Patients who have colon cancer are at least 20 times more likely to die within five years if they have a history of heavy cannabis use, according to a research team at the University of California San Diego’s School of Medicine.

The study was published online in the Annals of Epidemiology.

“This study adds to a growing body of evidence suggesting that heavy cannabis use may have under-recognized impacts on the immune system, mental health and treatment behaviors — all of which could influence cancer outcomes,” lead author Raphael Cuomo, an associate professor in the school’s Department of Anesthesiology, said in a news release from the university.

Researchers analyzed more than 1,000 electronic health records of colon cancer patients treated within the University of California health system between 2012 and 2024. Besides considering documented use of cannabis before being diagnosed with the disease, they also controlled for age, sex and indicators of disease severity such as tumor staging and cancer biomarkers,” per the release.

They found that:

• Among patients with a history of cannabis use disorder, 55.8% died within five years, compared to 5.05% of patients who didn’t have the disorder.
• Those with the disorder before their cancer diagnosis were 24.4 times more likely to die within five years of diagnosis, compared to those without the disorder.

The researchers said their work adds to growing evidence that while studies document medical value in some cases, “real world use is more complex,” so there’s a need for more studies on the long-term effect of cannabis in “medically vulnerable populations.”

“High cannabis use is often associated with depression, anxiety and other challenges that may compromise a patient’s ability to engage fully with cancer treatment,” said Cuomo, who is also a member of UC San Diego Moore’s Cancer Center.

“However, this isn’t about vilifying cannabis. It’s about understanding the full range of its impacts, especially for people facing serious illnesses. We hope these findings encourage more research — and more nuanced conversations — about how cannabis interacts with cancer biology and care.”

A need for caution?

This study is not the first to urge caution with heavy cannabis use. In March, the American College of Cardiology noted “mounting evidence that people who use cannabis are more likely to suffer a heart attack than those who do not use the drug, even among younger and otherwise healthy adults.”

That finding was based on a retrospective study of more than 4.6 million people that was published in the Journal of the American College of Cardiology, as well as a meta-analysis of 12 earlier studies presented at the group’s scientific session.

The researchers in that study effort added that they were not able to account for possible confounding factors, including how long and how much people used cannabis and whether they also used tobacco or other drugs.

Another recently published study of cannabis use among young adults classified as recent or heavy users said that working memory is impacted.

The study, published in JAMA Network Open, looked at more than 1,000 adults ages 22 to 36 using brain imaging tools, and found that 63% of heavy lifetime users of cannabis had reduced brain activity while performing a working memory task. The same was true of 68% of recent users.

The researchers, from the University of Colorado Anschutz Medical Campus, in a release noted that “this decline in brain activity was associated with worse performance on working memory — the ability to retain and use information to perform tasks. For example, working memory allows a person to follow instructions they’ve just been given or to mentally visualize and manipulate information, like solving a math problem.”