There are limits to predictability

Clinical review p 520

In short succession two recently published papers have shown not only a very high relative risk of cerebral sinus thrombosis in users of oral contraceptives but an even stronger effect of contraceptives among women who carry hereditary prothrombotic conditions. 1 2 This raises several questions: Why this sudden wave of publications? Why the very high relative risks? What are the lessons for clinical practice and prevention?

The field of investigation into oral contraceptive use and the occurrence of venous thrombosis has been undergoing a rapid paradigm shift over the past couple of years. That oral contraceptives may cause venous thrombosis has been known since the 1960s, but ever since there have been controversies about the size of the risk, the role of bias in observational research, the role of hormonal dosage, the type of hormones implicated, and—particularly—the glaring absence of any biochemical explanation. Some have even doubted the reality of the association. The discovery of first the factor V Leiden mutation and then the factor II mutation has changed the picture.

These prothrombotic mutations strongly enhance the risk of oral contraceptives causing venous thrombosis, which, firstly, proves …