Harvard Women's Health Watch

Timing of hormone therapy influences breast cancer risk

Many postmenopausal women and their clinicians are avoiding hormone therapy (HT) because of the health risks, but there's been some hope that it might be safer in younger women near or at menopause. In the case of breast cancer, at least, that isn't so, according to results from the Million Women Study in the United Kingdom. In this study, taking HT in early menopause was actually associated with a higher than average risk of breast cancer. Also important — for the many women wondering about the impact of their past HT use — results showed that soon after quitting HT, a woman's breast cancer risk was no greater than that of a woman who never had HT. Finally, the study found little or no increased breast cancer risk among women taking estrogen-only HT who waited at least five years after menopause to start taking hormones. Results were published online Jan. 28, 2011 in the Journal of the National Cancer Institute.

Other research has consistently found higher rates of breast cancer in women taking HT — in particular, combined estrogen and progestin. But few studies have investigated the timing of HT and how that influences breast cancer risk. The Million Women Study is the first to look specifically at the relationship between breast cancer incidence and when women started HT.

The study. Researchers at Oxford University analyzed data from 1.13 million women who were ages 50 to 64 when they enrolled in the study between 1996 and 2001. Participants answered periodic questionnaires about menopausal status, hormone use, lifestyle, and other factors. Cancer incidence and deaths were tracked through medical records, hospital admissions, and national health registries.

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