Plan B: how does it work and how effective is it?

Things happen. Condoms are forgotten in the heat of the moment, or they slip off or break. Contraceptive pills can be missed. If pregnancy isn’t in your plans right now, but you had unprotected vaginal sex within the last 72 hours (3 days), you can reduce the chance of getting pregnant by taking a morning-after pill called Plan B.

Read on to learn more about Plan B One Step, including its side effects, where to get it, and more.

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Plan B One Step, commonly called “the morning after pill,” is emergency contraception. It is an over-the-counter (OTC), single-dose tablet that you can take to reduce the chance of pregnancy after unprotected sex or after another birth control method fails or wasn’t used correctly (for example, if a condom breaks during sex) (FDA, 2019). 

It is called Plan B “One Step” because you take one tablet by mouth, one time. The tablet contains 1.5 milligrams (mg) of levonorgestrel, a hormone ingredient that’s commonly found in birth control pills. Previously, with the “original” Plan B, you had to take two 0.75-mg tablets of levonorgestrel, 12 hours apart. The two-step version of Plan B was discontinued and replaced by Plan B One Step. So, any reference to “Plan B” for the rest of this article refers to Plan B One Step (FDA, n.d.; FDA, 2018).

Plan B works by reducing the chance that sperm will fertilize an egg, and it can delay ovulation (the release of an egg from the ovary). If an egg is not released, it can’t be fertilized by sperm. It is also possible that Plan B may prevent a fertilized egg from implanting in the lining of the uterus (womb). This is beneficial because pregnancy can’t occur if a fertilized egg does not implant (FDA, 2015; Turok, 2021).   

After a fertilized egg has implanted, Plan B will not do anything to reverse or interfere with the pregnancy. Also, keep in mind that Plan B does not protect against sexually transmitted infections (STIs).

Timing matters when taking Plan B, and while taking it does not guarantee that you will not get pregnant, the sooner you take it after sex, the better it works to prevent an unwanted pregnancy. 

Ideally, this means taking Plan B within three days (72 hours) of having unprotected sex. But if you miss the 72-hour mark, you can still take Plan B for up to five days after unprotected sex. It just may not be as effective. 

When taken within 72 hours, experts estimate that the chance of pregnancy ranges from 1% to 7%. This means that out of 100 people who take Plan B, 1 to 7 people will become pregnant anyway (Turok, 2021). 

What makes Plan B less effective?

Several factors might make Plan B less effective. For example, ovulation is the most fertile time of a woman’s menstrual cycle and typically occurs about 14 days before the next menstrual period. If you already ovulated before or during unprotected sex, Plan B may not work as well.

It may also not be as effective if you are taking certain herbal supplements or prescription drugs, such as (FDA, 2009):

• Certain seizure medications, such as carbamazepine, felbamate, oxcarbazepine, and phenytoin

• Griseofulvin, an antifungal drug

• Rifampin, an antibiotic

• St. John’s wort, an herbal remedy thought to help with depression

• Topiramate, a drug with several different uses, such as treating epilepsy bipolar disorder, and preventing migraines  

Additionally, Plan B may be slightly less effective in people with obesity, which is a body mass index (BMI) greater than 30 (Festin, 2017).

It isn’t possible to determine immediately if Plan B worked or not. Shortly after conception (when a fertilized egg implants into the lining of the uterus), the placenta begins producing a hormone called human chorionic gonadotropin (hCG). But it takes time—usually about 12 to 14 days after conception—for this hormone to build up enough to become detectable in the urine by home pregnancy tests (American Pregnancy Association, n.d.).

If your period is more than a week late, it could be a sign that Plan B did not work, and you may be pregnant. If this occurs, you should take a pregnancy test or reach out to a healthcare provider (FDA, 2019). 

Keep in mind that if you test too early, the results may not be accurate. Having patience can be challenging, but the longer you wait to test, the more accurate your results will be. Most healthcare professionals recommend that you wait to take a pregnancy test until the first day of your missed period.

You can purchase Plan B One Step over the counter at drugstores or at big box stores or supermarkets that have pharmacies. (Some drug stores may keep the product in a secured location, so just ask for it at the counter if you don’t see it near the condoms or pregnancy tests.) You can also get it from family planning clinics. 

Online retailers also sell Plan B, but keep in mind that the sooner you take Plan B, the better it works to prevent pregnancy. So, it’s best to visit your local pharmacy if you need it right away or keep one at home just in case.

It also does not require a prescription. In fact, anyone can purchase Plan B. There are no age restrictions, and you do not have to show your ID (HHS, 2019). 

Plan B usually costs around $45 to $50 (GoodRx, n.d.). Some insurance and medical assistance programs will cover Plan B for free if your healthcare provider orders you a prescription for it. Also, if you have an HSA or FSA, contraceptives, including Plan B, are typically considered eligible expenses. 

Plan B does not cause long-term side effects or affect your future fertility and is generally considered safe (WHO, 2021). Most people do not experience severe side effects, but some people get some temporary side effects, which are usually mild, such as (FDA, 2009; FDA, 2019):

• Changes in your menstrual cycle, such as your next period being earlier or later than expected, or it may be heavier or lighter than usual

• Nausea

• Fatigue

• Headache

• Dizziness

• Vomiting

• Breast pain

• Pain in your lower abdomen

However, if you develop severe abdominal pain, especially a couple of weeks after taking Plan B, you should seek medical care right away. This could be a sign of an ectopic pregnancy (FDA, 2019).

Several other brands and generic emergency contraception pills are available OTC that contain levonorgestrel, the same active ingredient in Plan B One Step. Some examples include (Drugs@FDA, n.d.):

• Athentia Next

• Fallback Solo

• Her Style

• Next Choice One Dose

• Opcicon One-Step

Another option is to reach out to a healthcare provider as soon as possible after having unprotected sex. They can give you personalized medical advice and tell you more information about other emergency contraception options.

No, emergency contraception like Plan B is not the same as an "abortion pill". If you hear otherwise, you might have come across misinformation by groups that oppose safe and legal abortion. These so-called “abortion pills” contain a different medication (​​mifepristone) and terminate an existing pregnancy. Plan B just prevents pregnancy. 

You can get mifepristone from a healthcare professional or a Planned Parenthood health center.

Plan B One Step is meant to be taken one time within 72 hours after unprotected sex or after another birth control method fails or isn’t used correctly. For example, condoms may break or slip off, or you may miss taking birth control pills. 

Taking extra Plan B tablets will not make it work better. In fact, this could make you more likely to have side effects like nausea and vomiting. And vomiting after you take Plan B could make it less effective; so if you vomit in the next two hours after taking Plan B, call your healthcare professional to find out if you should take another dose.

Morning after pills like Plan B are meant to help prevent pregnancy after one encounter of unprotected sex. If you engage in unprotected sex again, you may safely take Plan B again, whether it happens the next day, week, or month. There is no limit on how many times you can take Plan B in your life. 

However, a better option is to talk to a healthcare professional about other birth control methods. Plan B is called “Plan B” for a reason. It should not be your first defense—or your “plan A”—against an unwanted pregnancy. Several types of birth control may be more effective and likely less expensive than Plan B, such as birth control pills or non-hormonal birth control.

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Current version

January 20, 2022

Written by

Patricia Weiser, PharmD

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.