Even when drugs are approved by the FDA, it may not be entirely clear how they work, just that they do. And sometimes, the FDA label describing how they work is actually wrong, as is the case with the emergency contraceptive pill Plan B.
The pill and others like it, which are to be taken as a last-ditch birth control effort after unprotected sex, deliver a one-time dose of a hormone that prevents pregnancy. Because the label suggests that the pill may prevent pregnancy by keeping a fertilized egg from implanting in the uterus, which falls under some definitions of abortion, Plan B and other emergency contraceptives have popped up in political debate, with Mitt Romney calling them "abortive pills" and other Republican politicians making similar statements. The New York Times investigated the claim on the label, however, and found that it had been placed there by the FDA despite the fact that there was no evidence that the drug did so. Citing confidentially, the FDA will not say why.
Plan B was approved by the FDA in 1999. In the last 13 years, research into Plan B's mechanism has advanced, and scientists are now able to say with some certainty that the drug works by delaying ovulation, keeping the egg and the sperm from being in the same place at the same time, so the egg is never fertilized. Women who take Plan B after they've ovulated still get pregnant. And in lab tests using cultured cells, the drug does not keep a fertilized egg from attaching to a layer of uterine cells.
Government agencies and medical sites that update their information to reflect the most recent research, though, are being attacked by abortion opponents. More scientific clarity, it seems, does not always resolve conflict.