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Data concerning the effect of estrogen-only therapy on the risk for breast cancer are conflicting. To examine the effect of different doses, constituents, and routes of administration of estrogen-only therapy on the risk for breast cancer, researchers followed all postmenopausal women over age 50 in Finland who had purchased oral or transdermal estradiol (84,729 women), oral estriol (7941 women), or vaginal estrogens (18,314 women) for at least 6 months from 1994 to 2001.
Subjects were identified from the Finnish medical reimbursement register; data on breast cancer cases came from the Finnish Cancer Registry. Through 2002, 2171 women developed breast cancer. Women who took estradiol for less than 5 years were no more likely to develop breas…