The manufacturer, lawmakers and other advocates said they will press the government to let minors purchase the pills over the counter.
The Food and Drug Administration (FDA) said women 18 and older and men purchasing for their partners may buy the Plan B pills without a doctor's note, but only from pharmacies.
Girls 17 and younger still will need a prescription to buy the pills, the FDA told manufacturer Barr Pharmaceuticals Inc., in ruling on an application filed in 2003.
The compromise decision is a partial victory for women's advocacy and medical groups, which say easier access could halve the nation's 3 million annual unplanned pregnancies.
"While we are glad to know the FDA finally ended its foot-dragging on this issue, Planned Parenthood is troubled by the scientifically baseless restriction imposed on teenagers. The U.S. has one of the highest rates of teen pregnancy in the Western world ¡ª anything that makes it harder for teenagers to avoid unintended pregnancy is bad medicine and bad public policy," president Cecile Richards said.
Opponents contend that nonprescription availability could increase promiscuity and promote use of the pills by sexual predators.
"If the FDA thinks that enacting an age restriction will work, or that the drug company will enforce it ... then they are living in a dream world," said Wendy Wright, president of Concerned Women for America, who led the opposition.
Plan B contains a concentrated dose of the same drug found in many regular birth-control pills. Planned Parenthood estimates 41 other countries, including Australia and Canada, already let women buy emergency contraception without a prescription.
If a woman takes Plan B within 72 hours of unprotected sex, she can lower the risk of pregnancy by up to 89 percent. Plan B is different from the abortion pill: If a woman already is pregnant, Plan B has no effect.
The earlier the pills are taken, the more effective they are. Allowing nonprescription sales means women won't have to run themselves ragged to get a prescription, something especially difficult on weekends and holidays, advocates said.
The FDA's long delay in deciding on Barr's application ensnared President Bush's nominee to head the regulatory agency, Dr. Andrew von Eschenbach. On Thursday, two senators said they would lift their block on his nomination, making it likely he will win confirmation as FDA chief, perhaps next month.
In recent weeks, anti-abortion groups, angered that approval was imminent, had urged Bush to withdraw von Eschenbach's nomination. Bush stood behind his nominee Monday.
Barr hopes to begin nonprescription sales of Plan B by the end of the year. The pills will be sold only from behind the counter at pharmacies, but not at convenience stores or gas stations. Pharmacists will check photo identification.
There isn't enough scientific evidence that young teens can safely use Plan B without a doctor's supervision, von Eschenbach said in a memo. Over-the-counter use is safe for older teens and adults, the acting FDA commissioner added in explaining the age cutoff.
"This approach should help ensure safe and effective use of the product," wrote von Eschenbach.
Barr and others were disappointed that FDA imposed the age restriction. Bruce L. Downey, Barr's chairman, pledged to continue working with the agency to try to eliminate it.
The age restriction remains controversial even inside FDA, agency drugs chief Dr. Steven Galson told The Associated Press. Galson has acknowledged overruling his staff scientists, who concluded in 2004 that nonprescription sales would be safe for all ages.
"There still are disagreements," Galson said in an interview. "There were disagreements from the first second this application came in the house."
The Center for Reproductive Rights said a lawsuit filed last year to do away with all age restrictions would continue.
As a condition of approval, Barr agreed to use anonymous shoppers and other methods to check whether pharmacists are enforcing the age restriction.
"I'm sure the FDA will follow through on that and make sure these important conditions are established and enforced," said White House spokeswoman Dana Perino.
Barr hasn't said if it will raise the price of the pills, which now cost $25 to $40 in prescription form.
Planned Parenthood, the largest dispenser of the pills, expects some insurers to continue covering prescription sales. Whether that would be cheaper will depend on a woman's insurance.
Nine states ¡ª Alaska, California, Hawaii, Maine, Massachusetts, New Hampshire, New Mexico, Washington and Vermont ¡ª already let certain pharmacies sell Plan B without a doctor's prescription to women of any age.
Minors won't see any change in those states, because the pharmacist already technically writes the prescription, the American Pharmacists Association said.
The FDA approved prescription-only sales of Plan B in 1999. The quest to change its status began in 2003. That year, agency advisers endorsed nonprescription sales for all ages, and FDA's staff scientists agreed.
Higher-ranking officials rejected that recommendation, citing concerns about young teens using the pills without oversight. Barr reapplied, asking that women 16 and older be allowed to buy Plan B without a prescription. Then last August, the FDA postponed a final decision indefinitely, saying the agency needed to determine how to enforce the age restrictions.
FDA's handling of Plan B sparked a firestorm, with allegations of political meddling, high-profile resignations, lawsuits and congressional investigations.
The controversy appears to have helped Plan B sales, which are up an estimated 30 percent this year, according to IMS Health Inc., a health care consulting company. Barr estimates pharmacists dispense about 1.5 million packs a year.