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How Effective is Plan B?

  • The Pill Club
    Reviewed By: Julie Lamonoff, CNM, OBGYN-NP, Shannon DeVita DNP, FNP-BC, CPNP-PC

If you think your birth control failed or you didn't use birth control, you may be feeling a little anxious, which is totally understandable. Everybody needs a plan B sometimes, especially when the stakes are high. Your plan B for preventing an unwanted pregnancy might be an emergency contraceptive (EC) pill called Plan B® One-Step, or the morning-after pill.

In the past we've covered how taking Plan B can affect your menstrual cycle. But how effective is Plan B at preventing pregnancy? In this article we'll talk about how effective it is, how to take it, and what you can expect from it.

Short answer: How effective is Plan B?

Time is of the essence when it comes to the effectiveness of Plan B. The sooner you take Plan B One-Step, aka the "morning-after pill," the more effective it is. If taken within 3 days of unprotected sex, Plan B can lower your chances of getting pregnant by 75-89%.

Some studies have shown that Plan B may still work for up to 120 hours, or five days, after unprotected sex. But taking it within 24 to 72 hours is definitely your best bet at preventing pregnancy.

What is Plan B and other emergency contraception?

Emergency contraception is used as a time-sensitive way of preventing pregnancy. The Plan B pill, ella®, and Paragard® copper IUD are three methods of EC.

Plan B One-Step

Plan B One-Step is the most common brand name for the levonorgestrel 1.5mg tablet. The same product is sold under other brand names including:

  • Take Action®

  • My Choice®

  • My Way®

  • Next Choice®

  • One Dose®

  • Option 2®

These are all emergency contraceptive pills taken by mouth once as soon as possible after unprotected sex. These are generally the easiest type of EC for most women to get because they are reasonably priced and can be purchased over-the-counter.

ella morning-after pill

Another pill used for EC is called ella, or ulipristal acetate. This pill works similarly to Plan B to prevent pregnancy, and can be taken for up to five days after unprotected sex. Unlike Plan B, ella requires a prescription from your health care provider and can be more effective than Plan B if you weigh at least 155 pounds. It may not work as well in people who weigh 195 pounds or more.

Copper intrauterine device (IUD)

Paragard copper IUD is the third and most effective form of EC. Paragard is a small plastic device that is inserted into the uterus and prescribed by your health care provider. It works to prevent pregnancy if used within five days of unprotected sex. The copper from the IUD makes it much harder for sperm to get into the uterus and fertilize an egg.

If you continue to use Paragard, it can protect against pregnancy for up to 10 years as long as it stays in.

Can you still get pregnant after taking Plan B?

The timeline for a pregnancy can be difficult to predict and is different for everyone. Taking Plan B within 72 hours after unprotected sex provides you with the greatest chance of preventing pregnancy, but since it's not 100% effective, you still have a chance you may become pregnant. If your next period does not come within a week of when you are expecting it, it's a good idea to take a pregnancy test.

Keep in mind that Plan B is a medication that acts to prevent pregnancy from unprotected sex within the last 72 hours, not future unprotected sex. Plan B will not help you prevent pregnancy if you have unprotected sex after taking it.

Who is Plan B for?

Plan B is for women who have had unprotected sex for any reason such as failure to use birth control or not using it correctly, such as missing two or more birth control pills in a row, finding a hole in a condom, or a condom or cervical cap falling off during sex. Plan B is also for people who have experienced a sexual assault.

Some women have questions about higher BMIs and Plan B effectiveness. For those who are worried, know that the FDA says that "all women, regardless of how much they weigh, can use these products to prevent unintended pregnancy following unprotected sexual intercourse or contraceptive failure."

There are a few types of medications that can interact with Plan B and may make it less effective. These include:

  • Medications to prevent seizures such as topiramate, carbamazepine, oxcarbazepine or phenytoin

  • St John's Wort: An herbal remedy used to treat mental health disorders

  • Rifampin: An antibiotic that treats tuberculosis

How does Plan B work?

Levonorgestrel is a progestin hormone that works by preventing ovulation. It is similar to the hormone progesterone that occurs naturally in your body. Ovulation is when an egg is released from the ovary to be fertilized by sperm. When Plan B is taken and ovulation is prevented, there is no egg released for the sperm to fertilize, and therefore no pregnancy.

Plan B also works by thickening the fluid on your cervix, which makes it much more difficult for the sperm to travel and reach an egg to fertilize.

You may wonder whether Plan B stops an existing pregnancy. The answer is no, because that's not how Plan B works. Pregnancy does not occur immediately after unprotected sex. Fertilization and implantation must happen first, which can take days. Plan B is not an abortion pill, and if you're already pregnant, may not stop a pregnancy from occurring.

How to take Plan B?

As soon as possible after you have unprotected intercourse, simply swallow the pill in the Plan B package. If you're taking Plan B because you missed two or more days of your normal birth control pills (BCP), continue taking the rest of your BCP as normal. In this case you will also need to use a backup method of birth control (like a condom) for the next seven days.

Side effects of Plan B are rare, but can include nausea and vomiting. If you throw up less than two hours after taking Plan B, you most likely need to take another pill. You can contact your health care provider or pharmacist for their recommendation if you're unsure whether this is necessary.

Emergency contraception should not be used as your regular form of birth control. If you are having sex regularly, there are many birth control options available to prevent pregnancy such as non-hormonal birth control and the vaginal ring.

Keep in mind that pills, IUDs, rings, implants, or shots used for birth control protect against sexually transmitted infections (STIs). It's a good idea to use a condom unless you and your partner only have sex with each other and have both been checked for STIs.

What are the side effects of Plan B?

The most common side effects of Plan B include:

  • Nausea and vomiting

  • Lower abdominal pain

  • Fatigue

  • Dizziness

  • Headaches

  • Breast tenderness

These side effects occur in less than 20% of people that take Plan B.

Where to get Plan B?

Since Plan B works better the sooner you take it, you can keep it on hand for future use. Everyone has to make their own choices regarding their health. If you choose not to be on a regular form of birth control, keeping Plan B handy for emergencies may be the best way to save yourself stress, anxiety, or a pregnancy.

You can get Plan B emergency contraception pills at drugstores without a prescription. Planned Parenthood clinics also offer access. With health insurance, the cost of Plan B could be as low as $0. Without insurance, the cost of the brand name Plan B One Step pill is usually around $40-$50. The Pill Club also offers Plan B to our members.

How do you know if Plan B worked?

If you take Plan B within 72 hours after unprotected sex, your chance of becoming pregnant is low, but not impossible. The only way to know for sure whether you're pregnant is to take a pregnancy test. If your next period does not come within a week of when you expect it, it's a good idea to take a pregnancy test and contact your OB/GYN or health care provider.

Recap: How effective is Plan B?

Plan B is very effective for occasional instances where your regular birth control method fails. As long as you can take it within 72 hours of unprotected sex, or even better, within 24 hours, you have a good chance of avoiding an unwanted pregnancy.

It's smart to be prepared and have a plan in place to prevent a pregnancy that you don't want. Consider having Plan B readily available as a backup for when slip-ups happen, as they can for many women.


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  2. Piaggio G, Kapp N, von Hertzen H. Effect on pregnancy rates of the delay in the administration of levonorgestrel for emergency contraception: a combined analysis of four WHO trials., 2011. Contraception. Published July 2011.

  3. Haeger K, Lamme J, Cleland K. State of emergency contraception in the U.S., 2018. Contracept Reprod Med. Published September 5, 2018.

  4. Rosato, Elena et al. Mechanism of Action of Ulipristal Acetate for Emergency Contraception: A Systematic Review., 2016. Frontiers in pharmacology. Published January 12, 2016.

  5. Planned Parenthood. What's the ella morning-after pill? Reviewed June 15, 2021.

  6. Mayo Clinic. Copper IUD (ParaGard). Published Feb. 11, 2020.

  7. Office on Women's Health. Womenshealth.gov. Emergency contraception. Updated April 24, 2017.

  8. Plan B One-Step. Package insert. Duramed Pharmaceuticals; 2005.

  9. Eggertson, Laura. Plan B emergency contraceptive may be ineffective for heavier women., 2014. CMAJ. Published January 7, 2014.

  10. Hsiang, Diana, and Sheila Dunn. Emergency contraception., 2016. CMAJ. Published December 6, 2016.

  11. Peck R, Rella W, Tudela J, Aznar J, Mozzanega B. Does levonorgestrel emergency contraceptive have a post-fertilization effect? A review of its mechanism of action., 2016. The Linacre quarterly. Published February 2016.

  12. Turok, David. Patient education: Emergency contraception (Beyond the Basics). In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA. (Accessed on June 06, 2021.)

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