Thrombosis with low-estrogen oral contraceptives.

A retrospective study of the relationship of oral contraception to thrombosis was made in 104 idiopathic cases, 357 other thrombotic cases (exclusive of cerebrovascular thrombosis), and 1302 matched controls. The relative risk for idiopathic case users was 7.2 times that for non-users; for the whole series it was 1.9. The apparent low risk for non-idiopathic cases is considered to be due to selective prescription of this method of contraception. The risk for women using formulations containing 100 mug or more of estrogen was higher than that for users of lower doses. Among the controls, the risk of gallbladder surgery was twice as high for users as non-users. Several theoretical sources of bias were searched for and shown not to be present. Histories of oral contraceptive use were found to be accurate. A definite risk of thrombosis exists even with the low-estrogen dosage currently employed in oral contraceptive steroids.