Bias in studies of venous thromboembolism in relation to the use of new formulations of oral contraceptives.

The "attrition of susceptibles" was a plausible noncausal hypothesis (that sought to account for the observed association between current use of OCs containing desogestrel or gestodene and the incidence of VTE). However, there are now several pieces of evidence suggesting that this hypothesis can explain, at most, only a small part of the association. For the time being, when weighing the advantages and disadvantages of use of various types of OCs, it is probably prudent to assume that relative to the risk of VTE in users of levonorgestrel, there truly is a heightened risk in women who currently take desogestrel- or gestodene-containing OCs.