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Design Flaws Cast Doubt on Million Woman Study

Last Updated: January 19, 2012.

The largest study linking hormone replacement therapy to breast cancer, the Million Women Study, had flaws in its design and the findings do not satisfy the principles of causation, according to an evidence review published online Jan. 16 in the Journal of Family Planning & Reproductive Health Care.

THURSDAY, Jan. 19 (HealthDay News) -- The largest study linking hormone replacement therapy (HRT) to breast cancer, the Million Women Study (MWS), had flaws in its design and the findings do not satisfy the principles of causation, according to an evidence review published online Jan. 16 in the Journal of Family Planning & Reproductive Health Care.

Samuel Shapiro, Ph.D., of the University of Cape Town in South Africa, and colleagues assessed the evidence for causality for estrogen plus progestogen (E+P) and for unopposed estrogen therapy (ET) in the MWS, using generally accepted causal criteria.

The researchers found that the findings for E+P and for ET did not sufficiently satisfy the criteria for time order, information and detection bias, confounding, statistical stability and strength of correlation, duration-response, internal and external consistency, or biological plausibility, in spite of the massive size of the MWS. The apparent risks reported would have been nullified had detection bias resulted in the identification of 0.3 additional cases of breast cancer in ET users (women aged 50 to 55 years) per 1,000 per year, or 1.2 cases in E+P users.

"HRT may or may not increase the risk of breast cancer, but the MWS did not establish that it does," the authors conclude.

Several authors disclosed financial ties to the manufacturers of products mentioned in the review.

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