Plan B: How It Works, Side Effects, and More

Sep 2, 2025

young woman taking the plan b pill
Even with protection, there’s a chance of pregnancy. If you’ve turned to Plan B to prevent an unplanned pregnancy, understanding its effectiveness and what happens if it fails is essential. This guide covers how Plan B works, its risks and side effects, and your next steps if pregnancy occurs.

 

How Does Plan B Work?

Plan B is a form of emergency contraception that lowers the risk of pregnancy by preventing or delaying ovulation. It’s often used when birth control fails or in cases of sexual assault.

However, Plan B doesn’t always prevent ovulation,[1][2][3] but may also work by disrupting the hormonal environment and interfering with the embryo’s attachment to the uterus, resulting in its death.[4]

It’s important to know that Plan B and all other forms of emergency contraception should not be used as primary methods of birth control.[5]

 

Is Plan B the Same as the Abortion Pill?

While they seem similar, Plan B and the abortion pill are not the same things.

The difference is that Plan B aims to keep you from getting pregnant in the first place and in some cases, disrupt a newly formed embryo from continuing to grow.[6] The abortion pill is designed to end a pregnancy every single time it is used.

If you’ve been sexually active in the weeks before taking Plan B, consider a pregnancy test or ultrasound to confirm you’re not already pregnant.

 

Can Plan B Cause a Miscarriage?

A miscarriage is the unintended loss of a pregnancy before 20 weeks. If an embryo has formed when Plan B is taken, it may interfere with uterine attachment, potentially causing the embryo’s termination.[7]

If that is not your intent, then Plan B may not be for you.

 

How Effective Is Plan B? When Should I Take It?

Plan B is most effective when taken within 72 hours after unprotected sex. While it significantly reduces the risk of pregnancy, it is not 100% effective.[8]

 

What Are the Side Effects of Plan B?

Common side effects of Plan B include[9]:

  • Dizziness
  • Headache
  • Abdominal pain or cramps
  • Nausea or vomiting
  • Breast tenderness
  • Delayed period, irregular bleeding, or heavier menstrual bleeding[10]

These side effects are typically mild and resolve within a few days. However, if you experience prolonged bleeding, spotting beyond a week, or severe abdominal pain, contact your healthcare provider immediately. These symptoms could indicate a miscarriage or ectopic pregnancy.[11]

 

What to Do If Plan B Fails

If you’ve taken Plan B and missed your period, it could mean that you’re pregnant.[12] Take a pregnancy test to confirm. If the result is positive, our compassionate team at A Better Choice is here to guide you through your pregnancy options, empowering you to make an informed decision. We offer confidential, free services to support your next steps.

Give us a call at (316) 685-5757 or make an appointment online today. All services are confidential and free of charge!

Make an Appointment

Please be aware that A Better Choice does not provide or refer for abortion services. 

Sources

    1. Hapangama D, Glasier AF, Baird DT. “The effects of peri-ovulatory administration of levonorgestrel on the menstrual cycle.” Contraception. 2001 Mar;63(3):123-9. DOI:10.1016/s0010-7824(01)00186-x https://www.contraceptionjournal.org/article/S0010-7824(01)00186-X/fulltext 
    2. Croxatto, HB, Brache V, Pavez M, Cochon L, Forcelledo ML, Alverez F, Massai R, Faundes A, and Salvatierra AM, 2004. “Pituitary-ovarian Function Following a Standard Levonorgestrel in Emergency Contraception Dose or a Single 0.75 mg Dose Given on the Days Preceding Ovulation.” Contraception. 2004 Dec;70(6):442-50. DOI:10.1016/j.contraception.2004.05.007 https://www.contraceptionjournal.org/article/S0010-7824(04)00156-8/fulltext 
    3. AAPLOG Committee Opinion #7. (2020, January). Embryocidal Potential of Modern Contraceptives. https://aaplog.org/wp-content/uploads/2024/05/Committee-Opinion-7-updated.pdf 
    4. See #3. 
    5. Levonorgestrel (Oral Route) Side Effects. Mayo Clinic. (2024, September 1). https://www.mayoclinic.org/drugs-supplements/levonorgestrel-oral-route/side-effects/drg-20074413 
    6. Kahlenborn, C., Peck, R., & Severs, W. B. (2015). Mechanism of action of Levonorgestrel emergency contraception. The Linacre Quarterly, 82(1), 18-33. https://doi.org/10.1179/2050854914y.0000000026
    7. See source #6.
    8. See source #5.
    9. Mayo Foundation for Medical Education and Research. (2022, June 3). Morning-after pill. Mayo Clinic. https://www.mayoclinic.org/tests-procedures/morning-after-pill/about/pac-20394730#
    10. See sources #5 and #9.
    11. See sources #5 and #9.
    12. See sources #5 and #9.
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