Objective: To determine the relative risk of developing a first acute myocardial infarction after treatment with oestrogens alone or oestrogen-progestogen combinations.
Design: Prospective cohort study utilizing a prescription-based and record linkage system for a follow-up period from 1977 to 1983. Average individual observation time was 5.8 years.
Setting: The entire female population of the Uppsala Health Care Region (1.4 million inhabitants), one-sixth of the total Swedish population.
Subjects: 23,174 women aged 35 years and older, identified from pharmacy records as having been prescribed non-contraceptive oestrogens during 1977-1980.
Outcomes: Admissions to hospitals for first acute myocardial infarctions.
Results: Overall, 227 cases of a first acute myocardial infarction were observed as against 281:1 expected, RR = 0.81 (95% confidence limits 0.71 to 0.92). Women who were younger than 60 years at entry into the study and prescribed oestradiol compounds (1-2 mg) or conjugated oestrogens (0.625-1.25 mg) showed a significant 30% reduction of the relative risk (RR = 0.69, 0.54 to 0.86). Those prescribed a combined oestradiol-levonorgestrel brand also demonstrated a significantly lowered relative risk (RR = 0.53, 0.30 to 0.87). The risk estimates were near unity during the first year of follow-up but decreased during subsequent years. Exposure to the weak oestrogen oestriol did not alter the risk.
Conclusion: Hormonal replacement therapy with oestrogens alone, and maybe also when cyclically combined with progestogens, can reduce the risk of acute myocardial infarction.