Oral contraceptives and myocardial infarction: results of the MICA case-control study

Abstract

Objectives: To determine the association between myocardial infarction and use of different types of oral contraception in young women.

Design: Community based case-control study. Data from interviews and general practice records.

Setting: England, Scotland, and Wales.

Participants: Cases (n=448) were recruited from women aged between 16 and 44 who had suffered an incident myocardial infarction between 1 October 1993 and 16 October 1995. Controls (n=1728) were women without a diagnosis of myocardial infarction matched for age and general practice.

Main outcome measures: Odds ratios for myocardial infarction in current users of all combined oral contraceptives stratified by their progestagen content compared with non-users; current users of third generation versus second generation oral contraceptives.

Results: The adjusted odds ratio for myocardial infarction was 1.40 (95% confidence interval 0.78 to 2. 52) for all combined oral contraceptive users, 1.10 (0.52 to 2.30) for second generation users, and 1.96 (0.87 to 4.39) for third generation users. Subgroup analysis by progestagen content did not show any significant difference from 1, and there was no effect of duration of use. The adjusted odds ratio for third generation users versus second generation users was 1.78 (0.66 to 4.83). 87% of cases were not exposed to an oral contraceptive, and 88% had clinical cardiovascular risk factors or were smokers, or both. Smoking was strongly associated with myocardial infarction: adjusted odds ratio 12.5 (7.29 to 21.5) for smoking 20 or more cigarettes a day.

Conclusions: There was no significant association between the use of oral contraceptives and myocardial infarction. The modest and non-significant point estimates for this association have wide confidence intervals. There was no significant difference between second and third generation products.

PIP: The association between myocardial infarction and use of oral contraceptives (OCs) was investigated in a case-control study conducted in England, Scotland, and Wales. 448 women 16-44 years old who had suffered an incident myocardial infarction during 1991-95 were identified from general practice records and matched by age and general practice with 1728 healthy controls. 85% of cases were not taking OCs in the 3 months before their myocardial infarction. The adjusted odds ratio (OR) for myocardial infarction was 1.40 (95% confidence interval [CI], 0.78-2.52) for all combined OC users, 1.10 (95% CI, 0.52-2.30) for users of second-generation formulations, and 1.96 (95% CI, 0.87-4.39) for third-generation OCs. Subgroup analysis by progestogen content did not show any significant difference from 1, and there was no effect of duration of OC use. 88% of cases had clinical cardiovascular risk factors or were smokers, or both. The adjusted OR for third-generation vs. second-generation OC users was 1.78 (95% CI, 0.66-4.83). The adjusted OR was 12.5 (95% CI, 7.29-21.5) among women who smoked 20 or more cigarettes a day. These findings indicate that OCs, including third-generation formulations, do not increase a woman's risk of myocardial infarction. Women who are concerned about their cardiovascular health should be urged to stop smoking, however.

Figure

Case accrual and interview rates (*30 women died at time of myocardial infarction and five died subsequently)