Morning-after pill: Emergency birth control

What can you tell me about the morning-after pill? How does it work?

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Mayo Clinic breast-health specialist Sandhya Pruthi, M.D., and colleagues answer select questions from readers.

Answer

The morning-after pill — a form of emergency birth control — is used to prevent a woman from becoming pregnant after she has had unprotected vaginal intercourse. Morning-after pills are generally considered safe, but many women are unaware that they exist. Here's how the morning-after pill works.

Human conception rarely occurs immediately after intercourse. Instead, it occurs as long as several days later, after ovulation. During the time between intercourse and conception, sperm continue to travel through the fallopian tube until the egg appears. So taking emergency birth control the "morning after" isn't too late to prevent pregnancy.

The active ingredients in morning-after pills are similar to those in birth control pills, except in higher doses. Some morning-after pills contain only one hormone, progestin (Plan B), and others contain two, progestin and estrogen. Progestin prevents the sperm from reaching the egg and keeps a fertilized egg from attaching to the wall of the uterus (implantation). Estrogen stops the ovaries from releasing eggs (ovulation) that can be fertilized by sperm.

The morning-after pill is designed to be taken within 72 hours of intercourse with a second dose taken 12 hours later. Side effects may include nausea, vomiting, abdominal pain, fatigue and headache. According to the Food and Drug Administration, the morning-after pill is 80 percent effective in preventing pregnancy after a single act of unprotected sex.

Morning-after pills aren't the same thing as the so-called abortion pill, or mifepristone (Mifeprex). Emergency contraceptive pills prevent pregnancy. The abortion pill terminates an established pregnancy — one that has attached to the uterine wall and has already begun to develop.

Plan B is available to women 18 years and older without a prescription at most pharmacies. Women must show proof of age to purchase Plan B. For women 17 years old and younger, Plan B is available with a doctor's prescription.