Updated on April 28, 2016.
While postmenopausal women with type 2 diabetes may have a heightened risk for invasive cancers and cancer mortality, use of metformin — particularly long-term use — may lower the risk for dying from cancer, according to results from a large prospective study.
Researchers from Roswell Park Cancer Institute and the University of Buffalo analyzed information from 145 826 postmenopausal women aged 50 to 79 years from 40 US clinical centers who participated in the Women’s Health Initiative (WHI) between 1993 and 1998.
They found that women with diabetes were more likely to die from invasive cancer when compared with women without diabetes. In addition, among women with diabetes, the risk for cancer death differed significantly between metformin users and nonusers. The researchers also noted that risk for cancer death tended to decrease with longer metformin use.
“As expected, in this large cohort of postmenopausal women, we found that women with diabetes are at higher risk to develop cancer and are at greater risk of death from these diseases,” said study author Zhihong Gong, PhD, assistant professor of oncology in the department of cancer prevention and control at Roswell Park Cancer Institute in Buffalo, New York.
“However, our results did suggest that long-term metformin use was related to a lower risk of developing certain cancers. We also found that women with diabetes who were treated with metformin were at lower risk of dying from cancer compared to non-metformin users, suggesting that metformin may be associated with better survival in diabetes patients with cancer.”
Metformin and Cancer Risks in Diabetes
Overall, the researchers found that women with diabetes had a 13% higher risk for developing invasive cancer when compared with those without diabetes (hazard ratio [HR]=1.13; 95% CI, 1.07-1.19).1
In terms of cancer types, women with diabetes faced a 37% higher risk for cancers of the digestive organs and peritoneum (HR=1.37; 95% CI, 1.23-1.53), including a 24% increased risk for colon cancer (HR=1.24; 95% CI, 1.06-1.46) and a more than 2-fold increased risk for liver and pancreas cancers, as compared with women without diabetes. Risk for reproductive tract cancer was also 22% higher for women with diabetes vs those without diabetes (HR=1.22; 95% CI, 1.05-1.42), especially endometrial cancer (HR=1.36; 95% CI, 1.12-1.65). The researchers also observed a suggestion of a higher risk for malignant neoplasms of lymphatic and hematopoietic tissue, particularly non-Hodgkin lymphoma (HR=1.26; 95% CI, 1.02-1.55).
Breast cancer risk was elevated among women with diabetes only after BMI and waist-to-hip ratio were included as covariates (HR=1.11; 95% CI, 1.01-1.20).
Women with diabetes also had a 46% increased risk for death from invasive cancer overall (HR=1.46; 95% CI, 1.34-1.60) and a 44% increased risk for death from colorectal cancer (HR=1.44; 95% CI, 1.06-1.96).
In women with diabetes vs those without the disease, risk for total invasive cancer was not significantly different between metformin users (HR=1.13; 95% CI, 1.04-1.23) and users of other medications (HR=1.10; 95% CI, 1.00-1.21), according to the data. However, metformin use was associated with better survival. Women with diabetes who used metformin had a significantly lower risk for dying of cancer (HR=1.09; 95% CI, 0.91-1.28) than those using other medications (HR=1.45; 95% CI, 1.25-1.69), relative to women without diabetes. Data also showed that women taking metformin were less likely to die from breast cancer (HR=0.50; 95% CI, 0.22-1.13) than those taking other medications (HR=1.29; 95% CI, 0.71-2.33).