The emergency contraception drug Plan B One-Step will be available for women as young as 15 without a prescription, the Food and Drug Administration said on April 30.

The drug, also known as the morning-after pill, was previously available without a prescription only to women ages 17 and over. Consumers who buy the product will need to provide proof of age, the FDA said.

On May 1, the Obama administration indicated that it will challenge this new FDA ruling in federal court.

Plan B One-Step contains levonorgestrel, a hormone found in most birth control pills, but at a higher dose. It should be taken within 72 hours of unprotected sexual intercourse and is intended to prevent pregnancy if other forms of contraception fail, or if no contraception was used, according to Teva Women's Health, the company that makes Plan B.

The drug mainly prevents pregnancy by stopping the release of an egg from the ovary, and by preventing the fertilization of an egg, Dr. Christopher Estes, assistant professor of obstetrics and gynecology at the University of Miami Miller School of Medicine, told MyHealthNewsDaily in a 2011 interview.

While the drug causes changes to the lining of the uterus, it does not interfere with the implantation of a fertilized egg, Estes said. In fact, in order to stabilize the pregnancies of women who've suffered miscarriages, doctors give a drug very similar to Plan B, Estes said.

The drug will not work if a woman is already pregnant. (It has no effect on an existing pregnancy, according to Teva.) Emergency contraception will not cause an abortion.

If taken within 72 hours of unprotected sex, Plan B is 89 percent effective at preventing pregnancy, according to Planned Parenthood. The drug is less effective after three days, but can work up to five days after unprotected sex.

Plan B should not be taken in place of regular birth control, and cannot prevent infection with sexually transmitted diseases, according to Teva.

Another type of emergency contraception, called Ella (ulipristal), manufactured by Watson Pharma Inc., requires a prescription and can prevent pregnancy if taken within five days (120 hours) of unprotected sex, according to the FDA. This drug is 85 percent effective if taken within five days of unprotected sex,  Planned Parenthood said.

Emergency contraception can cost between $10 and $70, Planned Parenthood said.

Follow Rachael Rettner @RachaelRettner. Follow MyHealthNewsDaily @MyHealth_MHND, Facebook & Google+. Originally published on MyHealthNewsDaily.

Related on MyHealthNewsDaily and MNN:

This story was originally written for MyHealthNewsDaily and was republished with permission here. Copyright 2013 MyHealthNewsDaily, a TechMediaNetwork company.