The decision by FDA administrators to keep Plan B available by prescription-only went against the recommendation of the agency's own panels of expert advisers in December. That led many abortion- rights advocates to complain that the decision was a case of "politics trumping science."

Plan B is packaged as two pills, with one taken as soon as possible within 72 hours and the other taken 12 hours later. The pills are made of the same hormones in regular birth-control pills, but at higher doses.

Abortion opponents object to the pills for different reasons. Some groups say the pills could stop a fertilized egg from implanting in the uterus. Since they believe pregnancy starts with fertilization, they equate the interference with an abortion. Other groups say pregnancy starts when an egg is implanted in the uterus.