Marijuana users frequently tout its beneficial effects of helping people feel relaxed and happy. These can be attributed to tetrahydrocannabinol (THC), a chemical in the plant that mimics naturally occurring brain chemicals known as cannabinoids that stimulate dopamine release. This activates the brain’s reward system and induces pleasurable sensations.
But THC also exerts numerous other effects on the brain. It disrupts the hippocampus and frontal cortex, which control memory, attention and focus. This is why the National Institute on Drug Abuse reports that “using marijuana causes impaired thinking and interferes with a person’s ability to learn and perform complicated tasks.”
In addition, THC alters the function of the cerebellum and basal ganglia, which regulate balance, coordination and reaction time. Though marijuana use is not as clearly linked to motor vehicle accidents as alcohol, it alters judgment, slows reactions and distorts perception. Federal health guidance is clear that driving under the influence of marijuana, like alcohol, is dangerous and illegal. Yet in a 2019 report 12 million Americans reported driving under the influence of marijuana in the prior 12 months.
Then there are the long-term effects of cannabis use, starting with the fact that marijuana itself can be addictive. A large study of American adults, published in JAMA Psychiatry, found about 30 percent of people who use the substance suffer from marijuana use disorder. These individuals recognize marijuana is harming their ability to function at work and at home, but are unable to stop.
Regular use also has many ill effects, as David Jernigan, an addiction researcher and professor at Boston University’s School of Public Health, told me. “If we had a prescription drug with this many side effects, people wouldn’t want to use it,” he said. One Lancet Psychiatry article reported that daily use of marijuana increased the chance of developing psychosis nearly fivefold.
The detrimental impacts are particularly profound for young people. A large study from New Zealand found that frequent use of marijuana during adolescence was linked to an average loss of six IQ points by mid-adulthood. A JAMA Internal Medicine analysis tracked more than 5,000 young adults for 25 years and reported that cumulative lifetime exposure of marijuana was correlated with worse memory.
The damaging consequences occur during fetal development, too. THC can pass through the placenta into the fetus’s bloodstream, and maternal cannabis use is associated with children having poorer attention, visual functioning, language comprehension, impulse control and distractibility. The adverse outcomes can persist into young adulthood, with reduced executive function noted among college-age people who were exposed in utero. Some studies have also linked marijuana use in pregnancy with higher rates of babies needing intensive care, preterm birth and stillbirth.
Despite abundant scientific evidence documenting the short- and long-term dangers of cannabis use, there is widespread public perception to the contrary. A national survey of Americans aged 12 to 17 found that more than 56 percent believed there was no risk to monthly cannabis use. In other studies, as many as 30 percent of pregnant women said they did not believe cannabis use in pregnancy to be harmful to the baby. Women cited marijuana as being “natural” and therefore safe, and some even preferred it over prescription medicines to treat pregnancy-related nausea.
Cannabis proponents promote its medicinal benefits, though the Food and Drug Administration has only approved THC compounds for limited uses in treating chemotherapy-associated nausea and AIDS-related weight loss. Advocates also argue that other drugs, such as tobacco, alcohol and opioids, cause greater harm than cannabis. That might be true, but it’s hardly a ringing endorsement for marijuana’s safety. As Jernigan told me, “This is not the worst drug that we know, but it’s a serious drug that can cause a whole constellation of problems.”
Like me, Jernigan is in favor of decriminalizing marijuana possession, though he is also concerned that the discussion of marijuana’s health effects has been lost in the rapid push for legalization. “Making another addictive and harmful drug widely and commercially available is not the way to address social justice, especially when cannabis outlets are clustering in areas most affected by the war on drugs,” he said.
As access to marijuana becomes easier and distribution more widespread, the public health community should step up and be far more vocal on its risks. Marijuana possession shouldn’t be a crime, but neither should it be normalized and encouraged. Our societal perception must shift from the extremes of condemning or championing marijuana to treating it like tobacco and alcohol — legal substances that should be carefully regulated, including with clear warnings about their potential for serious and lasting harm.
