The method has been available by prescription to European women for years.
The FDA's announcement paves the way for pharmaceutical companies to specially package birth control pills here for women to take in case of emergency - such as rape, the failure of a birth control method or the failure to take precautions "in the heat of the moment."
So far contraceptive manufacturers have declined to sell the emergency contraception in the United States, citing fears of litigation and political fallout.
But the small, New Jersey-based Gynetics company is developing a specially packaged version of birth control that it hopes to sell for emergency use next year.
It is legal now for doctors to prescribe birth control in this form for emergencies and the American College of Obstetrics and Gynecology endorsed the method in December.
The FDA's instructions were detailed enough to give doctors and family-planning clinics the correct dosage for women.
In fact, some doctors and clinics have been handing out "morning after" birth control for years.
"It's been around for decades," Planned Parenthood of Maryland spokeswoman Diane Billings said.
In the last 20 years, "it has increased in usage and acceptability," Hagerstown Reproductive Services Director Diane Silas said. "It's not terrifically common."
Over the past three to five years, however, it has become more acceptable, she said.
Silas estimated that her clinic prescribes emergency contraception once or twice a week. "It represents a small percentage of what we do here," she said.
Both Planned Parenthood and Hagerstown Reproductive Services do a sensitive pregnancy test to determine if the woman already could be pregnant from an earlier encounter. If she is, the method is not used, clinic officials said.
The emergency contraceptive method relies on "a whopping hormonal dosage," Silas said.
The FDA has said that for the emergency contraceptive to work, women must take two to four birth control bills, depending upon the brand, within 72 hours of unprotected sex, and then repeat the dose exactly 12 hours later.
The pills prevent pregnancy by blocking a fertilized egg from implanting in the uterus so it can grow into an embryo.
That's why some people who oppose abortion also oppose emergency contraception.
"We do not support it because we feel life starts at conception," said Sherry Cline, director of the Waynesboro, Pa., Center of Pregnancy Ministries Inc.
"We consider it abortion any time after the egg has been fertilized," Gloria Smith, director of the nonprofit, church-sponsored Crisis Pregnancy Center in Hagerstown.
Smith wondered about the side effects of such a method and what the impact on the fetus would be if the method is unsuccessful.
"I think there's probably a lot left unknown," she said.
One side effect is nausea, which is why Hagerstown Reproductive Services also prescribes something for that when they give a woman the birth control pills, Silas said.
But the FDA's approval and the one-time nature of the method mean it can be used safely, physicians say.
"It's a one-shot thing so the long-term impacts are probably minimal," said Dr. Robert Parker, Washington County Health Department Health Officer.
Physicians and clinic operators say they don't know why emergency contraception has not been more widely promoted.
Hagerstown Obstetrician/Gynecologist Dr. Jay Greenberg said it's described in the standard textbook of gynecology and "it's been available forever and ever ... There's nothing new about it."
But in his 17 years of practice, Greenberg said, he has prescribed it fewer than five times.
"Patients don't seem to fit into that category too often," he said, referring to the 72-hour limit.
Women should not try this method with their own birth control pills but should consult a doctor or clinic for the appropriate brand and dosage, Silas said.
"Don't try this on your own," she warned.