Estrogen and estrogen-progestin replacement therapy and risk of postmenopausal breast cancer in Canada

Objective: To examine the association between hormone replacement therapy (HRT) use and breast cancer incidence and to determine whether the association differs according to type of regimen. Method: Data were collected in Ontario from 404 incident cases and 403 age frequency-matched controls, between 1995 and 1996, using a self-administered questionnaire. Results: Multivariate analyses revealed an elevated odds ratio among long-term (greater than or equal toten years) HRT users (odds ratio (OR) = 1.80, 95% confidence interval (CI) 1.06-3.06). Risk among long-term estrogen-progestin users was substantially higher (OR = 3.48, 95% CI 1.00-12.11) than risk among long-term users of estrogen alone (OR = 1.74, 95% CI 0.93-3.24). Among both estrogen and estrogen-progestin users, positive associations were not observed for durations of use less than ten years. Conclusion: These data suggest that long-term use of HRT increases the risk of breast cancer and that estrogen-progestin therapy may be more detrimental than estrogen use alone.