How Marijuana Affects Health

A marijuana leaf, and a joint
Marijuana may be linked with heart problems, according to a new study. (Image credit: Marijuana photo via Shutterstock)

Many people think that smoking pot is harmless, but there's good evidence that the drug has at least some negative effects on health, a new review says.

Some people who smoke marijuana can become addicted, and use of the drug in the teen years has been linked with abnormalities in certain brain areas important for learning and memory, the review said. And even the immediate short-term effects of the marijuana, such as impaired thinking and coordination, can have consequences, including difficulty in learning in school and an increased risk of car accidents, the review said.

Regular marijuana smokers are also more likely than nonsmokers to have symptoms of chronic bronchitis, such as daily cough and phlegm production.

But whether the drug has long-lasting effects on cognition in adults remains controversial, with some studies suggesting the effects are persistent, and others saying the effects may be reversible, said Dr. Nora Volkow, the director of the National Institute on Drug Abuse, who wrote the review along with her colleagues. More research is needed on this topic to provide a definitive answer, she said. [Trippy Tales: The History of 8 Hallucinogens]

Legal drugs, such as alcohol and tobacco, still account for a greater burden of disease than marijuana, but this is because legal drugs are more accessible, and thus more widely used, not necessarily more dangerous, Volkow said.

"As policy shifts toward legalization of marijuana, it is reasonable and probably prudent to hypothesize that [marijuana] use will increase and that, by extension, so will the number of persons for whom there will be negative health consequence," the researchers wrote in the June 5 issue of the New England Journal of Medicine.

Here are a few more highlights from the review:

  • Despite the popular belief that the marijuana is not addicting, about 9 percent of those who experiment with the drug, and up to 50 percent who use it every day, will become addicted.
  • Smoking marijuana in the teen years is linked with brain abnormalities, such as fewer neural fibers in certain brain areas, decreased brain activity and a smaller hippocampus, an area important for learning and memory. These studies show an association, and cannot prove that marijuana is the cause of the brain abnormalities, or that the abnormalities are harmful. Still, one study found that people who used marijuana heavily as teens had IQ scores that were 8 points lower, on average, than those who didn't use the drug.
  • People who use marijuana are at greater risk of abusing other drugs later in life, suggesting that marijuana maybe a "gateway drug." However, it could be that people who are more susceptible to drug use in general tend to start with marijuana because it is more accessible, and then move on to other drugs.
  • In people who are genetically at risk for schizophrenia, smoking marijuana is linked with an increased risk of developing the condition. However, its possible factors other than marijuana are responsible for the link.
  • A person's risk of a car accident doubles if that individual drives shortly after smoking marijuana.
  • It's not clear whether smoking marijuana increases the risk of lung cancer compared with people who don't smoke, but studies suggest that the risk of lung cancer is lower in marijuana smokers than in tobacco smokers.

More research is needed on the ways in which government policies on marijuana affect public health, the researchers said. For example, it's not known if legalizing pot will lead to an increase in car accidents or an increase in the number of teens who use the drug, Volkow said.

The researchers noted that the potency of marijuana has increased over the last few decades — from about 3 percent THC (tetrahydrocannabinol, the active ingredient of marijuana) in 1980, to 12 percent in 2012. Because older studies were based on lower-potency marijuana, it's possible that more-harmful health effects may occur with today's marijuana.

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Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.