This method of emergency birth control uses high doses of the hormones estrogen and progestin to prevent or delay ovulation and prevent a fertilized egg from implanting in the uterus. This treatment can be taken anytime up to five days (120 hours) after the date of intercourse. However, the sooner treatment is started after intercourse, the greater likelihood of preventing pregnancy.

For emergency birth control using combination birth-control pills: Several pills of either estrogen-progestin or progestin-only pills are taken as soon as possible after unprotected sex and repeated in 12 hours.

Combination birth-control pills available for emergency contraception include:

• Ovral — two tablets as soon as possible after unprotected sex, two more in 12 hours
• Alesse — five tablets as soon as possible after unprotected sex, five more in 12 hours
• Lo Ovral, Nordette, Levlen, Triphasil, Tri-Levlen, Seasonale — four tablets, as soon as possible after unprotected sex, four more in 12 hours
• Preven (a prepackaged kit available by prescription, also called an emergency contraceptive kit) — Four tablets are in each kit and each tablet contains 50 micrograms of ethinyl estradiol and 0.25 mg of levonorgestrel. Two tablets are taken immediately, followed by two more in 12 hours.

Progestin-only birth-control pill regimens include:

• Norgestrel mini-pill — 20 tablets as soon as possible after unprotected sex, 20 more in 12 hours
• Plan B (a prepackaged kit available by prescription, also called an emergency contraceptive kit) — There are two pills per kit, each containing 0.75 mg levonorgestrel. Instructions are to take one immediately, the other in 12 hours. Levonorgestrel is highly effective and easier to tolerate than either the combination pills or 40 pills of norgestrel.

Emergency hormonal contraception can fail and results in pregnancy in 0.1 percent to 3 percent of cases, depending on the regimen and how soon after intercourse it is used. Emergency hormonal contraception is most effective when used within 24 hours of intercourse, but may be used up to five days afterward.

Advantages

• It is still possible to prevent a pregnancy even though birth control either was not used or failed.
• Fertility is not altered by the use of emergency hormonal contraception.

Disadvantages

• A prescription is needed to purchase emergency contraception almost everywhere in the United States, but there are some states in which it can be purchased without a prescription. Check with your local pharmacist or ask your health-care provider.
• Emergency contraception can cause significant nausea, vomiting, breast tenderness, headache and dizziness. Nausea and vomiting may be relieved and prevented with anti-nausea medication. If vomiting occurs within the first hour after taking emergency contraception, take another dose, because vomiting may decrease the effectiveness of the first dose.
• Women with a personal or close family history of thrombosis or thrombophlebitis should not use combination birth-control pills for emergency contraception because of the potential complications associated with the estrogen in these pills.
• Women who cannot take estrogen products may use the progestin-only preparations or the copper IUD approach.
• Emergency contraception can alter the timing, flow or length of the next menstruation in 10 percent to 15 percent of women. The period may arrive a few days sooner or a few days later than expected. If the period is more than seven days late, consider the possibility of pregnancy and get tested.
• This form of birth control does not protect against sexually transmitted diseases.