Inconclusive: The evidence is too limited to determine the likelihood of an association with increased or decreased risk of endometrial cancer.

There is no conclusive evidence that taking metformin* is associated with a decreased risk of endometrial cancer. Only a small number of studies have been conducted and these have been of limited quality.

It has been suggested that metformin could lower the risk of endometrial cancer by lowering the level of insulin and other factors in the blood that may affect cancer cell growth. 

*Metformin is a drug used to treat type 2 diabetes.