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We investigated whether the occurrence of venous thrombosis in young women who use oral contraceptives might be explained by the factor V Leiden mutation, which leads to resistance to activated protein C and enhances susceptibility to thrombosis.

We compared 155 consecutive premenopausal women, aged 15 to 49, who had developed deep venous thrombosis in the absence of other underlying diseases, with 169 population controls. The risk of thrombosis among users of oral contraceptives was increased 4-fold (relative risk 3·8 [95% Cl 2·4-6·0]). The risk of thrombosis among carriers of the mutation compared with non-carriers was increased 8-fold (7·9 [3·2-19·4]). Compared with women who did not use oral contraceptives and were not carriers of the mutation, the risk of thrombosis among those with both risk factors was increased more than 30-fold (34·7 [7·8-154]).

Recalculation of population incidences …