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Whether you had unprotected sex or your contraception failed, Plan B is a form of emergency contraception that might be able to help. While it’s not a replacement for contraception, and only works for a relatively brief period of time, it’s beneficial for most people who don’t want to get pregnant.

Here’s what you need to know about Plan B, from how it works and how much it costs to the side effects you might experience from taking the medication.

Plan B is the brand name of the morning-after pill. It contains a 1.5-mg dose of levonorgestrel, a synthetic form of the hormone progestin.

Dr. Amy Roskin, a practicing OB-GYN and the medical director of The Pill Club, says, “Plan B, also known as the morning-after pill or emergency contraception, has a higher dose of hormones (levonorgestrel) than birth control pills and helps [people] significantly lower their pregnancy risk after unprotected sex.”

However, as Roskin points out, “It shouldn’t be used as a replacement for birth control. You get more efficacy and control by utilizing a more regular form of birth control.”

Plan B can help prevent pregnancy if taken correctly, but it’s important to be aware of the possible side effects and contraindications before taking it.

Because Plan B works by delaying ovulation, it should be taken as soon as possible after unprotected sex or after contraception failed — within 72 hours. After that time period, it won’t be effective.

A 2011 study concluded that levonorgestrel had no effect in stopping pregnancy if given on the day of ovulation or after ovulation. As a result, if you’re planning on taking the medication, it’s crucial to do quickly after unprotected sex.

“Plan B works by stopping the ovary from releasing an egg (ovulation),” Roskin says. “Plan B only works before pregnancy officially begins. If the egg and sperm have already met, fertilized, and implanted into the uterus lining, it’s pretty much a done deal.” Plan B will not be effective if the person is already pregnant, but it will not harm an existing pregnancy.

Plan B is taken orally as a single tablet. The sooner it’s taken after unprotected sex, or failed contraception, the more effective it’s likely to be.

Plan B must be taken within 72 hours of having sex. Still, it’s worth remembering that, the sooner you take it, the more likely it is that Plan B will work.

While it’s difficult to accurately assess the effectiveness of Plan B without knowing exactly how many people got pregnant after taking it, studies have estimated the efficacy of emergency contraception.

“Plan B can lower your chance of getting pregnant by 75 to 89 percent if you take it within 3 days of unprotected sex,” Roskin says.

According to Plan B’s website, “When used as directed, about 7 out of every 8 women who could have gotten pregnant did not become pregnant after taking Plan B.”

Plan B is most effective in people with a body mass index (BMI) that’s less than 25, and has been shown to not be as effective in people with a BMI of more than 30.

As with any medication, Plan B may cause some side effects. It’s important to be aware of any contraindications that might occur when taking other medications.

“Plan B is FDA-approved, safe, and effective. Any side effects should be mild and short-term. It’s pretty common to have irregular periods, which can come early or be delayed by a couple of days,” Roskin explains.

“It could also affect your next month’s period. The effects usually resolve on their own, but if you don’t get your period within a week of when you expect it to come, it’s probably a good idea to get a pregnancy test or get medical advice.”

If you experience vomiting after taking emergency contraception, consult your doctor or pharmacist right away. It may be necessary to take another dose.

There are a few reasons to not take Plan B, according to Kecia Gaither, MD, MPH, FACOG, a double board certified physician in OB-GYN and maternal fetal medicine and the director of perinatal services at NYC Health + Hospitals/Lincoln.

Gaither recommends not taking Plan B if you:

  • think you’re pregnant
  • have hypersensitivity to ingredients
  • have a history of abnormal bleeding
  • have any condition your health professional says is contraindicated

Some herbal products and medications could impact how well Plan B works, so ensure you mention to your doctor or pharmacist if you’re taking any of the following:

  • certain HIV/AIDS medications
  • St. John’s wort
  • barbiturates
  • topiramate
  • bosentan
  • carbamazepine
  • felbamate
  • Griseofulvin
  • oxcarbazepine
  • phenytoin
  • rifampin

As a branded form of emergency contraception, Plan B generally costs between $40 and $50. Other generic levonorgestrel pills are less expensive, contain the same ingredients, and work in the same way.

According to Roskin, “you might also be able to get a low-cost form of emergency contraception from a family planning clinic, like Planned Parenthood.”

Plan B is available over the counter, and no ID or prescription is required to purchase it. In 2013, the FDA approved Plan B One-Step’s plan to make the medication available to any woman of any age, without restrictions.

In addition to finding Plan B in the family planning aisle of major retailers, it’s available to purchase online via health services, like Nurx. This means you can have emergency contraception discreetly delivered to your door.

If you’d rather speak to a doctor or pharmacist, or attend Planned Parenthood, they’re able to advise you about your options.

There are multiple versions of the morning after pill, containing the hormone levonorgestrel, available in the United States, including:

  • Plan B One-Step
  • levonorgestrel (generic Plan B)
  • Athentia Next
  • Next Choice One Dose
  • Fallback Solo
  • Her Style
  • EContra EZ
  • My Way
  • Opcicon One-Step
  • React

There’s also a version of emergency contraception available containing the hormone ulipristal. It’s sold under the brand name ella.

ella is another commonly sold emergency contraception pill that should be taken as quickly as possible and within 5 days of unprotected sex. Both are safe to use to prevent pregnancy. You can get Plan B over-the-counter, but you’ll need a prescription for ella,” Roskin says.

ella is a better choice for those who had intercourse within the last 5 days, or if their BMI is more than 30. For those with BMI of more than 35, ella isn’t very effective.

How long do I have to take Plan B for?

Plan B One-Step consists of one pill, which should be taken as soon as possible after having unprotected sex, missing a contraceptive pill, or contraception failing.

The pill can be taken with or without food, and should be swallowed whole, preferably with water. Plan B should be taken within 72 hours of unprotected sex, for a higher efficacy rate.

Is Plan B the same as the ‘abortion pill’?

Plan B is not an “abortion pill.”

Levonorgestrel works to halt pregnancy before it takes place. “Plan B prevents ovulation, so if you’re already pregnant, it won’t work,” Roskin says.

How will I know if Plan B worked?

Plan B is most effective if taken within 72 hours. However, if ovulation has already occurred, the medication might not be able to prevent pregnancy.

“If your period is more than a week late after taking Plan B, you may be pregnant. You should take a pregnancy test and follow up with a healthcare professional,” says Dr. Betsy Koickel of GoHealth Urgent Care.

Plan B and generic versions of the hormone levonorgestrel are important options for those who may have been let down by birth control and are hoping to prevent an unwanted pregnancy.

This form of emergency contraception is available to all. But if you have any specific questions about the medication, make sure to consult a doctor or pharmacist before taking it.

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Amy Mackelden is the weekend editor at Harper’s BAZAAR, and her bylines include Cosmopolitan, Marie Claire, ELLE, The Independent, Nicki Swift, Bustle, xoJane, and HelloGiggles. She’s written about health for MS Society, MS Trust, The Checkup, The Paper Gown, Folks, HelloFlo, Greatist, and Byrdie. She has an unhealthy love for the “Saw” movies and previously spent all her money on Kylie Cosmetics. Find her onInstagram.