Emergency Contraception: Plan B

What is Emergency Contraception?

Emergency contraception (EC)is a term for medications that prevent pregnancy after unprotected sex, birth control failure, or sexual assault by delaying ovulation and preventing sperm and egg from meeting. When taken as directed, EC can effectively prevent 95% of pregnancies.

There are currently four different types of of emergency contraception on the market:

1. Levonorgestrel (LNG) (97.9%-98.8% effective)
2. Combined oral contraceptive pills (COCs)
3. Copper IUD (99% effective if placed within 120 hours of unprotected sex)
4. Ulipristal acetate (UPA) (98.8% effective)

Today, we’ll cover levonorgestrel (LNG), most commonly known as Plan B One-Step.

What is Plan B?

Plan B One-Step (Plan B) is an progestin-only emergency contraceptive that is taken in a single pill up to 72 hours after having unprotected sex. Plan B is available over the counter, making it  the most widely available form of EC worldwide. Other brands that contain levonorgestrel include Take Action, MyWay, and Aftera.

How Does Plan B Work?

Levonorgestrel works by preventing pregnancy by delaying ovulation temporarily (if ovulation has not already occured). Plan B contains the same active ingredient (LNG) as many birth control pills, just at a higher dosage (1.5 mg). In addition to delaying ovulation, Plan B thickens the cervical mucus, making it more difficult for sperm to reach the egg.

How Effective is Plan B?

When Plan B was taken within 72 hours of unprotected sex, studies found that 7 out of 8 women did not get pregnant. Effectiveness decreases over time, so the sooner you can take it, the better. In women who took Plan B within the first 24 hours following their sexual encounter the success rate was 95%, whereas its effectiveness dropped to only 61% when taken between 48 and 72 hours later. 

If you take emergency contraception after you’ve already ovulated, Plan B will not be effective since the egg has already entered the fallopian tubes. Think of Plan B as a temporary stop sign – if you put the sign up before a car (egg) reaches the intersection (ovulation), the car will stop before continuing on its way. But if the sign is placed after the car has already gone through the intersection, it doesn’t affect the car at all and it will continue on its journey as normal.

ECs containing ulipristal acetate (UPA) have appeared to be more effective in delaying ovulation than LNG but are not as well known and less available over the counter, making them harder to obtain. Given the time-sensitive nature of EC, LNGs such as Plan B or Take Action are more commonly used.

Does Weight Affect Plan B’s Effectiveness?

Plan B (LNG) has been found to be 3 times less effective in individuals with a body mass index (BMI) of over 26 or who weigh over 165 pounds. If a woman’s BMI is above this threshold, other emergency contraception options should be considered. Studies have shown that in individuals with a BMI over 26, pregnancy occurred at the same rates regardless of whether or not the woman took Plan B or a similar form of LNG emergency contraception.

In contrast, ella and other UPA-containing medications appear to be less affected by weight, with only 2.6% failure as compared to Plan B’s 5.8%. Because this method of emergency contraception requires a prescription, you should reach out to your primary care physician as soon as possible if you are concerned about an unprotected sexual encounter or birth control failure. 

The effectiveness of the copper IUD has been found to be unaffected by the patient’s weight. 

Who Should Not Take Plan B?

Plan B is generally safe for most women. However, if any of the following is true for you, Plan B should not be taken.

• You are currently taking antiepileptic drugs such as phenytoin or carbamazepine
• You are currently taking antituberculosis medications including rifampin
• You are currently taking St. John’s Wort
• You have an allergy to Plan B’s ingredients
• It has been more than 5 days since you had unprotected sex

If you have taken ella since your last period, it is not recommended that you take Plan B. 

Does Plan B Cause Side Effects?

Possible side effects include spotting, nausea and/or vomiting, fatigue, dizziness, and headache. If you vomit within two hours of taking Plan B, it’s best to take another dose to ensure that you didn’t throw up the medication. Taking Plan B may also cause a change in your current cycle, delaying the onset of menstruation. Some women may experience abdominal pain, breast sensitivity, spotting, or a change in the duration of their period. Plan B side effects should only last for a few days and your cycle should return to normal the following month.

If your period is delayed by over a week, it’s time to take a pregnancy test.

FAQs:

Is Plan B the same as the abortion pill?

No. Plan B and the abortion pill (also called RU486, a medication abortion, at-home abortion, Plan C, or its medication name: Mifepristone) contain different active ingredients and are taken for different purposes. Plan B works to delay ovulation and prevent pregnancy. If you have already conceived, Plan B will not harm the pregnancy. The abortion pill is taken with the purpose of ending a pregnancy that is already developing.

Can Plan B Work for Transgender or Nonbinary People?

Pregnancy is possible when a person who has a uterus and ovaries has receptive penis-in-vagina sex with a partner who produces sperm, regardless of either person’s gender identity. Testosterone is not a method of birth control for people who are able to become pregnant even if it has begun to produce other bodily changes or stopped their period. If taken as directed, Plan B works  the same way as it does in cisgender patients and has not been shown to interfere with continued gender-affirming treatments.

Will Taking Plan B Harm My Future Fertility?

No. When taken correctly, Plan B works to delay ovulation temporarily to prevent sperm and egg from meeting. In a review of 33 studies, researchers found “In a review of 33 studies, researchers found that rates of inhibited ovulation and of conception were similar among women who had taken LNG in the past and women who had never taken it Additionally, the researchers found no increased risk of miscarriage, stillbirth, or changes in fetal development.

Can I Use Plan B as My Primary Form of Birth Control?

Using Plan B as your primary form of birth control is not recommended as it’s not as effective as other forms of birth control such as the hormonal implant, the copper or hormonal IUD, birth control pills, or condoms. Taking Plan B more than once during your cycle does not increase its efficacy, and could increase your risk of side effects. 

Taking emergency contraception too often can interrupt your menstrual cycle and actually make pregnancy more likely because you are unable to track your cycle effectively. It can also decrease the effectiveness of the medication, underscoring the importance of using a primary, reliable form of contraception.

Where Can I Buy Plan B?

One-pill ECs like Plan B can be found at many drugstores like Walgreens or CVS or stores like Walmart and Target, and are available over the counter without age restrictions. Plan B is legal in all 50 states. There are no age or gender restrictions on who is able to purchase EC, meaning that someone else can purchase it for you if needed.

How much does Plan B Cost?

Branded versions of LVG such as Plan B One-Step cost an average of $50, whereas generic versions cost about $35-$40. 

Can I OverDose on Plan B?

Taking more than one dose of Plan B is not recommended and can increase your risk of side effects, a disrupted cycle, and decrease in medication efficacy. If you have taken more than one and are experiencing symptoms, it’s important to talk to your provider and monitor your symptoms. Your doctor may recommend an alternative form of emergency contraception. Always use Plan B as directed and ask your doctor about any questions or concerns you may have.

The Bottom Line

In case of birth control failure or unprotected sex, Plan B or other versions of levonorgestrel EC act as a relatively safe and effective way to prevent pregnancy. However, LVG does not prevent the spread of STIs, and condoms should be used to protect yourself and your partner.

As its name suggests, Plan B is meant to act as a backup to your primary form of birth control and is not intended to be used as long-acting birth control or taken regularly. If you’re sexually active, talk to your doctor about your birth control options and be sure to get tested for STIs regularly. 

If you have taken Plan B or another form of EC, yet suspect that you may be pregnant, call Alpha Center Fort Collins to schedule a free pregnancy testing or pre-abortion screening appointment.

Written May 2025, authored by Kristen Long and reviewed by Nurse Manager Meghan Downing.

Citations:

1. World Health Organization. Emergency contraception. Who.int. Published November 9, 2021. https://www.who.int/news-room/fact-sheets/detail/emergency-contraception
2. The Efficacy and Accessibility of Plan B Emergency Contraception: Medical and Public Health Perspectives – IJIRCT. Ijirct.org. Published 2021. Accessed May 15, 2025. https://www.ijirct.org/viewPaper.php?paperId=2410066
3. ‌Edelman AB, Cherala G, Blue SW, Erikson DW, Jensen JT. Impact of obesity on the pharmacokinetics of levonorgestrel-based emergency contraception: single and double dosing. Contraception. 2016;94(1):52-57. doi:10.1016/j.contraception.2016.03.006
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7. Eggertson L. Plan B emergency contraceptive may be ineffective for heavier women. CMAJ. 2014;186(1):E21-E22. doi:10.1503/cmaj.109-4671
8. Cleland K, Wagner B, Smith NK, Trussell J. “My BMI is too high for Plan B.” A changing population of women seeking ulipristal acetate emergency contraception online. Women Health. 2020;60(3):241-248. doi:10.1080/03630242.2019.1635560
9. V. Brache, L. Cochon, C. Jesam, R. Maldonado, A.M. Salvatierra, D.P. Levy, E. Gainer, H.B. Croxatto, Immediate pre-ovulatory administration of 30 mg ulipristal acetate significantly delays follicular rupture, Human Reproduction, Volume 25, Issue 9, 1 September 2010, Pages 2256–2263, https://doi.org/10.1093/humrep/deq157
10. 1. FSRH CEU Statement: Contraceptive Choices and Sexual Health for Transgender and Non-Binary People. The Faculty of Sexual & Reproductive Health. October 2017. Accessed May 22, 2025. https://www.fsrh.org/Common/Uploaded%20files/documents/contraceptive-choices-and-sexual-health-for-transgender-non-binary-people-oct-2017.pdf. 
11. 1. Emergency contraception for transgender and nonbinary patients. American Society for Emergency Contraception. Accessed May 22, 2025. https://www.ec-ec.org/wp-content/uploads/2021/01/ASEC-Factsheet_EC-for-transgender-patients.pdf. 
12. Edelman A, Hennebold JD, Bond K, et al. Double dosing ulipristal acetate emergency contraception for individuals with obesity: a randomised crossover trial. BMJ Sex Reprod Health. 2025;51(1):27-35. Published 2025 Jan 6. doi:10.1136/bmjsrh-2024-202401 https://pubmed.ncbi.nlm.nih.gov/39004442/

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