Cigarette smoking, use of oral contraceptives, and myocardial infarction.

The excess risk of nonfatal myocardial infarction among users of oral contraceptives observed in England and Wales can be explained in terms of the high proportion of smokers in the study population. Among nonsmokers, the relative risk associated with the use of oral contraceptives is estimated to be 2 to 1, which is not statistically significant (P = 0.28, Fisher's exact probability test). The present analysis suggests that smoking be considered as another contraindication for the prescription of oral contraceptives. The results further indicate that those women who smoke as well as use oral contraceptives and are interested in reducing the risk of nonfatal mycardial infarction should be encouraged to give up smoking. The relative risk of nonfatal myocardial infarction associated with the use of oral contraceptives observed in a society can vary between 0.9 to 1 and 11.7 to 1, depending upon the proportion of smokers. From the public health point of view, the reduction in the excess risk of nonfatal myocardial infarction achieved by eliminating smoking is estimated to be much more than can be achieved by eliminating the use of oral contraceptives.