Growing questions about marijuana smoking and cancer risk

While a growing body of research suggests a link between heavy marijuana smoking and cancer risk, there remains many unanswered questions. – Image credit Keck Medicine at USC

CANCER DIGEST – July 26, 2026 – With marijuana being legalized in more states, questions about health and safety remain, and a growing body of research is showing that heavy marijuana smoking may raise the risk of lung cancer and several types of head and neck cancers. Nevertheless, even as preliminary indications suggest a link, there are many unanswered questions including what dose constitutes heavy use.

"What we don't know right now is the dose relationship," Brooks Udelsman, MD, a thoracic surgeon at USC’s Keck Medicine said in a press release. "So, if someone smokes marijuana occasionally once a week, once a month or a few times a year, do they still have that same risk? My suspicion is that there is probably minimal risk. All we know right now is that people who smoke a lot of marijuana – to the point that they develop a dependency on it or require hospital care or evaluation for it – do appear to have a higher cancer risk."

In a series of studies led by Niels Kokot, MD at the University of Southern California Keck School of Medicine, it has been found that people who smoke large amounts of marijuana appear to have a higher risk of both small cell and non-small cell lung cancer. In another study Kokot’s team found that people who smoked marijuana daily were 3.5 to 5 times more likely to develop head and neck cancers.

At the same time the studies have found little evidence linking edible marijuana products to increased lung cancer risk says Udelsman.

"The risk is harder to track (with edibles) because there isn't a lot of data on it yet," Udelsman said, "but as of now there does not seem to be a relationship between edibles and lung cancer. We don't yet know if there is a link to other types of cancers, however."

In addition to lung cancers and head and neck cancers researchers are looking at whether heavy marijuana smoking might also be linked to bladder and gastrointestinal cancer, which has been shown to be the case with tobacco smoking.

"With tobacco, we do see an increased prevalence of bladder cancer," Udelsman said, "Whether that relationship is also the same for heavy marijuana smoking, we don't know. That's why we're trying to determine what the risks are so that people can know what risks they're taking," he says.

A common mechanism of action linked to developing cancer between tobacco smoking and marijuana smoking is chronic or long-lasting inflammation. Tobacco smoke contains more than 7,000 chemicals, and 70 of them have been linked to cancer, Udelsman says. Some of those same chemicals are found in marijuana smoke.

In addition, THC (tetrahydrocannabinol) the active ingredient in marijuana is linked to the conversion of a family of hydrocarbons that also trigger inflammation and DNA damage. Some studies have shown that non-small cell lung cancer appears more frequently among people who smoke marijuana.

While vaping has only been around for 15 years, doctors are already seeing serious inflammatory lung diseases linked to vaping, although not specifically cancer.

"The data on vaping is very new, so we don't know yet, but I'd worry about anything you're breathing into your lungs because it infiltrates the cells and air sacs in your lungs, which can cause damage and put you at a higher risk for cancer," Dr. Udelsman says.

Until more is known about dosing and chemical links to cancer in marijuana smoke, Udelsman advises people who use marijuana heavily to discuss their habits with their healthcare providers, who can help evaluate personal cancer risk, and whether additional screening may be appropriate.

Source: Based on original press release from the Keck Medicine of USCQuestions about marijuana smoking and cancer risk are growing