Is breast cancer linked to estrogen use?

A new study of 28,000 women conducted by Harvard researchers demonstrated that unopposed estrogen therapy for 10 years or more was linked to a higher risk of developing breast cancer in postmenopausal women. Estrogen therapy is used to prevent or treat osteoporosis.

According to the study, the multivariate relative risks (RRs) and 95% confidence intervals (CIs) for breast cancer with current use of unopposed estrogen were as follows (P for trend .001):

• Less than five years, RR 0.96 (95% CI, 0.75-1.22);
• Five to 9.9 years, RR 0.90 (95% CI, 0.73-1.12);
• 10 to 14.9 years, RR 1.06 (95% CI, 0.87-1.30);
• 15 to 19.9 years RR 1.18 (95% CI, 0.95-1.48);
• 20 years or longer RR 1.42 (95% CI, 1.13-1.77).

Based on these results, the risks of developing breast cancer is slightly higher at 10-14.9 years, and higher in women taking estrogen for more than 15 years.

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Wendy Y. Chen, et al, "Unopposed Estrogen Therapy and the Risk of Invasive Breast Cancer," Archives of Internal Medicine 2006;166:1027-1032.