How Plan B Works to Prevent Pregnancy

If your regular method of birth control fails, or if you did not use birth control at all, you may be concerned about an unplanned pregnancy. In the United States, unplanned pregnancies make up almost 45% of the pregnancies announced each year. If you’re not ready to be a parent, don’t panic and know that you have options.

After having unprotected sex, you can reduce your chances of pregnancy by taking Plan B emergency contraception. This pill is also sometimes called the “morning after pill.”

How It Works

Plan B is a one-time pill available over-the-counter that uses a hormone called levonorgestrel (a synthetic form of progesterone) to prevent or delay ovulation. It’s important to note that this is not an abortion pill and won’t terminate a pre-existing pregnancy. Instead, levonorgestrel delays ovulation and creates a hostile uterine environment unsuitable for conception.

After taking Plan B, if a sperm makes its way to the uterus, it will usually have trouble finding and fertilizing an egg. Plan B also thins the uterine lining, making it difficult for a fertilized egg to attach.


When considering emergency contraception like Plan B, remember that taking it sooner is better than later. Unlike the name suggests, you definitely don’t need to wait until the next morning if your regular birth control fails. The sooner the pill is taken, the more effective it can be at preventing an unwanted pregnancy.

Luckily, Plan B is generally quite effective: if taken within 3 days of unprotected sex, it can reduce the risk of pregnancy by over 85%. If taken within 1 day, the effectiveness is even greater, closer to 90%.

Despite these encouraging percentages, Plan B isn’t intended to replace your usual contraception. Over time, it’s not as effective as regular birth control options, such as IUDs or condoms. It also provides no protection against STDs. And if you think that’s not a big deal, consider that about 20% of the United States population has an STD on any given day!

Side Effects

Almost no medication is free of side effects and Plan B is no exception. While many women experience no symptoms after taking the pill, others may have nausea, stomach pain, dizziness, headache and tiredness. Levonorgestrel may also cause menstrual changes such as bleeding between periods, late periods, absent periods or heavy periods.

Women who have a history of vaginal bleeding, an allergy to an ingredient in Plan B, or are already pregnant should not take the pill. Nor should women who take certain drugs such as barbiturates, St. John’s wort and certain HIV/AIDS medications.

Where to Buy

Plan B is available over-the-counter at most pharmacies in the United States. You don’t need a prescription or proof of age to purchase.

If your regular method of birth control fails, you don’t need to panic. Options such as Plan B offer women a safe and effective way to prevent unwanted pregnancies. Just one pill within 3 days of unprotected sex can prevent over 85% of pregnancies.

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