Researchers at the University of California San Diego School of Medicine have found that individuals with colon cancer and a documented history of high cannabis use were more than 20 times more likely to die within five years of diagnosis compared to those without such a history.

“This study adds to a growing body of evidence suggesting that heavy cannabis use may have underrecognised impacts on the immune system, mental health and treatment behaviours — all of which could influence cancer outcomes,” said lead author Raphael Cuomo, Ph.D., associate professor in the Department of Anesthesiology at UC San Diego School of Medicine.

The research team drew on electronic health records from more than 1,000 colon cancer patients treated across the University of California Health system between 2012 and 2024.

They evaluated how cancer outcomes differed based on patients’ documented cannabis use before diagnosis, controlling for age, sex and indicators of disease severity such as tumour staging and cancer biomarkers.

The analysis found:

• Patients with a history of cannabis use disorder (CUD) had a substantially higher five-year mortality rate (55.88 %) compared to patients without CUD (5.05 %).
• Patients diagnosed with CUD prior to cancer diagnosis were 24.4 times more likely to die within five years of diagnosis compared to those without CUD.

While some laboratory studies have shown anti-tumour effects from certain cannabis compounds, the authors note that real-world use is more complex.

Given the increasing prevalence and social acceptance of cannabis use, they also emphasise the need to further investigate its long-term effects 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 Centre.

“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.”

The study was published online in the Annals of Epidemiology.

No funding or conflicts of interest were reported for this study.

Source: University of California - San Diego