Third generation oral contraceptives and heritable thrombophilia as risk factors of non-fatal venous thromboembolism.

Third generation oral contraceptives (OCs) are apparently stronger risk factors for venous thromboembolism (VTE) than other OCs, however, the increased risk may be due to confounding by indication related to differences in prescription behaviour. We estimated the risk of VTE associated with use of OCs with and without the presence of Factor V Leiden mutation, protein C-, protein S- or antithrombin deficiency. Sixty-seven cases with VTE were compared with 134 controls. The risk of VTE in the presence of thrombophilia was of the same magnitude for third generation OC users as for users of other OCs; OR: 52.5 (95% CI: 3.7-738.1) and OR: 63.3 (95% CI: 6.2-648.4), respectively. It is unlikely that confounding by indication entirely explains the risk of VTE associated with third generation OCs since the combined effect exceeds what could be explained if this source of error was the only determinant of the association.