Breast Cancer Incidence and Mortality in Relation to Hormone Replacement Therapy Use Among Postmenopausal Women: Results From a Prospective Cohort Study.

Hormone replacement therapy (HRT) is associated with increased risk of breast cancer (BC), but little evidence assesses the effects of potential effect-modifiers on HRT-related BC. We sought to examine the relationship of different HRT types/method use and risk of BC in US postmenopausal women. In total, 689 BC cases and 81 BC deaths were identified during 372,210 person-years of follow-up. Cox regression and competing risk regression were used to estimate multivariable-adjusted hazards ratios (HRs) and 95% confidence intervals (95% CIs) by HRT status (never, former, current) for risk of BC incidence and mortality. The total current HRT use was associated with an increased risk of BC (HR current vs. never, 1.67; 95% CI, 1.33, 2.11), but did not associate with risk of death from BC (HR current vs. never, 0.85; 95% CI, 0.40, 1.78). Furthermore, underweight women (BMI 30 kg/m2, HR current vs never, 1.19; 95% CI, 0.73, 1.97). This study suggests that HRT use was associated with an altered risk of the occurrence of BC in the US postmenopausal women, especially for underweight women.