Women's Health

Plan B

Plan B is a type of emergency contraception. This is birth control that may prevent pregnancy after unprotected sex. People sometimes call it the "morning after pill." But you don't have to wait until the morning after sex to take it. In fact, Plan B is more effective the sooner you take it.

What's the Difference Between Plan B and Plan B One-Step?

In 1999, the FDA approved Plan B. It is a two-dose regimen: you take two pills 12 hours apart. Each pill contains 0.75 milligrams of the progestin levonorgestrel.

This is a synthetic hormone that has been used in birth control pills for more than 35 years. But the levels of progestin in Plan B (and Plan B One-Step) are higher than those in birth control pills.

In 2009, the FDA approved Plan B One-Step. It is a one-dose regimen: you take one pill. The pill contains 1.5 milligrams of levonorgestrel. Research has shown that taking Plan B One-Step up to 72 hours after unprotected sex works just as well as Plan B. And it doesn't cause increased side effects. So Plan B One-Step is replacing Plan B, although it may still be available in some pharmacies and health centers until supplies run out.

Beginning in 2009, Plan B and Plan B One-Step became available without a prescription to women aged 17 and older. But you must show proof of age. They are available by prescription to women younger than age 17.

How Does Plan B or Plan B One-Step Work?

Depending upon where you are in your cycle, Plan B or Plan B One-Step may work in one of these ways:

• It may prevent or delay ovulation.
• It may interfere with fertilization of an egg.

It is also possible that this type of emergency birth control prevents implantation of a fertilized egg in the uterus by altering its lining.

Plan B or Plan B One-Step is not the same as RU-486, which is an abortion pill. It does not cause a miscarriage or abortion. In other words, it does not stop development of a fetus once the fertilized egg implants in the uterus. So it will not work if you are already pregnant when you take it.

How Effective Is Plan B or Plan B One-Step?

Plan B or Plan B One-Step is more effective than emergency contraceptive pills (ECPs) that contain both estrogen and progestin.

If you take it within 72 hours after you've had unprotected sex, Plan B One-Step can reduce the risk of pregnancy by up to 89%. If you take Plan B One-Step within 24 hours, it is about 95% effective.

But you should know that Plan B or Plan B One-Step is not as effective as regular contraception. So don't take it as your main form of birth control. And, it does not protect you against sexually transmitted diseases. Think of it as a backup -- not for routine use. That's why it's called Plan B.