Does hormone replacement therapy cause breast cancer? An application of causal principles to three studies. Part 1. The collaborative reanalysis

Background: Concern that hormone replacement therapy (HRT) may cause breast cancer has existed since the time it was introduced, and based on evidence in three studies, the Collaborative Reanalysis (CR), the Women's Health Initiative (WHI) and the Million Women Study (MWS), it is claimed that causality is now established. Objective To evaluate the evidence for causality in the three studies. Methods: Using generally accepted causal criteria, in this paper the authors begin with an evaluation of the CR. Analogous evaluations of the WHI and MWS will follow. Results: The findings in the CR did not adequately satisfy the criteria of time order, bias, confounding, statistical stability and strength of association, dose/duration-response, internal consistency, external consistency or biological plausibility. Conclusion: HRT may or may not increase the risk of breast cancer, but the CR did not establish that it does.