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Plan B isn’t the same thing as the abortion pill. It doesn’t cause abortion or miscarriage.

Plan B, also known as the morning-after pill, is a form of emergency contraception (EC) that contains levonorgestrel, a synthetic form of the hormone progestin.

Plan B can help prevent pregnancy if taken within 120 hours (5 days) after sex. It doesn’t work if you’re already pregnant.

Read on to learn more about the important differences between Plan B and the abortion pill.

There is some debate about how Plan B pills work. To add to the confusion, people disagree about when pregnancy begins.

After sexual intercourse, it can take up to a week to become pregnant. This process involves a complex series of steps, including:

  1. The release of an egg from the ovaries (ovulation)
  2. The penetration of an egg by a sperm (fertilization)
  3. The embedding of a fertilized egg, or zygote, in the uterus (implantation)

Medical organizations such as the National Institutes of Health (NIH) and the American College of Obstetricians and Gynecologists (ACOG) define pregnancy as beginning with implantation, the third step listed above.

But others believe that pregnancy begins with fertilization.

The confusion around Plan B appears to be related to the possibility that it could work after fertilization. But most research indicates that Plan B does not work after fertilization.

Plan BMedication abortion
What is it?Medication that prevents pregnancy soon after sexMedication that ends an early pregnancy
Can it be used to prevent a pregnancy?YesYes
Can it be used to end a pregnancy? NoYes
How does it work? Delays or prevents the release of an egg from the ovaryStops a pregnancy from growing and forces it from the uterus
How long does it take? Works for several days4 to 5 hours
How effective is it? 75 to 95 percent98 to 99 percent
How safe is it? About as safe as taking the birth control pillSafer than carrying a pregnancy to term
Does it have side effects?Yes — menstrual cycle irregularities, spotting, nausea, and vomitingYes — cramping, bleeding, nausea, vomiting, and diarrhea

Current research suggests that Plan B works mainly by delaying or preventing ovulation. It may also prevent fertilization.

As far as we know, Plan B is no longer effective once an egg is fertilized. It doesn’t prevent a fertilized egg from implanting in the uterus or interfere with an implanted zygote.

Many researchers have challenged this idea, including the authors of a 2015 literature review. They argued that Plan B is too effective to work solely at the ovulation stage and concluded that it probably has effects after fertilization.

We don’t know whether this is true or not.

Indeed, the authors of a 2015 literature review indicated that it might not be scientifically possible to prove that Plan B has no effects after an egg is fertilized.

They stressed that according to the best evidence we have, EC pills do not appear to work after fertilization.

In addition, keep in mind that according to the standard medical definition, pregnancy begins with implantation.

Vaginal bleeding isn’t a common side effect of Plan B, but it can happen. It’s caused by the hormones in Plan B and other EC pills. Usually, bleeding is light and goes away.

In rare cases, bleeding could be caused by something more serious. You should seek medical attention if you experience:

  • unusually heavy bleeding
  • unexpected bleeding that lasts more than a few days
  • bleeding that’s accompanied by other symptoms, such as cramps or dizziness

Because Plan B prevents pregnancy, measuring its effectiveness is hard. That would require knowing how many women would have become pregnant had they not taken Plan B, which isn’t possible.

As a result, most measures of Plan B’s effectiveness are estimates. Plan B’s manufacturers claim that Plan B is:

  • 95 percent effective when taken within 24 hours of sex
  • 61 percent effective when taken between 48 and 72 hours after sex

Researchers have questioned these estimates. Studies suggest Plan B and other progestin-only pills are between 52% and 100% effective at preventing pregnancy.

In addition, Plan B’s manufacturers recommend taking it within 72 hours. However, recent research suggests that it may still be somewhat effective for up to 120 hours after sex.

A medical abortion involves two drugs.

The first drug is mifepristone. It works by blocking progesterone, a hormone necessary for a pregnancy to continue growing.

The second drug is misoprostol. Typically taken after mifepristone, it works by causing contractions that push the pregnancy from the uterus.

Plan B won’t work if you’re already pregnant.

Though few studies have assessed the effects of taking Plan B during pregnancy, there’s moderate evidence that it won’t harm a growing fetus.

Plan B doesn’t affect fertility. It won’t prevent you from becoming pregnant in the future or increase your risk of miscarrying if you eventually become pregnant.

In addition, there’s no limit to how often you can take Plan B.

If you can safely take birth control pills, you can probably take Plan B.

In fact, according to the medical eligibility criteria provided by the Centers for Disease Control and Prevention (CDC), the benefits of taking EC pills nearly always outweigh the risks.

A few recent studies suggest that Plan B isn’t as effective among people who have a body mass index (BMI) above 25.

In particular, a 2011 study reported that compared to people with a BMI under 25, people with a BMI over 30 were three times more likely to become pregnant despite taking EC.

A 2014 study reported that, in general, higher BMIs are associated with a decreased effectiveness of Plan B and other progestin-only EC pills.

According to a 2016 study, a double dose might improve Plan B’s effectiveness among people with a BMI over 25.

This doesn’t mean you should avoid taking Plan B altogether if you have a BMI over 25.

If it’s the only option available to you, it may be more effective than taking nothing.

With that said, the EC options discussed later in this article are more effective for people who have a BMI over 25.

Side effects from Plan B are typically mild. They can include:

  • dizziness
  • fatigue
  • headaches
  • irregular menstruation
  • mild abdominal pain or cramps
  • nausea
  • vomiting
  • unusual spotting or bleeding

Plan B isn’t your only option. Ulipristal acetate is another EC pill sold under the brand name ella. It appears to be more effective than Plan B.

A 2012 study based on clinical trial data suggests that ella maintains approximately the same level of effectiveness for up to 120 hours after sex. It’s probably better if you’ve waited more than 24 hours to take EC.

In addition, its effectiveness doesn’t change according to your BMI. As a result, it’s a more effective option for people with a BMI of 25 or higher.

Another option is a copper intrauterine device (IUD), which can be inserted up to 5 days after ovulation to prevent pregnancy.

Copper IUDs are the most effective form of emergency contraception. When inserted within 5 days of sex, they’re 99% effective in preventing pregnancy.

Plan B and other progestin-only EC pills are available over the counter, which means you can buy them at a pharmacy without a prescription.

You don’t have to show your ID. The cost ranges from $35 to $60.

Generic brands tend to be less expensive and are just as effective in preventing pregnancy. In addition, family planning clinics sometimes offer lower cost or free EC pills.

The cost for ella is about $50. It requires a prescription but is more likely to be covered by insurance.

Copper IUDs also require a prescription. You’ll have to see a doctor to have a copper IUD inserted. This is often covered by insurance.

If you’re worried about the cost, contact your insurance company to find out what forms of EC it covers.

If you don’t have insurance, call your local health department or family planning clinic to discuss your options. They may be able to provide the services you need at little-to-no cost.

You still have options, whether terminating the pregnancy or carrying it to term.

If you’re unsure about continuing the pregnancy, there are several resources available to help you make an informed decision.

You’re not alone. Call or visit a reproductive health clinic to learn more about your options or speak to a counselor.

Plan B isn’t the same as the abortion pill. The abortion pill ends an early pregnancy.

In contrast, Plan B can be used only to help prevent pregnancy if taken within 5 days of sex. It works by delaying or stopping ovulation.