Myocardial infarction in users of low-dose oral contraceptives

To determine the relationship between the use of low-dose (less than 50 micrograms estrogen) oral contraceptives (OC) and myocardial infarction.

In this population-based case-control study, all incident myocardial infarctions in women, ages 15-44 years who were members of the Kaiser Permanente Medical Care Program, Northern and Southern California regions were ascertained during a 39-month period from 1991 through 1994. For each woman with myocardial infarction, up to three age- and facility-matched controls were chosen at random from female members. Information about OC use (predominantly low-dose preparations) was obtained in face-to-face interviews.

There were 187 incident cases of myocardial infarction during 3.6 million woman-years of observation (incidence rate, 5.2 per 100,000 woman-years). The prevalence of several risk factors for myocardial infarction was lower in controls who were current users of OCs than in controls who were noncurrent (past and never) users. The odds ratio for myocardial infarction in current OC users compared with noncurrent users was 1.65 (95% confidence interval 0.45, 6.06) after adjustment for major risk factors and for race and ethnicity, corresponding to an excess risk of less than one case per 100,000 woman-years. The study had 80% power to detect a relative risk of 2.3 (one-sided test, alpha = .05). The odds ratio of myocardial infarction in past OC users was not elevated.

With respect to myocardial infarction, low-dose oral contraceptives can be used safely by women who lack risk factors for coronary heart disease.