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
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
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
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
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.
Substantial evidence from animal research and a growing number of studies in humans indicate that marijuana exposure during development can cause long-term or possibly permanent adverse changes in the brain. Rats exposed to THC before birth, soon after birth, or during adolescence show notable problems with specific learning and memory tasks later in life. 32–34 Cognitive impairments in adult rats exposed to THC during adolescence are associated with structural and functional changes in the hippocampus. 35–37 Studies in rats also show that adolescent exposure to THC is associated with an altered reward system, increasing the likelihood that an animal will self-administer other drugs (e.g., heroin) when given an opportunity (see “Is marijuana a gateway drug?”).
Overall, the results seem to support researchers’ belief that the benefits for older mice are a result of stimulating the brain’s endocannabinoid system, a biochemical pathway in both mice and human that grows less active over time. The scientists noted, “T o ge t h e r , t h e s e re s u lt s re ve a l a pr of ou n d , l on g – l a s t i n g improve me nt of c o g n it iv e p e r for man c e res u lt i ng f rom a l ow d o s e of TH C t re at m e nt i n m atu re an d ol d a ni m a l s .”
As has been similarly observed with humans, younger animals excelled at the tests when ‘sober’ but tended to struggle significantly under the influence of THC. “Mature” and “old” mice, on the other hand, struggled with tasks as consistent with their brain ages at first, but saw a huge increase in performance with THC infusions that raised their skill level up to young-mouse (drug free) standards and continued for weeks afterward. Meanwhile, The Guardian reported, “None of the mice displayed the strange effects one might expect from doses of THC.”
Researchers at the University of Bonn and Hebrew University have discovered that low, regular doses of tetrahydrocannabinol (THC), one of the main active ingredients or cannabinoids found in marijuana, may help to keep our brains from ‘slowing down’ as we get older. Published today in the journal Nature Medicine, the German study revealed that while younger mice suffered a performance drop under the influence of THC, the psychoactive chemical gave older mice a considerable performance boost, even putting them on par with younger mice who’d abstained.
The team plans to explore the potential impact of THC on older human brains with a clinical trial later this year, being one of few to focus on more aged subjects so far. Previous research with mice by the Universities of Bonn and Mainz also suggested that the brain’s main cannabis receptor and neural pathways are closely related to brain health in later life, and seem to play a role in preventing brain degeneration when active.
Despite your average Shaggy and Scooby-style stereotypes, researchers believe that cannabis could actually help to sharpen our minds later in life.
To test the chemical’s effect on brains of different ages, researchers put mice that were two months, one year, and 18 months old on a daily regimen of THC over the course of a month. The mice were then tested on their abilities to recognize familiar objects, and to navigate a water maze in known and new configurations.
Co-author Andras Bilkei-Gorzo told the Guardian, “If we can rejuvenate the brain so that everybody gets five to 10 more years without needing extra care then that is more than we could have imagined.”