title: Does hormone replacement therapy cause breast cancer? An application of causal principles to three studies. Part 4: the Million Women Study.
abstract: BACKGROUND
  Based principally on findings in three studies, the collaborative reanalysis (CR), the Women's Health Initiative (WHI) and the Million Women Study (MWS), it is claimed that hormone replacement therapy (HRT) with estrogen plus progestogen (E+P) is now an established cause of breast cancer; the CR and MWS investigators claim that unopposed estrogen therapy (ET) also increases the risk, but to a lesser degree than does E+P. The authors have previously reviewed the findings in the CR and WHI (Parts 1-3).
  
  
  OBJECTIVE
  To evaluate the evidence for causality in the MWS.
  
  
  METHODS
  Using generally accepted causal criteria, in this article (Part 4) the authors evaluate the findings in the MWS for E+P and for ET.
  
  
  RESULTS
  Despite the massive size of the MWS the findings for E+P and for ET did not adequately satisfy the criteria of time order, information bias, detection bias, confounding, statistical stability and strength of association, duration-response, internal consistency, external consistency or biological plausibility. Had detection bias resulted in the identification in women aged 50-55 years of 0.3 additional cases of breast cancer in ET users per 1000 per year, or 1.2 in E+P users, it would have nullified the apparent risks reported.
  
  
  CONCLUSIONS
  HRT may or may not increase the risk of breast cancer, but the MWS did not establish that it does.
authors.affiliation: Department of Epidemiology, University of Cape Town, Cape Town, South Africa. samshap@mweb.co.za
authors.forename: Samuel
authors.initials: S
authors.lastname: Shapiro
authors.forename: Richard D T
authors.initials: RD
authors.lastname: Farmer
authors.forename: John C
authors.initials: JC
authors.lastname: Stevenson
authors.forename: Henry G
authors.initials: HG
authors.lastname: Burger
authors.forename: Alfred O
authors.initials: AO
authors.lastname: Mueck
mesh_terms: D015982:Bias; D001943:Breast Neoplasms / Q000139:chemically induced* / Q000453:epidemiology; D015986:Confounding Factors, Epidemiologic; D004359:Drug Therapy, Combination; D015914:Estrogen Replacement Therapy / Q000009:adverse effects*; D004967:Estrogens / Q000008:administration & dosage / Q000009:adverse effects*; D005260:Female; D006801:Humans; D008875:Middle Aged; D045167:Progesterone Congeners / Q000008:administration & dosage / Q000009:adverse effects*; D015203:Reproducibility of Results; D013997:Time Factors; D016387:Women's Health
chemical_list: D004967:Estrogens; D045167:Progesterone Congeners