Not so long ago, pot was made out to be a taboo “gateway drug” that would tar your lungs and damage your brain forever. But pot isn’t taboo anymore. Eighteen states and Washington, D.C., have legalized it, and more American adults are using the drug than any time since Ronald Reagan was president.
And just as the legality and culture around pot has changed, so has some of the science. Epidemiologists are still alarmed at just how many Americans are smoking pot, but they admit that pot isn’t the health menace we were once told it was — and there’s some evidence that a little bit of pot could be a good thing for adults.
Pot use has soared among Americans over the age of 26 — over 10 percent of them reported getting high in a given month in 2020, according to the National Survey on Drug Use and Health (NSDUH), the country’s largest survey on drug use. That’s more than double the rate of monthly use reported in 2010.
But what are the actual dangers of using cannabis? To help you decide whether to load another bowl or put the bong down for good, we’ve summarized the latest science on marijuana’s health risks; the areas where there’s an absence of evidence that pot is risky (think: lung cancer); and, finally, the surprising reason why the director of the the federal agency in charge of studying how drugs are bad thinks adults could possibly benefit from using cannabis.
The risks are real
Many of pot’s effects are tangled in contradictory research, but there are a few clear health risks to consuming the drug. Smoking cannabis regularly can cause bronchitis-like symptoms, and research published last month found that chronic cannabis users, defined as people who used pot at least four times a week for more than three years, had impaired pancreatic function. There have also been cases of daily cannabis users developing pancreatitis without having any other obvious risk factors.
Regular pot use has also been associated with higher rates of depression, anxiety and poorer life outcomes like being unemployed, but causality has not been established because other factors could predispose someone both to using cannabis and having a mental illness or not having a job.
There’s also evidence that cannabis can be dangerous when used in certain situations, like during pregnancy or while driving a vehicle. A recent study linked increasing rates of childhood leukemia to an increase in cannabis use, and a separate study found a correlation between women using cannabis while pregnant and their children having higher rates of anxiety. There’s also evidence that using pot while pregnant can lead to lower birth weights, although that evidence is still considered limited. And driving a car while high has been shown to moderately increase the risk of getting into a motor vehicle accident.
Addiction can be an issue as well. Some people who smoke pot develop what’s called cannabis use disorder (CUD), a clinical diagnosis of problematic and uncontrollable cannabis use. There’s evidence that CUD rates have increased since 2008, but Dr. Kevin Hill, an addiction psychiatrist and professor at Harvard Medical School, told FiveThirtyEight in an email that “it is still important to point out that most people who use cannabis don’t have a problem with it.”
The 2020 NSDUH found that 4.1 percent of people ages 12-17 met the criteria for CUD,1 13.5 percent of people ages 18 to 25 had the disorder, and 4 percent of people over age 26 had the disorder. Yet those numbers were below rates of alcohol use disorder across all age groups in 2020’s survey.
Deborah Hasin, an epidemiologist at Columbia University, said she is very concerned about adults’ increasing use of cannabis because CUD is associated with poorer quality of life, cognitive decline and impaired educational and occupational employment. Hasin’s research has found that 19.5 percent of people who use cannabis met the criteria of CUD in their lifetimes.
“It’s clear that not everybody who smokes marijuana has all of these problems, but the risk is there, and it’s a greater risk than people assume,” Hasin said.
Using cannabis frequently increases the risks of developing CUD, and frequent pot use is growing among adults. Monthly use for 26-to-34-year-olds has more than doubled since 2008, and the share of people getting at least five days a week increased from 5.8 to 13.8 percent between 2008 and 2019, according to NSDUH survey results.
Some risks have been overblown, though
The popular image of a pothead is loaded with negative health connotations, from joint smoke causing cancer to forgetful stoners damaging their brain with weed. But the actual science behind these popular insinuations is far from settled, and in some ways the data points to pot not actually being the harmful culprit.
While pot smoke does contain cancer-causing chemicals, recent research has found that cannabis use is not associated with a higher risk of lung cancer or emphysema. Using pot is in fact associated with an increased forced vital capacity, a measure of lung function. (That doesn’t mean that people’s lungs are necessarily healthier if they use pot — just that they aren’t unhealthy by this metric.)
Pot has long been accused of causing psychosis or schizophrenia, whether it’s a fable from the Middle Ages, a film from the 1950s Reefer Madness era, or a 2019 Malcolm Gladwell feature in The New Yorker. And while modern studies have found that people diagnosed with schizophrenia have higher rates of cannabis use than people without the condition, there is no scientific consensus that cannabis can cause psychosis or schizophrenia in people who wouldn’t have otherwise developed the illnesses.
A 2019 study attracted widespread attention after it found an association between daily consumption of higher THC potency cannabis and developing schizophrenia, ostensibly giving more evidence that the potent cannabis sold in America’s dispensaries was especially risky. But what wasn’t mentioned in news reports was that the study’s authors did not determine that cannabis had a direct causal relationship with schizophrenia. There’s strong evidence that genetic factors predict both schizophrenia and cannabis use, and a 2021 study examining hundreds of thousands of genetic records found that particular genetics carried a stronger risk for developing schizophrenia than smoking cannabis.
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Carl Hart, a neuroscientist at Columbia, said that despite the attention the 2019 potency and schizophrenia study received, it didn’t actually convince experts that higher potency pot was causing schizophrenia.
“Those studies are poor and crappy — they’re correlation studies,” Hart said. “What they're calling strong correlations are about the same correlations when you look at cat ownership in childhood and likelihood to go on and have a psychiatric illness.”
Pot’s long-term effects on the brain are also more complicated than the caricature of a burned-out pothead. Brain studies on regular cannabis users have detected noticeable changes in brain structure and mild reductions in cognitive function, but a 2017 review of 56 brain studies found that it was unclear whether those changes were caused by or preceded cannabis use, and if those structural changes actually affected brain function.
Pot has surprising benefits, too
Government agencies like the National Institute on Drug Abuse (NIDA) have spent over $1 billion vilifying cannabis as a uniquely damaging substance, even though pot use has not been linked to higher overall mortality while alcohol and tobacco use have. And there is a robust body of evidence that cannabis can provide health benefits. That’s perhaps not very surprising, given that the drug is one of the world’s oldest medicines. Modern epidemiological and animal studies show that cannabinoids, the active compounds in pot, have great promise for treating a wide range of diseases, including cancer and neurological decline, and even the Food and Drug Administration has recognized its medical potential by approving a cannabis-derived drug to treat rare forms of epilepsy.
Some studies that set out to measure pot’s damaging effects have paradoxically found evidence of its benefits. A 2016 study that followed a group of New Zealand adults for 20 years found that cannabis use was associated with worse gum health, but better cholesterol levels, lower BMI and reduced waist circumference.
Those results were further substantiated in a 2020 study that looked at cannabis use among people over the age of 60. Cannabis users in the study exercised more often and had a significantly lower BMI than non-users.
While there's evidence that BMI, which measures only weight and height, is not the best way to gauge health for people who are normal weight or are slightly overweight, very high BMI scores are significantly associated with mortality.
The growing evidence that cannabis use is associated with lower BMI scores in adults is one reason why Dr. Nora Volkow, the director of NIDA, said cannabis could end up having benefits for adults.
“BMI is lower in marijuana users, and that was very surprising, and yet we know that high BMI, particularly the older you get, can have negative effects,” Volkow said. “This is why we need to study it.”
Volkow reiterated that she is “absolutely” concerned about increasing rates of cannabis consumption because long-term, daily consumption of cannabis, particularly the high-THC products sold at legal dispensaries, “can have harmful effects even on the adult brain.”
But for adults who use cannabis occasionally and in moderate doses -- the most common way American adults use cannabis — Volkow said it wasn’t clear if there were any negative effects. “There’s no evidence to my knowledge that occasional [adult] marijuana use has harmful effects. I don’t know of any scientific evidence of that. I don’t think it has been evaluated,” Volkow said. ”We need to test it.”
American adults are not waiting for those tests to be done. Instead, we’ve launched a haphazard clinical trial in which millions of adults dose themselves with cannabis every month. For some people, that may lead to a smoker’s cough and problematic cannabis use, but for others, using pot more often could be a good thing.