Oral contraceptives, smoking and risk of myocardial infarction in young women. A longitudinal population study in eastern Finland.

The use of oral contraceptives (OC) and smoking habits were studied by using a questionnaire in a random population sample of women from two counties of Eastern Finland in 1972. The participation rate was 95%. Data on use of OC and smoking were collected for 2,653 women aged 35-49 years. During the next seven years, 27 of them had developed an acute myocardial infarction (AMI). Smokers had a 2.6-fold (95% confidence interval (CI)=1.2-6.0) crude risk of developing AMI compared with non-smokers. Women taking OC had a 1.3-fold (95% CI = 0.4-6.9) risk of developing AMI compared with those who did not. Women who both smoked and used OC had a 7.2-fold (95% CI = 2.1-24.7) risk of AMI compared with non-smokers and non-users of OC. The results indicate that use of OC is associated with an excessive risk of AMI among female smokers.