Duration of estrogen replacement therapy in relation to the risk of incident myocardial infarction in postmenopausal women

Background: There is little information about whether an increasing duration of estrogen replacement therapy is associated with a declining risk for myocardial infarction in postmenopausal women. Objective: To conduct a population-based, case-control study among enrollees of the Group Health Cooperative (GHC) of Puget Sound, Seattle, Wash. Subjects and Methods: Case subjects were all postmenopausal women who were enrolled in the GHC with an incident fatal or nonfatal myocardial infarction from July 1986 through December 1993. Control subjects were a stratified random sample of postmenopausal women who were enrolled in the GHC without myocardial infarction and matched to case subjects by age and calendar year. We reviewed the medical records of the 850 case subjects and 1974 control subjects and conducted telephone interviews with consenting survivors. Use of estrogen or estrogen and progestin was assessed using GHC's computerized pharmacy database. Results: Among women who were currently using estrogen, a longer duration of use was inversely associated with a risk for myocardial infarction after adjustment for age, year of identification, diabetes mellitus, angina, and smoking. For categories of increasing duration of estrogen use (never, >0-<1.8 years, 1.8-<4.2 years, 4.2-<8.2 years, and greater than or equal to 8.2 years), the odds ratios for myocardial infarction were 1.00 (reference), 0.91, 0.70, 0.65, and 0.55 (for trend among the current users, P=.05). Among women who had used estrogen in the past, there was no evidence of decreasing risk with increasing duration of estrogen use. Conclusion: In this study, a long duration of hormone replacement therapy among women currently using estrogen was associated with a reduced risk for first myocardial infarction.