A new prospective study has reported that both estrogen-only and combined hormone replacement therapy increase the risk of breast cancer. The study was designed to investigate the association between the use of different types of menopausal hormone therapy and breast cancer risk in the European women. Menopausal hormone therapy, of which the combined hormone replacement therapy (HRT) typically prescribed in the U.S. is one type, is characterized by the use of different compounds, regimens and routes of administration. The study included 133,744 postmenopausal women among whom new breast cancers were identified using population cancer registries or by active follow-up. The average follow-up period was 8.6 years. A total of 4,312 new breast cancer cases were found during 1,153,747 person-years of follow-up.

Risk of breast cancer was found to be 42 percent higher among current users of estrogen-only therapy compared to who had never used menopausal hormone therapy. The risk was even higher among current users of combined menopausal hormone therapy. Continuous combined regimens conferred a 43% higher risk than sequential regimens. No significant difference was found between progesterone- and testosterone-derivatives in sequential regimens. There was no significant variation in risk of based on whether the estrogenic component was administered orally or through the skin, or whether the estrogen was a form of estradiol or conjugated equine estrogen. The authors comment that additional studies are needed to disentangle the effects of the regimen and the progestin component.