Will Taking Plan B Cause a Miscarriage?

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Plan B One-Step (levonorgestrel) is a type of emergency contraception that's sold over the counter to prevent pregnancy after unprotected sex. It's also available under the brand names Fallback Solo, Next Choice One Dose, and Opcicon and may be referred to as the "morning-after pill." A common misunderstanding about Plan B is that it causes a miscarriage.

However, Plan B doesn't terminate an established pregnancy. It prevents conception from happening in the first place. By the time a pregnancy can be confirmed with a positive result on a home pregnancy test, an embryo will have already implanted in the walls of the uterus and taking Plan B will have no effect on it.

Plan B should be regarded as a backup form of birth control (never relied upon as a regular way to prevent pregnancy). But it is a safe and effective way to deal with a potential emergency if used correctly after inadvertently unprotected sex.

How Plan B Works

The active ingredient in Plan B and other morning-after pills is levonorgestrel—a progestin (a type of hormone) that is prescribed not only to prevent conception but also to treat endometriosis (a condition in which the tissue lining the uterus grows outside of the uterus), certain cancers, and appetite loss and severe weight loss caused by acquired immunodeficiency syndrome (AIDS) or cancer.

As emergency contraception, progestin works in three ways—by preventing the release of an egg from an ovary; by preventing an egg that is released from being fertilized; and by preventing implantation of a fertilized egg in the lining of the uterus.

Which of these functions the drug performs will depend on where you are in your menstrual cycle when you take it. For example, if you've already ovulated, levonorgestrel can't stop that from happening. But it can prevent fertilization if the egg you released comes in contact with sperm.

When to Use Plan B

You might be advised to use the morning-after pill in situations such as:

  • You had sex without using any birth control.
  • You forgot to insert your ring or apply your patch.
  • You missed taking at least two or three active birth control pills in a row.
  • Your partner didn't pull out in time.
  • You used a condom but it slipped off or broke.
  • You were forced to have sexual intercourse.
  • You were wearing a diaphragm that slipped out of place.

Be aware that Plan B and other morning-after pills will not protect you from sexually transmitted infections. The only way to prevent STIs is by always using a condom (even during oral or anal sex) or by not having sex at all.

How to Take Plan B

Some brands of levonorgestrel, including Plan B, are taken as one pill; others are taken as two. In order for the drug to be effective, you must take the single pill as soon as possible within 72 hours (three days) of having unprotected sex. If you're taking it in two doses, swallow the first pill within 72 hours and take the second one 12 hours later.

Planned Parenthood says that you can take emergency contraceptive pills as late as five days after having unprotected sex. Just know that the sooner you use it, the more likely it is to be effective.

Note that you should not take a double dose of levonorgestrel or two different brands of emergency contraception. Doing so will not increase the effectiveness of the drug and may even make you sick.

One of the possible minor side effects of taking levonorgestrel is nausea and vomiting. If you vomit within two hours of taking the pill, call your doctor. It's possible that you will need to take a second dose. Other side effects associated with taking emergency contraception include:

  • Breast pain or tenderness
  • Diarrhea
  • Dizziness
  • Fatigue
  • Headache

Although taking the pill will not cause a miscarriage (because it will prevent pregnancy from occurring in the first place), you also may notice that when you have your next period it's heavier than usual. It's just as likely, however, that your flow could be lighter. Some people have experienced spotting or bleeding between periods after using emergency contraception.

Your period may come a week earlier or later than usual after taking levonorgestrel. If you don't get your period within three weeks, do a home pregnancy test or have your doctor test you to make sure the pill worked to prevent conception.

The only serious side effect that's been reported is severe abdominal pain three to five weeks after taking levonorgestrel. If this happens to you, see your doctor.

Is Plan B Right For You?

There are some cases in which Plan B or other morning-after pills are less effective, such as in people who are overweight. The drug also may not be as effective for people who take certain medications. These include anticonvulsant drugs, antibiotics, the tuberculosis treatment rifampicin, the antimicrobial drug Mycobutin (rifabutin), Grifulvin V (griseofulvin) for treating certain skin infections, and St. John’s wort, a dietary supplement sometimes used to treat depression.

People who have abnormal vaginal bleeding or may be allergic to any of the pill's ingredients shouldn't take Plan B. And if you know you're already pregnant, you should not take Plan B. However, don't worry if you take the drug and then discover that you're expecting; it won't harm your pregnancy.

Alternatives to Plan B

If you're considering using emergency contraception, there are more effective options than Plan B and similar morning-after pills. These include having a ParaGard IUD inserted within five days or getting a prescription for Ella (ulipristal acetate), which also must be taken within five days after having unprotected sex but is as effective at that point as it is on day one.

Costs of Morning-After Contraception

One advantage of morning-after pills containing levonorgestrel is that they are available over the counter in pharmacies, supermarkets, and chain stores; can be purchased by anyone of any age or gender (you do not have to show an ID to buy a morning-after pill), and are relatively affordable. The average price of morning-after pills is $35 to $60; generic options cost 10% to 15% less than brand names.

Prices were generally lower at independent pharmacies than at chain stores, but the least expensive version of levonorgestrel is a product called AfterPill, which is available only online at afterpill.com for $20 plus $5 for shipping. AfterPill is good for 18 months, so it can be purchased ahead of time and stored in the event of an emergency.

A Word From Verywell

Morning-after pills should be not be used as regular contraception, but they can be useful to know about and have on hand for emergencies. If you're not sure what type of birth control is right for you, see your doctor or visit a Planned Parenthood or women's health clinic for guidance. There are lots of options, and you should be able to find one that works well for you.

9 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Krissi Danielsson
Krissi Danielsson, MD is a doctor of family medicine and an advocate for those who have experienced miscarriage.