Vandenbroucke and colleagues state that we present the "recency of market introduction" as a potential source of bias in evaluating the risk for venous thromboembolism associated with oral contraceptives containing the new progestogens. They misconstrued our argument as stating that the risk for thrombosis is "higher in the early periods" of use and then declines over time. We instead suggest that clinicians prescribe newly introduced drugs to a population different from the population receiving more established drugs. Thus, the relative risk for venous thromboembolism in users of new drugs may be elevated because the population to whom new drugs are prescribed has a higher baseline risk. The excess risk observed by Bloemenkamp and colleagues [1] in younger women (a population in which most new users will be found) further supports this argument.

With banner headlines carrying the government's warning, there can be little doubt that many women either switched formulations or discontinued use of oral contraceptives, leading to increased rates of unplanned pregnancies and abortion. The British Pregnancy Advisory Service, which provides nearly 20% of abortions in the United Kingdom, reported a 10% increase in the number of abortions performed after the government's warning. Of women surveyed, 40% reported discontinuing oral contraceptive use immediately after the warning [2]. A decrease in rates of thromboembolic disease after this sensational announcement has not been shown. A more moderate approach may have been better.

We agree that the evidence does suggest, although not confirm, an increased risk for thromboembolic disease associated with use of the newest progestogens. This apparent increase in risk cannot be dismissed as a result of bias, but neither has it been confirmed. The strength of the evidence and the appropriateness of response appear to be our area of disagreement.