Objective: We combined data from published reports of the relation between estrogen use and breast cancer. We specifically addressed the hypothesis that the addition of progestins to estrogen therapy reduces the risk of breast cancer.
Study design: A meta-analysis was performed.
Results: Replacement hormone therapy is not associated with increased risk of breast cancer in women who have ever used it (relative risk 1.02, 95% confidence interval 0.93 to 1.12). However, current use is associated with increased risk (relative risk 1.40, 95% confidence interval 1.20 to 1.63). Although there was no significant trend with increasing duration of use, women with > or = 10 years of estrogen use had a relative risk of 1.23 (95% confidence interval 1.08 to 1.40). However, data on use of estrogen plus progestins combined from four studies indicate that risk is not reduced and that the overall relative risk is 1.13 (95% confidence interval 0.78 to 1.64). Further, risk did not vary in strata of family history or benign breast disease.
Conclusion: Although these results exclude a large effect of hormone therapy on risk of breast cancer, we are unable to rule out some risk associated with current or long-term estrogen use.