By Dr. Jeannie Hilton, DO, OB-GYN
Medically Reviewed by Alan B. Copperman, MD, FACOG
Last Update: January 29, 2025
What is Plan B?
Plan B is a morning-after pill used to prevent pregnancy after unprotected sex or birth control failure. It contains levonorgestrel, a progestin that works by delaying ovulation, so no egg is released for fertilization. Without an egg, pregnancy cannot occur. It is not an abortion pill and does not affect an existing pregnancy. It simply provides a safe and effective backup option for those who need emergency contraception.
For the best results, take the tablet as soon as possible after unprotected sex. It is most effective within 24 hours, reducing the risk of pregnancy by up to 89%, but it can still work up to 72 hours after. The effectiveness decreases the longer you wait, so quick action is key. This levonorgestrel dose is available over the counter without a prescription, making it an accessible option for emergency contraception.
This emergency contraceptive is meant for occasional use and should not replace regular birth control methods like daily pills, IUDs, or condoms. It does not impact future fertility, so if you decide to get pregnant later, your cycle should return to normal quickly. Since Plan B prevents ovulation, it may temporarily shift your menstrual cycle, but your period should come as expected or slightly earlier or later. If your period is more than a week late, taking a pregnancy test is recommended.
FAQ
How soon must you take Plan B?
The pill works best when taken as soon as possible after unprotected sex. It is most effective within 24 hours, reducing the risk of pregnancy by up to 89%. However, it can still be taken up to 72 hours (3 days) after, though effectiveness decreases over time.
The sooner you take the levonorgestrel tablet, the better the results. Waiting beyond 72 hours may significantly reduce its ability to prevent pregnancy. If you need emergency contraception, taking the pill immediately after sex gives you the highest chance of success.
Is levonorgestrel estrogen or progestin?
How does it work?
Plan B works by preventing pregnancy before it starts. It is a progestin hormone that stops the body from releasing an egg (ovulation), so there is nothing for sperm to fertilize. It also thickens cervical mucus, making it harder for sperm to reach an egg, and changes the uterine lining to prevent implantation.
For emergency contraception, levonorgestrel is most effective when taken as soon as possible after unprotected sex, reducing the risk of pregnancy. It does not affect an existing pregnancy and does not provide long-term birth control.
What is the Plan B success rate?
This backup birth control is up to 89% effective at preventing pregnancy when taken within 72 hours after unprotected sex. Its success rate is highest within 24 hours, reducing the risk of pregnancy by up to 95%. The sooner you take it, the better it works.
If taken on time, most users will not get pregnant, but this unprotected sex remedy is not 100% effective. It works best before ovulation, as it prevents the release of an egg. If ovulation has already occurred, the tablet may be less effective. For the best results, take it immediately after unprotected sex.
How do you know if the pill worked?
The best way to know if Plan B worked is if you get your next period on time or within a week of when you expected it. Some people may notice their period comes a little earlier or later, but this is normal.
If your period is more than a week late, take a pregnancy test to be sure. Other signs that post-coital treatment worked include mild spotting or changes in your cycle, which show that your body is adjusting. Remember, this post-coital contraception is highly effective when taken on time, but if you’re unsure, checking in with a healthcare provider can give you peace of mind.
What are the signs that Plan B didn’t work?
If the pill didn’t work, the most obvious sign is a missed period. If your period is more than a week late, take a pregnancy test to be sure. Some people may also notice pregnancy symptoms like nausea, breast tenderness, fatigue, or frequent urination a few weeks after taking the pill.
This emergency birth control works best before ovulation, so if you were already ovulating, it may not have been as effective. If you’re unsure, listen to your body and check for any unusual changes. When in doubt, a pregnancy test or a visit to a healthcare provider can confirm if the medication did its job.
What are the side effects?
Plan B may cause temporary side effects as your body adjusts. Common side effects include:
- Heavier menstrual bleeding – 30.9% of users
- Nausea – 13.7%
- Lower abdominal pain – 13.3%
- Fatigue – 13.3%
- Headache – 10.3%
- Dizziness – 9.6%
- Breast tenderness – 8.2%
- Delayed period (over 7 days) – 4.5%
These effects are temporary and usually go away within a few days. If your period is more than a week late or symptoms feel severe, check with a healthcare provider.
Can I take the pill twice in 2 days?
Yes, you can take Plan B twice in 2 days if needed, but it’s not recommended as a regular practice. Taking multiple doses won’t make it more effective and may increase the chance of temporary side effects like nausea or irregular bleeding.
This pregnancy prevention pill is meant for emergency use only, not as a routine birth control method. If you find yourself needing it often, consider a long-term contraceptive option like the pill, IUD, or implant. If you’re unsure about what’s best for you, a healthcare provider can help find a solution that fits your needs.
Can the levonorgestrel tablet be taken while pregnant?
No, levonorgestrel tablets should not be taken during pregnancy, but if you take one without knowing you’re pregnant, it won’t harm the pregnancy. This drug is designed to prevent pregnancy before it happens by stopping ovulation. It does not work if pregnancy has already started and will not cause a miscarriage.
If you think you might be pregnant, taking a pregnancy test before using emergency contraception is a good idea. If you have concerns about pregnancy or birth control options, a healthcare provider can offer guidance based on your situation.
Can you be pregnant if you took this pill and got your period?
If you took Plan B and got your period, pregnancy is highly unlikely. Your period means the pill worked, and your body is shedding the uterine lining, which doesn’t happen during pregnancy.
This one-step contraceptive can sometimes make your period earlier or later, but as long as normal bleeding occurs, pregnancy is not a concern. If your period is much lighter or shorter than usual, taking a pregnancy test can confirm the result.
What causes Plan B to fail?
This pill can fail if certain factors reduce its effectiveness. Here are the main reasons Plan B might not work:
- Taking it too late – The next-day contraception is most effective within 24 hours and works up to 72 hours after unprotected sex. After that, its ability to prevent pregnancy drops.
- Already ovulating – If you were already ovulating when you took the pill, it may not stop pregnancy since it works by delaying ovulation.
- Higher body weight – Some research suggests levonorgestrel may be less effective for people over 165 lbs (75 kg).
- Drug interactions – Certain medications, like some seizure drugs, antibiotics (rifampin), and herbal supplements (St. John’s Wort), can make the hormonal emergency pill B less effective.
- Vomiting after taking the pill – If you vomit within 2-3 hours of taking the tablet, your body may not absorb it properly, reducing its effectiveness.
If you think this morning-after option may not have worked, watch for changes in your period. If your period is more than a week late, taking a pregnancy test can confirm whether it was effective.
How long after this pill do you bleed?
Bleeding after taking Plan B can happen, but the timing varies for each person. Some may experience light spotting or bleeding within a few days, while others won’t see changes until their next period.
Your next period may come on time, early, or up to a week late. If your period is more than a week late, taking a pregnancy test can confirm if the pill worked. Bleeding patterns can be different after emergency protection, but this is temporary and usually returns to normal in the next cycle.
Are there levonorgestrel interactions to avoid?
Some medications and supplements can make Plan B less effective. If you’re taking any of these, you may need an alternative emergency contraceptive or backup birth control.
- Certain antibiotics – Rifampin, rifabutin (do not affect most birth control pills but can reduce the contraceptive’s effectiveness).
- Seizure and epilepsy medications – Carbamazepine, phenytoin, phenobarbital, primidone, topiramate, oxcarbazepine can speed up hormone breakdown.
- HIV medications – Efavirenz, nevirapine, ritonavir may lower levonorgestrel levels.
- Tuberculosis treatments – Rifampin can interfere with hormone absorption.
- St. John’s Wort – This herbal supplement can reduce levonorgestrel’s effectiveness.
If you’re on any of these, consider a copper IUD or another emergency contraception option. If this hormonal backup is your only option, use extra protection like condoms until your next period.
Can the pill make you skip a period?
Yes, Plan B can make you skip a period, but this is not common. The pill works by changing your hormone levels, which can cause your period to be earlier, later, lighter, or heavier than usual.
Most people get their period on time or within a week of when they expect it. If your period is more than a week late, take a pregnancy test to be sure the pill worked. Missing a period after taking the emergency contraceptive doesn’t always mean pregnancy, but it’s important to check if your cycle doesn’t return to normal.
What to avoid after taking Plan B?
To ensure the morning-after pill works effectively, avoid these common mistakes:
- Taking another dose too soon – One dose is enough. Taking multiple doses won’t increase effectiveness and may cause more side effects.
- Skipping a pregnancy test if your period is late – If your period is more than a week late, take a pregnancy test to confirm this emergency contraceptive worked.
- Relying on Plan B as regular birth control – It’s for emergency use only and not as effective as regular birth control pills, IUDs, or implants.
- Taking medications that lower effectiveness – Some seizure drugs, rifampin, St. John’s Wort, and certain HIV medications can make the levonorgestrel tablet less effective.
- Ignoring unusual symptoms – Light spotting is normal, but if you have severe abdominal pain or very heavy bleeding, seek medical advice.
This fast-acting hormone is a safe and effective emergency option, but for long-term pregnancy prevention, consider a more reliable birth control method.
How much does it cost?
As of 2025, the Plan B One-Step emergency contraceptive pill typically costs between $40 and $50. However, generic versions containing the same active ingredient, levonorgestrel, are available at lower prices, ranging from $11 to $40. Opting for these generics can provide significant savings while offering the same effectiveness.
Prices can vary depending on the retailer and location. Some health insurance plans may cover the cost of emergency contraception, so it’s advisable to check with your provider. Additionally, clinics like Planned Parenthood or local health departments often offer these pills at reduced costs or even for free, making them accessible to those in need.
About the Author
Dr. Jeannie Hilton, DO, is a highly experienced OB-GYN committed to providing comprehensive, patient-centered care tailored to women’s unique health needs. She has practiced at esteemed institutions like Complete Women’s Care of Naples and Virginia Beach Obstetrics and Gynecology, where she emphasizes a supportive environment that empowers women to make informed health decisions. Dr. Hilton earned her Doctor of Osteopathic Medicine from the Philadelphia College of Osteopathic Medicine, where she was recognized for her leadership and academic excellence. She completed her Obstetrics and Gynecology residency through Coastal AHEC/UNC, gaining expertise in all aspects of women’s health. With additional training in osteopathic techniques and a strong foundation from Pennsylvania State University, she integrates holistic, evidence-based approaches to ensure optimal outcomes. Throughout her career, Dr. Hilton has remained a dedicated advocate for women’s health, blending compassion with clinical excellence.
Disclaimer: This material is for general informational use and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a licensed healthcare provider for medical concerns. In an emergency, seek immediate assistance from local emergency services. You are responsible for how you use this information, and any decisions made based on it are at your own risk.