Use of hormone-replacement therapy
Breast Cancer Risks: Page 3
The results of long-term studies show that women who took a combination of estrogen and progesterone to treat menopausal symptoms have both an increased risk of getting breast cancer and an increased risk of dying from breast cancer. However, the risk seems to apply only to women who are currently taking hormones or who stopped taking them within the past few years; if you stopped taking hormones five or more years ago, your risk level will have returned to that of the general population.
Estrogen-only therapy doesn't seem to increase breast cancer risk except in women who took the hormones for more than ten years. In this case, estrogen therapy increases the risk of both breast and ovarian cancer.