Washington ? By the end of the year, American women will be able to walk into any pharmacy and buy emergency contraceptive pills without a prescription as a result of a Food and Drug Administration decision announced Thursday.

The decision means women will not have to go to a doctor first as long as they can prove they are 18 or older to a pharmacist, who will keep the drugs behind a counter. Younger teens still will need a prescription, and the pills will not be sold at gas stations, convenience stores or other outlets that do not have pharmacists.

The approval marks the first time a hormonal contraceptive will be broadly available in the United States without a prescription. The pills, which will be sold as “Plan B,” probably will cost about $25 to $40 per dose, and men also will be able to purchase them.

The announcement was aimed at resolving one of the longest, highest-profile health controversies of the Bush administration, but opponents said they were considering trying to block the decision, either in court or in Congress.

“This decision has nothing to do with science or FDA rules but has everything to do with politics,” said Sen. Tom Coburn, R-Okla.

Coburn and other social conservatives said the high doses of hormones in the pills carried risks, and making them more easily available would encourage sexual activity and result in more unwanted pregnancies and sexually transmitted diseases. They also liken taking the pills to abortion, because they can sometimes prevent a fertilized egg from implanting in the womb.

“This is a bad decision for women, for girls, for parents and for public health,” said Wendy Wright of Concerned Women for America, which led a campaign to block the decision. “The FDA’s decision today will only make things worse for American women.”

Women’s health and family planning advocates, while criticizing the FDA for the age restriction, hailed the decision as a long-overdue milestone that will make it much easier for women to prevent unwanted pregnancy when they have unprotected sex or when other contraception, such as a condom, fails. It will be particularly valuable to rape victims, they said.

“This is great news for women and great news for women’s health,” said Cecile Richards of the Planned Parenthood Federation of America. “This provides women with another important option.”

The FDA’s move reverses a decision it made three years ago denying over-the-counter sales of the drug. That decision, which rejected the advice of the agency’s outside advisers and internal reviewers, triggered intense criticism that the administration was letting political ideology influence scientific decisions and undermining the credibility and independence of an agency charged with protecting the nation’s health.

Plan B, which consists of two pills containing a synthetic version of the hormone progestin used in standard birth control pills, is highly effective at preventing a pregnancy if taken within 72 hours of unprotected intercourse – long before pregnancy tests usually work.