The Food and Drug Administration decided Aug. 24 to make the “morning-after” contraceptive pill known as Plan B available without a prescription to people 18 and older, ending a three-year impasse that put the agency at the center of a polarizing debate over reproductive choice.

Girls 17 and younger will need a prescription to obtain the pills, which will be available only from pharmacists at drugstores and health clinics. Purchasers would be required to show proof of their age. The pill is expected to be available over-the-counter by November.

“What this means is now women 18 and over can go to the local Safeway for the pill,” said Dr. Tina Raine-Bennett of the University of California, San Francisco’s Center of Reproductive Health Research and Policy. “I think this is great news.”

Planned Parenthood President Cecile Richards said the FDA decision was a “great victory and long overdue,” saying that it would help prevent some of the 1.5 million unplanned pregnancies in the U.S. each year.

Social conservatives, however, reacted harshly to the decision, saying that making the pill easily available would only lead to more teenage promiscuity and sexually transmitted disease.

Wendy Wright, president of Concerned Women of America, said rapists could buy the drug for teens “to cover up their abuse” because men over 18 also may but the drug without a prescription. She also warned that the age restriction would be near impossible to police.

Her organization, which led the fight against wider availability of Plan B, called on President Bush to withdraw the nomination of acting FDA commissioner Andrew C. von Eschenbach as permanent commissioner, citing his “pandering to political activists and a drug company.”

Two Democratic senators angry over the agency’s procrastination on Plan B — Hillary Rodham Clinton of New York and Patty Murray of Washington — had blocked von Eschenbach’s nomination.

After Thursday’s decision, Clinton and Murray said they would remove their objections to a Senate vote.

“We are very pleased about this decision,” Clinton told reporters on a conference call. “It is an important step forward, and it is about something more than just Plan B. The non-decision out of FDA was about the integrity of the agency as a whole.”

Plan B, produced by Barr Pharmaceuticals Inc. of Woodcliff Lake, N.J., consists of two pills, taken 12 hours apart. It can prevent up to 89 percent of pregnancies if taken within 72 hours after unprotected sex.

The pill, which now sells for between $25 and $40, is simply a concentrated dose of regular birth control pills. It is thought to work by releasing a flood of synthetic hormone that prevents an egg from being released from the ovary or from being fertilized. It can also prevent implantation of a fertilized egg into the womb.

Side effects of Plan B include nausea and vomiting. It can be used by women for whom ongoing birth control pills are not advised.

The drug has never been widely used, with annual sales of just $30 million.

Plan B became caught up in abortion politics because some abortion opponents viewed it as a method for ending a pregnancy. The FDA classifies the drug as a contraceptive.

But unlike RU-486, the abortion pill, Plan B cannot end an established pregnancy.

The FDA approved Plan B as a prescription drug in 1999. In December 2003, an FDA advisory committee recommended that it be made available over-the-counter.

But the FDA said it could not approve Plan B for over-the-counter sale because it did not have enough information on how adolescents would use the drug. The agency said it was concerned that younger teens might engage in risky sex if the contraceptive became readily available.

For the next three years, the agency was unable to make up its mind about what would be the right cutoff age.