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

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.

Data were collected in Ontario from 404 incident cases and 403 age frequency-matched controls, between 1995 and 1996, using a self-administered questionnaire.

Multivariate analyses revealed an elevated odds ratio among long-term (> or = ten 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.

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.