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American Journal of Epidemiology Advance Access published online on November 28, 2006
American Journal of Epidemiology, doi:10.1093/aje/kwk038
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American Journal of Epidemiology Copyright © 2006 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.
Received May 10, 2006
Accepted July 26, 2006

ORIGINAL CONTRIBUTIONS

Use of Oral Conjugated Estrogen Alone and Risk of Breast Cancer

Shumin M. Zhang 1 *, JoAnn E. Manson 2, Kathryn M. Rexrode 3, Nancy R. Cook 1, Julie E. Buring 4, and I-Min Lee 1

1 Division of Preventive Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA; Department of Epidemiology, Harvard School of Public Health, Boston, MA
2 Division of Preventive Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA; Department of Epidemiology, Harvard School of Public Health, Boston, MA; Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA
3 Division of Preventive Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA
4 Division of Preventive Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA; Department of Epidemiology, Harvard School of Public Health, Boston, MA; Department of Ambulatory Care and Prevention, Harvard Medical School, Boston, MA

* To whom correspondence should be addressed.
Shumin M. Zhang, E-mail: shumin.zhang{at}channing.harvard.edu


  Abstract

The authors conducted a prospective cohort analysis in the Women's Health Study (1992-2004), a completed randomized trial assessing aspirin and vitamin E in the primary prevention of cancer and cardiovascular disease, to evaluate use of oral conjugated estrogen alone (0.625 mg/day) and breast cancer risk in a time-varying fashion. Over an average of 10 years of follow-up, 305 incident cases of breast cancer (258 invasive and 47 in situ cancers) were documented among 12,718 women aged 45 years or more who were either consistent current users of oral conjugated estrogen alone (0.625 mg/day) or never users of any type of postmenopausal hormones at baseline and during follow-up. The multivariable hazard ratios comparing "consistent current users" with "never users" were 1.11 (95% confidence interval: 0.79, 1.56) for total breast cancer and 1.13 (95% confidence interval: 0.77, 1.64) for invasive cases. No significant associations were observed for use of less than 8 and 8 years or more. Restricting the analyses to women with prior hysterectomy somewhat strengthened the associations, albeit still not significantly. These data, like those from the Women's Health Initiative, show no significant increase in breast cancer risk with use of oral conjugated estrogen alone (0.625 mg/day), but a small increase or decrease in risk cannot be excluded.

Keywords: breast neoplasms; estrogen replacement therapy.
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