(HealthDay News) — Postmenopausal hormone therapy with estrogen plus progestin is associated with an increased risk of breast cancer, according to a study published online March 29 in the Journal of the National Cancer Institute.

Noting that there have been mixed results on the association of estrogen plus progestin with breast cancer incidence in randomized clinical trials compared with observational studies, Rowan T. Chlebowski, M.D., Ph.D., from the Los Angeles Biomedical Research Institute in Torrance, Calif., and colleagues investigated breast cancer incidence in 41,449 postmenopausal women, where 25,328 did not use hormones and 16,121 used estrogen plus progestin.

After a mean of 11.3 years, the researchers identified 2,236 breast cancers. Breast cancer incidence was significantly higher in women taking estrogen plus progestin (annualized rate of 0.60 versus 0.42%; hazard ratio = 1.55).

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The highest risk was for women initiating hormone treatment closer to menopause. Both the hormone therapy and nonuser groups had similar survival after breast cancer, measured from diagnosis (HR, 1.03). Measured from cohort entry, the estrogen plus progestin group had more deaths from breast cancer (HR, 1.32; P = 0.15) and more all-cause deaths after breast cancer (HR, 1.65).

“In summary, estrogen plus progestin use is associated with increased breast cancer incidence, especially when its use is initiated close to menopause,” Chlebowski and colleagues conclude. “Because prognosis after a breast cancer diagnosis is similar for combined hormone therapy users and nonusers, increased breast cancer mortality on a population basis can be expected.”

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