Considered a form of emergency contraception, the morning-after pill works by preventing or delaying ovulation or blocking fertilization of the egg. It may also prevent implantation of a fertilized egg in the uterine wall. Although some brands of the morning-after pill are available over the counter without a prescription, it's important to learn more about how and why it works, its effectiveness, and the possible side effects.

What The Pill Contains

While some morning-after pills contain levonorgestrel -- a progestin hormone -- they are different from standard birth control pills in that they contain no estrogen. Progestin is synthetic progesterone -- a hormone secreted by the ovaries that plays a role in maintaining the menstrual cycle and uterine lining. The progestin hormone levonorgestrel, in particular, causes changes in the uterine lining, making it more difficult for a fertilized egg to attach.

How It Works

For it to be effective, you must take the morning-after pill as soon as possible but within 72 hours of having sexual intercourse. Since it can take up to six days for sperm to fertilize an egg, you still have time to prevent pregnancy following sex.

If you get the two-pill dosage regimen, take the second pill 12 hours after taking the first dose, although it's safe and just as effective to take both pills at the same time. While you have up to five days to take the morning-after pill, it becomes less effective the longer you wait before taking it.

The Pill's Effectiveness

Although a morning-after pill that contains levonorgestrel can reduce the risk of pregnancy up to 89 percent if you take it within 72 hours after having sexual intercourse, you should not rely on it as a regular form of contraception. The morning-after pill won't keep you from getting pregnant if you take the pill and then have unprotected sex.

Possible Side Effects

Any side effects you may suffer usually go away quickly. However, you should ask your doctor if the type of morning-after pill you choose could interact with other prescription or over-the-counter medications you are taking.

Common side effects you may have include:

  • Nausea and vomiting

  • Dizziness

  • Headache

  • Fatigue

  • Abdominal pain

  • Breast tenderness

  • Changes in your menstrual cycle (bleeding that is heavier or lighter than usual)

Despite possible side effects, no serious complications related to taking the morning-after pill have been reported. Still, although it has FDA approval and is generally considered safe, not all women can take the morning-after pill. You should not use the morning-after pill as emergency contraception if you:

  • Suspect you are pregnant

  • Have abnormal vaginal bleeding that hasn't yet been diagnosed but could be due to diabetes, hypothyroidism, pelvic inflammatory disease, uterine fibroid, a sexually transmitted infection, or cancer

  • Are allergic to any of the ingredients contained in the pill

  • Are overweight, causing the pill to be less effective

  • Are taking certain medications that can decrease its effectiveness

Doctors also caution that you may experience irregular periods if you use emergency contraceptives frequently. Ask your gynecologist to prescribe regular oral contraceptives, which are more effective.

For more information, contact Women's Healthcare of Illinois or a similar location.