How does the "morning after pill" work?

The U.S. Food and Drug Administration approved an emergency contraceptive to be sold over the counter to women at least 17 years old. The drug's generic name is levonorgestrel. It's also known by the brand name, Plan B. Levonorgestrel disrupts the woman's natural hormonal cycle. (Another option, called Plan B One Step, is available without a prescription.)

Although scientists aren't completely sure how it works, they believe that levonorgestrel prevents pregnancy either by stopping the ovulation process or by disrupting the ability of sperm and egg to meet in the fallopian tubes. Some speculate that the drug may prevent the fertilized egg from implanting as well, perhaps by making the lining of the uterus less receptive to the egg.

Levonorgestrel contains a synthetic form of the hormone progesterone. (Regular birth-control pills contain it in lower doses.) The high doses of progesterone in Plan B are disruptive enough to keep an egg from being fertilized or implanted.

>When you purchase Plan B, you get two pills. Each pill contains 0.75 milligrams of levonorgestrel. You take the first pill as soon as possible after unprotected sex, and you take the second pill 12 hours later.

During the reproductive process, if a sperm fertilizes an egg, and if conditions are right, the egg implants itself into the uterine lining and the woman is pregnant. If the egg goes unfertilized or if something prevents implantation, there is no pregnancy, and the woman begins menstruating about two weeks after ovulation.

There is a 24-hour window during which a sperm can fertilize an egg. However, sperm can live three to five days inside a woman's body. So if a woman has unprotected sex three days before ovulation, she has a good chance of getting pregnant.

If ovulation has occurred, levonorgestrel will be less effective. It will be most potent if taken before ovulation. This is why it's important to take Plan B as soon as possible after unprotected sex. Doctors advise women to take Plan B no later than 72 hours after unprotected sex. That's because Plan B will have no effect once a fertilized egg implants.

As a result of these variables, Plan B is not completely effective. In clinical trials, however, it has been found to be 89 percent effective.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.