title: Metformin and breast and gynecological cancer risk among women with diabetes.
abstract: OBJECTIVE
  We investigated if metformin lowers breast, endometrial, and ovarian cancer risk in women with type 2 diabetes mellitus compared with women who used other antidiabetic medications.
  
  
  METHODS
  We followed a cohort of 66 778 female patients with diabetes for a maximum of 12 years (median 6 years). We examined breast, endometrial, and ovarian cancer risk, and the composite cancer risk. We examined drug categories using pharmacy records: metformin only; metformin combination regimens; non-metformin regimens; and non-users. We used χ(2) analyses to examine categorical variables. We conducted multivariable Cox regression models with time-dependent drug use status.
  
  
  RESULTS
  Women who used metformin combination regimens versus metformin only had a 15% lower breast cancer risk (adjusted HR=0.85, 95% CI 0.69 to 1.04). After stratifying by glycated hemoglobin (HbA1c), the association attenuated in those who had poorly controlled HbA1c (adjusted HR=1.06, 95% CI 0.73 to 1.55). Given the small numbers of ovarian and endometrial cancer outcomes, we examined these as a composite. The risk of all cancers combined was similar in those who used metformin combination regimens versus metformin only (adjusted HR=0.92, 95% CI 0.78 to 1.10). We found no significant differences for breast cancer or all cancers combined when we compared risks in non-metformin users versus metformin only users.
  
  
  CONCLUSIONS
  Women who used metformin and other antidiabetic drugs had a lower breast cancer risk compared with women who used metformin only, but the results were not significant. We also found no difference in overall cancer risks when we compared women who used other antidiabetic drugs (no metformin) versus metformin users.
authors: Southern California Permanente Medical Group, Department of Research & Evaluation, Pasadena, California , USA.
authors: Diana
authors: D
authors: Soffer
authors: Southern California Permanente Medical Group, Department of Research & Evaluation, Pasadena, California , USA.
authors: Jiaxiao
authors: J
authors: Shi
authors: Southern California Permanente Medical Group, Department of Research & Evaluation, Pasadena, California , USA.
authors: Joanie
authors: J
authors: Chung
authors: Southern California Permanente Medical Group, Department of Research & Evaluation, Pasadena, California , USA.
authors: Joanne E
authors: JE
authors: Schottinger
authors: Southern California Permanente Medical Group, Department of Research & Evaluation, Pasadena, California , USA.
authors: Lauren P
authors: LP
authors: Wallner
authors: Division of Medical Oncology and Hematology , Harbor-UCLA Medical Center , Torrance, California , USA.
authors: Rowan T
authors: RT
authors: Chlebowski
authors: Department of Gynecologic Oncology , Kaiser Permanente Southern California , Los Angeles, California , USA.
authors: Scott E
authors: SE
authors: Lentz
authors: Southern California Permanente Medical Group, Department of Research & Evaluation, Pasadena, California , USA.
authors: Reina
authors: R
authors: Haque
keywords: Breast Cancer; Cancer; Cohort; Metformin