Combined conjugated esterified estrogen plus methyltestosterone supplementation and risk of breast cancer in postmenopausal women

Objectives Testosterone supplementation is being prescribed increasingly to treat symptoms of hormone deficiency in pre- and postmenopausal women; however, studies of the association of testosterone therapy, alone or in combination with estrogen, with risk of breast cancer are limited. The current study assessed the association of combination conjugated esterified estrogen and methyltestosterone (CEE + MT) use and breast cancer risk in postmenopausal women in the Women's Health Initiative (WHI). Study design At Year 3 of follow-up, women in the WHI observational study (N = 71,964) provided information on CEE + MT use in the past two years, duration of use, and the brand name of the product. In addition, in each of years 4-8, women were asked whether they had used CEE + MT in the previous year. After 10 years of follow-up, 2832 incident breast cancer cases were identified. Cox proportional hazards models were used to estimate hazard ratios (HR) and 95% confidence intervals (95% CI) for the association of CEE + MT use (irrespective of use of other hormones) and of exclusive CEE + MT use in relation to breast cancer risk. Results Neither CEE + MT use nor exclusive use of CEE + MT was associated with risk: multivariable-adjusted HR 1.06, 95% CI 0.82-1.36 and HR 1.22, 95% CI 0.78-1.92, respectively. Among women with a natural menopause, the HR for exclusive use was 1.32 (95% CI 0.68-2.55). There was no indication of an association when repeated measures of CEE + MT use were included in a time-dependent covariates analysis. Conclusion The present study, the largest prospective study to date, did not show a significant association of CEE + MT supplementation and risk of breast cancer.