Type 2 diabetes ("diabetes"), a disease characterized by insulin resistance and abnormally high levels of blood glucose, is associated with poorer breast cancer survival. One study reported that breast cancer patients with high fasting blood sugar levels at diagnosis were almost twice as likely to experience recurrence as those with normal levels.
Metformin has long been safely used to increase insulin sensitivity in people with type 2 diabetes and has also been shown to reduce the risks of cancer and cancer-related death.
Generally speaking, diabetics who use metformin have better outcomes than those who do not. On the other hand, there is some evidence that the use of insulin (especially long-acting types such as Lantus), or sulfonylurea derivatives such as Glipizide, could reduce breast cancer-specific survival, although these findings are controversial. Now a new study has reported that the risks of breast cancer recurrence and death among women enrolled in Medicare are lowest in those using metformin.

Latest research finds metformin reduces recurrence and death

The study referenced above was designed to investigate the influence of diabetes treatment on breast cancer relapse and survival. To conduct the study, the authors used data from the Surveillance, Epidemiology and End-Results (SEER)-Medicare database. The study included 14,766 women aged 66 to 80 newly diagnosed with stage I or II breast cancer during the period 2007 to 2011, of whom 627 experienced a recurrence and 237 died from breast cancer during the study period. Medicare Part D claims data was used to obtain information concerning diabetes-related medications. Metformin was used by 2,558 of the study population.
Treatment with metformin was found to be associated with 31% lower risk of recurrence and 49% lower risk of breast cancer-specific death. On the other hand, use of sulfonylureas was associated with 1.49 times higher risk of breast cancer-specific death and use of insulin was associated with 2.58-fold higher risk of death from breast cancer.
The authors comment that further research may be justified to determine whether metformin (1) is a preferred treatment for diabetes among breast cancer survivors; and (2) is beneficial for breast cancer patients without diabetes.
Please see our article on type 2 diabetes for more information.