Adults diagnosed with cannabis use disorder are nearly four times as likely to develop lung cancer as matched peers without the diagnosis. That is the headline number from a large U.S. study in the journal Lung Cancer. The relative risk came out at 3.87.
The catch is who the study actually measured. Cannabis use disorder is a formal clinical diagnosis, a pattern of heavy, compulsive use that causes significant impairment or distress. The paper says nothing about casual or infrequent users, and they should not assume the 3.87 figure applies to them.
How the numbers were built
Researchers pulled 20 years of electronic medical records from the TriNetX US Collaborative Network, a database covering more than 118 million patients at 67 health care organizations. They used propensity score matching, a statistical technique that builds comparable groups, and ended up with nearly 150,000 matched participants in each group. The study was published April 22, 2026, and ScienceDaily highlighted it on July 10.
The 95% confidence interval ran from 3.43 to 4.38. The excess risk was already visible at one year of follow-up (relative risk 3.48) and held at five years (3.92). It also showed up across the major lung cancer types: small cell carcinoma at 2.70, adenocarcinoma at 2.54, squamous cell carcinoma at 2.90 and non-small cell lung cancer at 2.75.
What it cannot say
The authors acknowledge limits. The study does not establish how much cannabis drives the risk. It also cannot fully separate out concurrent tobacco use, vaping or other inhaled exposures, which can occur together with cannabis use disorder. It shows an association, not a proven mechanism.
An earlier 40-year Swedish cohort study found about a two-fold lung cancer risk for heavy cannabis use that began in young adulthood, even after adjusting for tobacco and other factors. The new, larger number may reflect how severe the use is in people who carry a clinical diagnosis.
Dr. Brooks Udelsman, a thoracic surgeon at Keck Medicine of USC and a co-author, told ScienceDaily that the link between tobacco and lung cancer is well established while the picture with cannabis is far less clear, “but growing evidence suggests heavy use may carry significant risks.” Scientists are still working out the biology. Marijuana smoke contains combustion carcinogens also found in tobacco smoke, including benzene, naphthalene and vinyl chlorides, and some studies suggest it deposits more tar because users tend to inhale deeply and hold the smoke.
The authors called for more research on dose and on the mechanisms, and said clinicians should weigh these risks when advising patients about cannabis. Cannabis is now legally sold for recreational use in 24 U.S. states.
