[Handling small relative risks in science and management: the third-generation pill].

Small relative risks (a twofold or lesser increase in disease frequency) become scientifically acceptable with (a) repeated consistent findings from studies that (b) address the most important forms of bias and confounding, and (c) when there is a plausible biologic mechanism. The third-generation oral contraceptive controversy is an example of such a relevant but small relative risk and demonstrates the problem of interpretation and implementation into medical practice guidelines.