As more states legalize marijuana, many people believe it is relatively safe. But when it comes to cancer, the picture is less clear. Although it has long been established that smoking is a major cause of lung cancer, scientists are still working to understand exactly how marijuana affects cancer risk, said Brooks Yudelsman, M.D., a thoracic surgeon at Keck Medicine at the University of Southern California.
Researchers continue to investigate whether smoking marijuana increases the chance of developing cancer. A recent Keck Medicine study led by Niels Kokot, M.D., an otolaryngologist at the Caruso Department of Otolaryngology and Head and Neck Surgery at the University of Southern California, found that people who smoke heavy marijuana appear to have an increased risk of both small cell and non-small cell lung cancer.
In another study, Dr. Kokot and colleagues reported that people who used marijuana daily were 3.5 to 5 times more likely to develop head and neck cancer than people who did not use marijuana. These cancers include cancers that affect the mouth, pharynx, larynx, oropharynx (tongue, tonsils, back wall of the throat), and nearby salivary glands.
How much marijuana is too much?
Although evidence continues to grow linking heavy marijuana smoking to cancer, researchers still don’t know exactly how much use is required before the risk starts to increase.
“What we don’t know at this point is the dose relationship,” Dr. Yudelsman said. “So, if someone smokes marijuana occasionally, once a week, once a month, or even a few times a year, are they at the same risk? My suspicion is that the risk is probably minimal. All we know at this point is that people who smoke marijuana in large amounts to the point that they develop a dependence on marijuana or require hospital care and testing appear to be at increased risk for cancer.”
Scientists are also investigating whether heavy cannabis use is linked to other cancers, such as bladder and gastrointestinal cancers.
“For tobacco, we see an increased incidence of bladder cancer. We don’t know if that relationship is the same for heavy marijuana smoking,” he says. “That’s why we’re trying to determine what the risks are so people can know what risks they’re taking,” he says.
Are edible foods associated with lung cancer?
Smoking is not the only way people consume marijuana, but current evidence suggests that non-smoking methods, such as edibles, are unlikely to increase the risk of lung cancer.
“Probably not,” Dr. Yudelsman says. “The risk is even harder to track because there is still little data, but there doesn’t seem to be a link between edible foods and lung cancer at this time. However, we don’t yet know if there is a link with other types of cancer.”
Why smoking marijuana may increase cancer risk
Anything that causes long-term inflammation can increase your risk of cancer over time, says Dr. Yudelsman. Tobacco smoke contains more than 7,000 chemicals, about 70 of which are known to be associated with cancer. Some of the same chemicals are also found in marijuana smoke.
He also points out that THC (tetrahydrocannabinol), the main psychoactive ingredient in marijuana, is associated with the conversion of polycyclic aromatic hydrocarbons (PAHs). These compounds can cause inflammation and damage DNA.
“When inflammation and DNA changes occur, the risk of developing cancer increases,” he says.
What about second-hand marijuana smoke?
There is not enough evidence to confirm that second-hand marijuana smoke increases the risk of lung cancer. However, Dr. Yudelsman said it could potentially pose some risk, as people nearby could breathe in inflammatory particles that are thought to contribute to the development of cancer.
Which lung cancers are linked to marijuana?
Lung cancer is generally divided into two main categories: small cell lung cancer and non-small cell lung cancer.
Small cell lung cancer has a particularly strong association with smoking, but Dr. Yudelsman said it may also be linked to marijuana smoking.
“It’s almost unheard of to get small cell lung cancer without some sort of inhalation injury,” he added.
Non-small cell lung cancer also occurs more frequently among people who smoke marijuana.
“We have also seen an increase in non-small cell lung cancer in both cigarette and marijuana smokers, with increases in both types compared to non-smokers,” he added.
Is there a cancer risk from smoking marijuana?
When e-cigarettes first became popular, many people believed they were safer than vaping. But doctors are now treating people with a serious inflammatory lung disease linked to vaping, although not cancer itself.
“But we’re starting to see some very severe inflammatory diseases caused by e-cigarettes, benign diseases that are not cancerous,” Dr. Udelsman says.
E-cigarettes have only been popular for about 15 years, so researchers haven’t had enough time to determine whether they increase the risk of lung cancer. The same uncertainty applies to marijuana vaping.
“The data on e-cigarettes is so new that we don’t know yet, but we’re concerned about what we inhale into our lungs, because e-cigarettes can infiltrate cells and air sacs in the lungs, causing damage and increasing the risk of cancer,” says Dr. Yudelsman.
Do you use it occasionally or often?
Despite these findings, Dr. Yudelsman does not believe that recreational marijuana use causes the lung cancer epidemic.
“I wouldn’t worry too much about occasional use and the minor irritation it may temporarily cause. Your body will probably recover very quickly and there will be little damage,” he says. “I’m more concerned about people who have chronic, high-dose exposure. People who use marijuana every day, multiple times a day. I think that’s where the damage starts to accumulate. Today’s data shows that these people are at risk of developing cancer, just like tobacco.”
People who frequently use marijuana should discuss their habit with their health care provider. Doctors can help assess a person’s cancer risk and decide whether additional screening or follow-up care is appropriate.
