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Metformin May Lower the Risk of Colorectal Cancer

Treatment found to reduce incidence of adenomas in patients with seemingly dose-dependent effect.

Metformin is among the most common drugs prescribed to type 2 diabetes patients. It has numerous positive effects for patients, including decreased blood sugar without risk of hypoglycemia and facilitation of weight gain. Recent research reinforces the notion that metformin may have other benefits, such as helping to prevent certain cancers.

The potential for metformin to lower the risk of colorectal cancer has previously been studied. Colorectal cancers mostly stem from pre-cancerous adenomas. This retrospective cohort study was designed to see if metformin lowers the incidence of these precursors to colorectal cancer and thus the risk of colorectal cancer itself.

The study included more than 2,000 patients. The patients were between the ages of 40 and 89 years old and all had type 2 diabetes mellitus. All study participants also had at least one colorectal adenoma identified at baseline colonoscopy, and at least one colorectal adenoma remaining through repeat colonoscopy up to 10 years from the baseline colonoscopy diagnosis of adenoma.

Patients were followed for a median of 4.5 years. Overall, 1,117 patients were found to have at least one adenoma during repeat colonoscopy after the baseline adenoma was found. Patients not receiving diabetes medications (n = 1,578) had a higher incidence of adenoma recurrence than metformin-only patients (n = 457). Furthermore, the association between lower adenoma incidence and metformin use was improved with higher doses of metformin. The association was ascertained via Cox models. The researchers found the beneficial association persisted when controlling for confounding factors like age, ethnicity, sex, body mass index, and repeat examination indication.

Previous studies have supported these findings and noted the decreased mortality and colorectal cancer risk that metformin confers to patients. They have also suggested that the benefit of metformin to reduce colorectal cancer applies both to patients who have never had colorectal cancer and those who previously suffered from it. More research is needed to identify the mechanism of action by which metformin inhibits adenoma formation and to identify the ideal dosing to maximize this benefit.

Practice Pearls:

  • Pre-cancerous adenomas are the most common sources of colorectal cancer.
  • This study of over 2,000 patients found that metformin use reduces the incidence of these adenomas in patients with a seemingly dose-dependent effect.
  • More research is needed to figure out how metformin inhibits colorectal adenomas.

Marks AR, Pietrofesa RA, Jensen CD, Zebrowski A, Corley DA, Doubeni CA. “Metformin use and risk of colorectal adenoma after polypectomy in patients with type 2 diabetes mellitus.” Cancer Epidemiol Biomarkers Prev. 2015. E-published ahead of print.