Can metformin reduce the risk of colorectal cancer in diabetic patients?

This review looked at whether metformin (Glucophage) can reduce the risk of colorectal cancer for patients with diabetes. It found that patients who had been treated with metformin may have a lower risk of colorectal cancer.

Patients with diabetes are at increased risk of colorectal cancer. Colorectal cancer occurs in the large intestine, the final part of the digestive tract. It is not clear why people with diabetes have higher rates of colorectal cancer. It may be connected to higher levels of glucose (sugar) in the blood, or to more inflammation. Also, both conditions are connected to obesity.

Metformin is a treatment for diabetes which reduces the amount of glucose created by the liver. Metformin is often the first medication given to patients with type 2 diabetes. Some studies have found that metformin can reduce the risk of colorectal cancer for patients with diabetes, but other studies have found no effect. It is not clear whether metformin can reduce colorectal cancer for people with diabetes.

This analysis used the results of 17 studies with 1,092,074 patients with diabetes. Most of the patients had type 2 diabetes, though some had type 1 diabetes. 12 studies recorded whether patients were prescribed metformin, and tracked over time whether they developed colorectal cancer. One study randomly assigned some patients to receive metformin, and others to receive a placebo (inactive pill). The remaining four studies used patient records. 

Patients treated with metformin had a significantly lower risk of colorectal cancer (12% lower risk).

Seven studies conducted since 2016 accounted for more factors that can influence cancer, such as gender and diet. These studies also found that patients using metformin trended toward a somewhat lower risk of colorectal cancer (8% lower risk).

This study found that patients with diabetes treated with metformin may have a lower risk of colorectal cancer.

The patients in these studies used metformin for different amounts of time. Also, metformin may have different effects on patients with type 1 and type 2 diabetes.