Fatal breast cancer risk in relation to use of unopposed estrogen and combined hormone therapy

Use of combined hormone therapy (CHT) is associated with increased breast cancer incidence, but it is unclear whether this translates into increased breast cancer mortality. To address this question, we conducted a population-based nested case–control study in Saskatchewan, Canada, where a population-based prescription drug database has existed since 1975. We evaluated fatal breast cancer risk in relation to recency and duration of use of CHT and unopposed estrogen hormone therapy (EHT). A total of 1,288 cases and 12,535 controls were included in the analyses. Exclusive use of EHT was not associated with fatal breast cancer risk, either overall or within categories of recency or duration [odds ratio (OR) for current vs. never use = 1.1; 95 % confidence interval (CI) 0.8–1.3]. Use of CHT (includes women who had also used EHT) was also not associated with fatal breast cancer risk (OR for current vs. never use = 0.9; 95 % CI 0.7–1.3), except for a suggestion of an increased risk with current long-term use. Consistent with prior studies, we observed no increased risk of fatal breast cancer associated with use of EHT. To date only a few studies have evaluated fatal breast cancer risk in relation to use of CHT, and collectively the results are inconsistent.