Emergency contraception is available in several forms. The most effective, according to the Association of Reproductive Health Professionals, are those that contain a single female hormone, progestin. Two progestin-only pills are sold in the United States: Plan B and Ovrette. Only Plan B is specifically marketed for use as emergency contraception, but either can be used.
The active ingredient in Plan B and Ovrette, progestin, is one of the hormones commonly found in standard birth-control pills. However, Plan B contains a higher dose of progestin than a standard birth-control pill.
Plan B comes in a two-dose package. One pill is taken 12 hours after the first. To get protection with Ovrette, 40 of the "minipills" must be taken to equal the dose of progestin found in Plan B. Whichever is chosen, the sooner you take the pills, the lower your odds of getting pregnant.
Plan B recommends that to obtain optimal efficacy, the first tablet should be taken as soon as possible within 72 hours of unprotected intercourse.
Another type of emergency contraception uses a combination of two female hormones: estrogen and progestin. Outside the United States, this combination is packaged for sale as emergency contraception.
In the United States, doctors must prescribe regular estrogen/progestin birth-control pills (only certain kinds work for emergency use) and instruct teens and adults on how to take them.
Use of this type of emergency contraception reduces the odds of getting pregnant by 75 percent. As the ARHP Web site Not-2-Late.com explains, this doesn't mean that 25 percent of women using estrogen/progestin combinations will become pregnant.
"Rather, if 100 women had unprotected intercourse once during the second or third week of their cycle, about eight would become pregnant," according to the site.
Following treatment with an estrogen/progestin combination, only two women would become pregnant. In other words, a 75 percent reduction in the odds of becoming pregnant.
One of the controversies that surrounds this topic is the way these pills work inside the body.
Opponents of abortion rights say that the pills sometimes work to prevent the implantation of a fertilized egg into the uterus. They consider this action to be the same as an abortion. This raises the issue of when a pregnancy actually starts.
The U.S. government and leading medical societies define pregnancy as beginning at implantation, not fertilization. Other contraceptives also work by preventing implantation of fertilized eggs, blocking fertilization or inhibiting ovulation.
Emergency contraceptives are not to be confused with the so-called "abortion pill," RU-486, sold under the brand name Mifeprex. Mifeprex can only be used after a pregnancy has been established.