Risk of myocardial infarction, angina and stroke in users of oral contraceptives: an updated analysis of a cohort study.

OBJECTIVES

To investigate risk of myocardial infarction, angina and stroke in users of contraceptive pills compared with users of other methods of contraception.

DESIGN

Prospective cohort study, with recruitment between 1968 and 1974 and annual follow up until the age of 45 years. After this age, only women who had never used oral contraception or those who had used it for eight or more years continued to be followed up annually until July 1994.

SETTING

Seventeen family planning clinics in England and Scotland.

POPULATION

17,032 women aged between 25 and 39 years at entry to the study.

MAIN OUTCOME MEASURES

Occurrence of angina, myocardial infarction or stroke that was associated with either hospital admission or outpatient referral to hospital or death.

RESULTS

Increased risk of myocardial infarction in oral contraceptive users was observed only in women who were heavy smokers at entry to the study. In this subgroup the relative risk of a myocardial infarction was 4.2 (95% CI 1.4-16.6) in ever users of oral contraception compared with non-users, 4.9 (1.2-23.6) in current users, and 4.0 (1.3-16.2) in ex-users. In all current users the relative risk of angina was 0.5 (0.1-1.4), and the relative risk of ischaemic stroke was 2.9 (1.3-6.7). The increased risk of ischaemic stroke did not persist in ex-users.

CONCLUSIONS

Use of oral contraception is associated with increased risk of ischaemic stroke and increased risk of myocardial infarction (only in heavy smokers), but no increased risk of angina. These increased risks need to be considered within the context of the very low absolute risks of cardiovascular disease in this population. 5880 women need to take oral contraception for one year to cause one extra stroke, and 1060 women who are heavy smokers need to take it for one year to cause one extra myocardial infarction.