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

May 9, 2025

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 only be used as backups, not 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 may disrupt a newly formed embryo from continuing to grow[6], but 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 as soon as possible after unprotected sex, ideally within 72 hours. 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 Care Net Northern New York 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 (315) 782-5433 or make an appointment online today. All services are confidential and free of charge!

Make An Appointment

Please be aware that Care Net Northern New York 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.