Hormone replacement therapy (HRT) for post-menopausal women provides an excellent example of how observational research can mislead. In the 1980s and early 1990s, observational studies suggested that post-menopausal women benefited from HRT and that there were few risks associated with this intervention.
See Henderson, B.E.,
Paganini-Hill, A., and
Ross, R.K.,
“Decreased Morality in Users of Estrogen Replacement Therapy,” Archives of Internal Medicine 151, no.
1 (
1991):
75–
78;
Stampfer, M.J. and
Colditz, G.A., “Estrogen Replacement Therapy and Coronary Heart Disease: A Quantitative Assessment of the Epidemiologic Evidence,” Preventive Medicine 20, no. 1 (
1991):
47–
63;
Grady, D.,
Rubin, S.M.,
Petitti, D.B.,
et al., “Hormone Therapy to Prevent Disease and Prolong Life in Postmenopausal Women,”
Annals of Internal Medicine 117, no. 12 (
1992):
1016–
1037. There was a biologically plausible explanation for why women should benefit from HRT. However, in the 1990s a prospective, randomized, double-blinded placebo-controlled, clinical trial involving over 16,000 women showed that the benefits of HRT were smaller and fewer than people believed, and the risks were greater. The RCT was stopped early because the treatment group's incidence of invasive breast cancer exceeded the rate set by the data and safety monitoring board's prearranged stopping rule, and the statistic summarizing risks as compared to benefits indicated that the risks of HRT exceeded its benefits in the study population.
Hulley, S.,
Grady, D.,
Bush, T.,
et al., for
the Heart and Estrogen/Progestin Replacement Study (HERS) Research Group, “Randomized Trial of Estrogen Plus Progestin for Secondary Prevention of Coronary Heart Disease in Postmenopausal Women,” JAMA 280, no. 7 (
1998):
605–
613;
Writing Group for the Women's Health Initiative Investigators, “Risks and Benefits of Estrogen Plus Progestin in Healthy Postmenopausal Women: Principal Results from the Women's Health Initiative Randomized Controlled Trial,”
JAMA 288, no. 3 (
2002):
321–
333;
Writing Group for the Women's Health Initiative Investigators, “Risks and Benefits of Estrogen Plus Progestin in Healthy Post-menopausal Women: Principal Results from the Women's Health Initiative Randomized Controlled Trial,”
JAMA 288, no. 3 (
2002):
321–
333.
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