. This bumbling stereotype might lead you to believe that marijuana makes you dumber, but the scientific consensus seems to be that marijuana does not kill brain cells. This doesn’t mean that marijuana is harmless, however.
4 Ways That Marijuana Affects Cognition
Although this phenomenon isn’t exclusive to weed smokers, they do face the greatest risk. Approximately 15% of marijuana users experience these psychotic symptoms (otherwise known as a “bad high”). Most will only experience these adverse side effects while intoxicated, but others go on to find themselves permanently afflicted, which would then fall under the label of a true drug-induced mental illness.
More research will be needed to answer definitively whether marijuana use causes long-term IQ losses and whether factors that weren’t measured in the prior research, such as the increasing amounts of THC in cannabis and the emergence of new cannabis products, are relevant.
Distribution of cannabinoid receptors in the rat brain. Brain image reveals high levels (shown in orange and yellow) of cannabinoid receptors in many areas, including the cortex, hippocampus, cerebellum, and nucleus accumbens (ventral striatum).
Several studies, including two large longitudinal studies, suggest that marijuana use can cause functional impairment in cognitive abilities but that the degree and/or duration of the impairment depends on the age when a person began using and how much and how long he or she used. 41
Marijuana, Memory, and the Hippocampus
Among nearly 4,000 young adults in the Coronary Artery Risk Development in Young Adults study tracked over a 25-year period until mid-adulthood, cumulative lifetime exposure to marijuana was associated with lower scores on a test of verbal memory but did not affect other cognitive abilities such as processing speed or executive function. The effect was sizable and significant even after eliminating those involved with current use and after adjusting for confounding factors such as demographic factors, other drug and alcohol use, and other psychiatric conditions such as depression. 42
A large longitudinal study in New Zealand found that persistent marijuana use disorder with frequent use starting in adolescence was associated with a loss of an average of 6 or up to 8 IQ points measured in mid-adulthood. 43 Those who used marijuana heavily as teenagers and quit using as adults did not recover the lost IQ points. People who only began using marijuana heavily in adulthood did not lose IQ points. Two shorter-duration prospective longitudinal twin studies found that youth who used marijuana showed significant declines in verbal ability (equivalent to 4 IQ points) and general knowledge between the preteen years (ages 9 to 12, before use) and late adolescence/early adulthood (ages 17 to 20); however those who went on to use marijuana at older ages already had lower scores on these measures at the start of the study, before they started using the drug. Also, no predictable difference was found between twins when one used marijuana and one did not. 44
Memory impairment from marijuana use occurs because THC alters how the hippocampus, a brain area responsible for memory formation, processes information. Most of the evidence supporting this assertion comes from animal studies. For example, rats exposed to THC in utero, soon after birth, or during adolescence, show notable problems with specific learning/memory tasks later in life. Moreover, cognitive impairment in adult rats is associated with structural and functional changes in the hippocampus from THC exposure during adolescence.
Some studies have also linked marijuana use to declines in IQ, especially when use starts in adolescence and leads to persistent cannabis use disorder into adulthood. However, not all of the studies on the link between marijuana and IQ have reached the same conclusion, and it is difficult to prove that marijuana causes a decline in IQ when there are multiple factors that can influence the results of such studies, such as genetics, family environment, age of first use, frequency of use, having a cannabis use disorder, duration of use, and duration of the study. Key research in this area to date is described below.
Imaging studies of marijuana’s impact on brain structure in humans have shown conflicting results. Some studies suggest regular marijuana use in adolescence is associated with altered connectivity and reduced volume of specific brain regions involved in a broad range of executive functions such as memory, learning, and impulse control compared to people who do not use. 38,39 Other studies have not found significant structural differences between the brains of people who do and do not use the drug. 40