Heavy Cannabis Use and Colon Cancer: A Sobering New Study

Heavy Cannabis Use and Colon Cancer: A Sobering New Study

Recent research has sparked a heated debate about the risks of marijuana use, particularly for those battling serious illnesses like colon cancer. A study published on April 28, 2025, by researchers at the University of California San Diego School of Medicine found a striking correlation: colon cancer patients with a 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 finding, while alarming, raises important questions about cannabis’s impact on cancer outcomes and underscores the need for more research. Let’s dive into the details of the study, explore its implications, and consider the broader context of cannabis and cancer.

The Study: What the Researchers Found

The UC San Diego team analyzed electronic health records from over 1,000 colon cancer patients treated across the University of California Health system between 2012 and 2024. They focused on patients’ documented cannabis use prior to their cancer diagnosis, specifically those with a history of cannabis use disorder (CUD)—a clinical diagnosis indicating problematic, heavy use. After controlling for factors like age, sex, tumor staging, and cancer biomarkers, the results were stark: patients with CUD had a 56% chance of dying within five years, compared to just 5% for those without CUD. This translates to a 24-fold increased risk of mortality for heavy cannabis users.

Lead author Raphael Cuomo, an associate professor at UC San Diego’s Department of Anesthesiology, pointed out that heavy cannabis use may affect the immune system, mental health, and treatment behaviors—all of which could influence cancer outcomes. For instance, high cannabis use is often linked to depression and anxiety, which might hinder a patient’s ability to fully engage with their cancer treatment. The study suggests that these factors, rather than a direct biological effect of cannabis on cancer cells, may be driving the increased mortality risk.

The Context: Cannabis and Cancer Research

This study adds a new layer to the complex relationship between cannabis and cancer. On one hand, some research has suggested potential benefits. A 2019 study from Penn State College of Medicine found that certain synthetic cannabinoid compounds inhibited the growth of colon cancer cells in lab tests. Another 2020 study in mice showed that tetrahydrocannabinol (THC), the psychoactive compound in cannabis, could reduce inflammation and prevent colon cancer linked to ulcerative colitis by binding to cannabinoid receptor type 2 (CB2). These findings have fueled hope that cannabis might one day play a role in cancer treatment or prevention.

However, the UC San Diego study highlights a different side of the story. While lab studies show promise, real-world use of cannabis is far more complicated. The researchers noted that the high concentrations of DMS and DMDS detected on K2-18b, mentioned in a previous post, are thousands of times higher than on Earth, where they are produced by marine life. This comparison, while unrelated to cannabis, illustrates how context matters when interpreting chemical or biological effects. Similarly, the effects of cannabis on colon cancer patients may depend on usage patterns, individual health factors, and the presence of other conditions like mental health challenges.

A Critical Look: What’s Missing?

While the UC San Diego study is compelling, it’s not without limitations. First, it relies on retrospective data, meaning it can’t prove that cannabis use directly caused the higher mortality rates—it only shows an association. Patients with CUD might have other risk factors, like poor overall health or socioeconomic barriers, that weren’t fully accounted for. Second, the study doesn’t explore the mechanisms behind the increased mortality. Does cannabis weaken the immune system’s ability to fight cancer? Does it interfere with chemotherapy? Or is the issue more behavioral, with heavy users less likely to adhere to treatment plans? These questions remain unanswered.

Moreover, the study’s focus on heavy cannabis use—specifically CUD—means its findings may not apply to moderate or occasional users. A 2019 systematic review found no clear association between marijuana use and the development of colorectal cancer, though it noted increased risks for other cancers like testicular cancer among heavy users. This suggests that the risks may be dose-dependent, a nuance the UC San Diego study doesn’t fully address.

It’s also worth questioning the narrative around cannabis. The study has been widely shared on platforms like X, with posts framing it as a warning against marijuana use. Yet, this risks oversimplifying the issue. Cannabis isn’t a monolith—its effects vary depending on the strain, method of use, and individual factors. Vilifying cannabis outright, as some reactions suggest, ignores its potential benefits for symptom management, as noted by colorectal cancer survivors in a 2021 Fred Hutchinson Cancer Center study. Up to 40% of cancer patients report using cannabis to relieve symptoms like nausea and pain, often with positive outcomes.

Implications and the Path Forward

The UC San Diego study doesn’t claim cannabis causes colon cancer or directly worsens it. Instead, it highlights a concerning correlation that demands further investigation. With cannabis use on the rise—4.5 million young adults in the U.S. reported daily or near-daily use in 2023, according to the National Survey on Drug Use and Health—the stakes are high. This is especially true for medically vulnerable populations like cancer patients, who may turn to cannabis for relief but could inadvertently face risks if their use becomes heavy.

For patients, this study underscores the importance of open conversations with healthcare providers. If you’re a colon cancer patient considering cannabis, discuss your usage patterns with your oncology team. For researchers, the study is a call to action. We need prospective studies to track cannabis use before and after a cancer diagnosis, as well as research into the biological pathways—like immune system effects—that might explain these outcomes.

Conclusion: A Balanced Perspective

The UC San Diego study is a sobering reminder that cannabis, despite its growing acceptance, isn’t risk-free—especially for those with serious illnesses like colon cancer. Heavy use may be linked to worse outcomes, but the why and how remain unclear. At the same time, dismissing cannabis entirely ignores its potential benefits for symptom relief and even cancer prevention, as other studies suggest. The truth likely lies in the middle: cannabis can be both a help and a hindrance, depending on how it’s used and by whom. As we await more research, the best approach is caution, curiosity, and honest dialogue—both in the doctor’s office and in the public square.

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Sources

• University of California San Diego School of Medicine, April 28, 2025, published in Annals of Epidemiology.
• Penn State College of Medicine, 2019, published in Cannabis and Cannabinoid Research.
• Medical News Today, October 3, 2020, study on THC and colon cancer in mice.
• National Survey on Drug Use and Health, 2023 data on cannabis use among young adults.
• Fred Hutchinson Cancer Center, 2021, study on cannabis use among colorectal cancer survivors, published in Cancer Causes & Control.
• Systematic review on marijuana use and cancer risk, 2019, published in Cancer Epidemiology, Biomarkers & Prevention.