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Breast Cancer Risk and Timing of Hormone Therapy

Breast cancer risk has generally been shown to be greater among users of estrogen–progestin formulations than among users of estrogen-only formulations of hormone therapy, but the reported magnitudes of their effects vary. Beral et al. (p. 296 ) investigated how the timing of hormone therapy use was associated with breast cancer risk. They used data from the Million Women Study, a large prospective study on hormone therapy use and other factors relevant for breast cancer risk. They found substantial heterogeneity in breast cancer risk among current users of hormonal therapy. Risks were greater among users of estrogen-progestin than among users of estrogen-only formulations and greater among those who started hormonal therapy at around the time of menopause than among those who started it later. The authors conclude that the time between menopause and starting hormonal therapy has a substantial effect on breast cancer risk.