Long-Term Effects on the Brain
THC’s chemical structure is similar to a brain chemical called anandamide. This similarity allows the body to recognize THC and to alter normal brain communication—thereby affecting areas of the brain which influence pleasure, memory, thinking, concentration, movement, coordination, and sensory and time perception.
Studies on marijuana’s impact on the brain structure have shown conflicting results. Some studies suggest regular marijuana use in adolescence is associated with altered connectivity and reduced volume of the brain regions involved in memory, learning, and impulse control; while other studies have not found significant structural differences between the brains of people who do and do not use the drug.
Marijuana’s Rising Potency and the Brain
Marijuana potency has steadily increased over the past few decades. In the early 1990s, the average THC content in confiscated marijuana samples was less than 4%. In 2018, it was more than 15%.
This increasing potency, combined with the use of high-THC concentrates, raises concerns that the consequences of marijuana use today could be worse than in the past, particularly among those who are new to marijuana use and in young people, whose brains are still developing.
Cognitive Ability
Several studies suggest that marijuana use can cause functional impairment in cognitive abilities, but the degree and/or duration of the impairment depends on the age when a person began using and how much and how long they used.
Among nearly 4,000 young adults tracked over a 25-year period, cumulative lifetime exposure to marijuana was associated with lower scores on a test of verbal memory, but it did not affect other cognitive abilities such as processing speed or executive function. The effect was sizeable and significant even after eliminating those involved with current use and after adjusting for demographic factors, other drug and alcohol use, and other psychiatric conditions such as depression.
IQ
Some studies have 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 studies 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 results, 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.
One large study 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. 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.
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.
Memory
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.
As people age, they lose neurons in the hippocampus, which decreases their ability to learn new information. Chronic THC exposure may hasten age-related loss of hippocampal neurons.
Risk of Psychiatric Disorders
Several studies have linked marijuana use to increased risk for psychiatric disorders, including psychosis (schizophrenia), depression, anxiety, and substance use disorders, but whether and to what extent it actually causes these conditions is not always easy to determine. The only significant associations are increased risk of alcohol use disorders, nicotine dependence, marijuana use disorder, and other drug use disorders.
Smoking high-potency marijuana every day could increase the chances of developing psychosis by nearly five times (or seven times, for those who carry a specific gene variant) compared to people who have never used marijuana. The amount of drug used, the age at first use, and genetic vulnerability have all been shown to influence this relationship.
No association between marijuana use and mood and anxiety disorders has been found. Inconsistent and modest associations have been reported between marijuana use and suicidal thoughts and attempted suicide among teens. Marijuana has also been associated with an amotivational syndrome, a diminished or absent drive to engage in typically rewarding activities. It has been hypothesized that brain changes resulting from early use of marijuana may underlie these associations, but more research is needed.