THE AUGUST 24, 2006, DECISION BY THE US FOOD AND Drug Administration (FDA) to approve over-thecounter (OTC) sales of the emergency contraceptive Plan B—1.5 mg of levonorgestrel taken after unprotectedsexual intercourse—wasamajordevelopment ina long and contentious regulatory process. Since the drug is more effective the sooner it is used, the delay in obtaining it imposed by its prior prescription-only status limited its effectiveness. In the interest of making Plan B available more quickly when needed, the distributor (Women’s Capital Corp, Washington, DC) applied to the FDA in April 2003 for approval of OTC status. In December 2003, a joint FDA advisory committee voted 23-4 in favor of approval. It did so on the basis of evidence that the drug is both effective and safe and that it met all FDA criteria for OTC availability. The committee also took into account the likelihood that improved access could prevent at least some of the 3.1 million unintended pregnancies (including the 25 000 that result from rape)—and, hence, some of the more than 1 million elective abortions—that occur in the United States every year. Although the FDA almost always accepts its advisory committees’ recommendations, in the case of Plan B it twice refused to grant approval of OTC status; the current approval decision came nearly 3 years from the time of the committee vote. As 2 of the committee members who voted for approval, we like many others were surprised and troubled by the FDA’s extended inaction. The agency justified its position primarily on the grounds that the number of women aged 14 to 16 years in the manufacturer’s actual use study (5% of the 585 women actually enrolled in the study) was too small to allow meaningful inferences about the consequences of its use in younger women. Although that narrow statistical point may have some merit, it cannot explain the tortuous and highly irregular process of decision making, uncovered in a subsequent Government Accountability Office inquiry, that led the FDA to withhold initial approval. It is therefore difficult to avoid the conclusion that the Plan B regulatory process was “a story of the entanglement of politics, science and religious beliefs. At the heart of it is the question of whether emergency contraception is or could be a form of abortion.” Public assertions such as the recent claim that “the proven ‘anti-implantation’ action of the morning-after pill is really nothing other than a chemically induced abortion” certainly seem to support that interpretation. The authors of such claims have offered no supporting evidence regarding Plan B’s mechanism of action. On the contrary, what has been offered are assertions that “the exact mechanism of action of Plan B is unknown” and that “the dirty little secret is, nobody really knows.” Such sweepingstatements of uncertainty not only directly contradict the claim that Plan B’s anti-implantation action is “proven” but also promote general misunderstanding and misuse of doubt in science. Doubt is the very core of the scientific enterprise, an essential part of the process by which scientific hypotheses are tested; on the other hand, doubt is sometimes exploited as a powerful way of subverting sound science-based public policy. Driven by the belief by some that interference with implantation is a form of abortion, the politics of doubt about Plan B’s contraceptive mechanism appears to have contributed not only to the delay in its OTC availability but also to the continuing refusal by some emergency services to provide the drug to rape survivors as well as refusal by some pharmacists to make it available to individual patients. In fact, a great deal is known about how Plan B prevents pregnancy. Now that the drug will be more widely available, it is more important than ever that the scientific evidence on its contraceptive mechanisms be as widely known as possible, so that decisions about its use can be fully informed. A summary of that evidence follows.