Low-dose oral contraceptive use and the risk of myocardial infarction

Background: Studies of oral contraceptives (OCs) containing 50 mug or more of estrogen suggest an increased risk of myocardial infarction (MI) among c urrent users, particularly if they smoke heavily. Objective: To assess whether use of the newer lower-dose OCs increases the risk of MI. Methods: A case-control study was conducted from January 1985 through March 1999 in 75 hospitals in the greater-Boston and greater-Philadelphia areas. Data on OC use and MI risk factors were obtained by interview from 627 wom en with a nonfatal first MI (cases) and 2947 female hospital controls young er than 45 years. Results: The overall odds ratio (OR) for current OC use relative to never u sed was 1.3 (95% confidence interval [CI], 0.8-2.2). The OR was elevated, 2 .5 (95% CI, 0.9-7.5), among heavy smokers (greater than or equal to 25 ciga rettes per day) but close to 1.0 among lighter smokers (OR=0.8) and nonsmok ers (OR=1.3). For current OC use together with heavy smoking relative to no nuse and nonsmoking, the OR was 32 (95 % CI, 12-81), considerably greater t han that for heavy smoking alone, 12 (95% CI, 8.6-16). The ORs did not vary according to the type of formulation or the dose of estrogen; there were t oo fen users to assess the new 20-mug preparations. Past OC use was unrelat ed to risk. Conclusion: Current use of low-dose OCs in the United States is unrelated t o an increased risk of MI among nonsmokers and light smokers, but users who smoke heavily may be at greatly increased risk.