Key takeaways:
Plan B One-Step (levonorgestrel) is a popular brand name for an emergency contraceptive tablet. It’s about 89% effective for preventing pregnancy when taken within 72 hours (3 days) of unprotected sex.
Plan B works by delaying ovulation (the release of an egg). Research suggests it’s likely not effective if you take it after ovulation has already happened.
Taking Plan B more than 72 hours after sex greatly lowers its effectiveness. Certain medications, such as anti-seizure medications, can also make it less effective.
Plan B is available as a lower-cost generic. GoodRx can help you save over 70% off the average retail price at certain pharmacies.
Save on related medications
Emergency contraception (EC), such as Plan B (levonorgestrel), is a commonly used form of birth control. Unlike most birth control methods, Plan B is meant to be used after having unprotected sex. And while not intended for regular use, it works well for preventing pregnancy when used correctly.
But just how effective is Plan B? And what can make it less effective? Let’s discuss.
What is Plan B, and how does it work?
Plan B One-Step (often simply called Plan B) is the most well-known brand of EC on the market. It is a single-dose medication that contains the hormone levonorgestrel. It’s meant to be taken shortly after having unprotected sex. Plan B prevents pregnancy by temporarily stopping your body from ovulating (releasing an egg). No ovulation means there will be no egg present for sperm to fertilize.
Plan B and its generic versions are not forms of abortion. They will not stop a pregnancy that has already started. If you are already pregnant, you should not take this medication. It has to be taken within 72 hours (3 days) of unprotected sex to work, but the sooner you take it, the better it will work.
How effective is Plan B?
Plan B and other levonorgestrel-containing ECs are very effective at preventing pregnancy. It’s also very easy to use, as there is just one pill you have to take. When taken correctly, Plan B is up to 89% effective. Some clinical trials have reported even better effectiveness (98% in some studies). But it’s important to remember that typical effectiveness is often a bit lower than what’s seen in clinical trials.
How long is Plan B effective for?
Plan B doesn’t hang around in your system too long. Within a few days, levonorgestrel starts to leave your body. While this helps lower the risk of side effects and makes it safer long term, it also means its protection doesn’t last.
You will need to take a dose each time you have unprotected sex. Because of this, you should not use Plan B as your primary form of birth control. Plan B is also less effective than many other primary forms of contraception, including birth control pills, patches, and intrauterine devices (IUDs).
What kind of things can make Plan B less effective?
As with all medications, there are certain times when Plan B can be less effective, such as:
Using it more than 72 hours after unprotected sex: Once that window of time passes, there is no guarantee on how well Plan B (or its generics) will work. If it’s been longer than 3 days since you’ve had unprotected sex, contact your healthcare team to see if there are any prescription options available to you.
Taking it with certain medications: Levonorgestrel interacts with other medications, some prescription and some over-the-counter (OTC). Double check with your pharmacist to make sure what you’re taking doesn’t interfere with Plan B. If you take any of the following, reach out to a healthcare professional before taking Plan B, as it won’t be as effective as it should be:
Carbamazepine (Tegretol)
Oxcarbazepine (Trileptal)
Phenytoin (Dilantin, Phenytek)
Phenobarbital and other barbiturates
Griseofulvin (Gris-Peg)
Rifampin (Rifadin)
Some HIV medications, including efavirenz (Sustiva)
Vomiting within 3 hours of taking it: While vomiting is a possible Plan B side effect, throwing up too soon after taking it can prevent your body from absorbing all of the medication. If this happens, contact a healthcare professional right away to see if you should take it again.
Storing it at extreme temperatures for too long: Some people buy Plan B to have on hand just in case they need it. While this is perfectly fine to do, be sure to store it properly. Generally speaking, keeping medications in places that are too hot or too cold (like your car in the summer or winter) for days can cause them to lose their effectiveness. Storing Plan B at room temperature is always best.
Taking it if it’s past its expiration date: Again, buying Plan B to have in case of an emergency is a great idea. Just keep an eye on the expiration date on the package. Once that day passes, it will be less effective and you should restock with a fresh box.
How effective is Plan B during and after ovulation?
In short: Plan B likely isn’t effective during or after ovulation. Ovulation typically happens about 2 weeks after your period starts, although this can vary. You’re most likely to conceive if you’ve had sex 3 days before or on the day of ovulation.
Previously, experts thought that Plan B also thickened cervical mucus (slowing sperm) and thinned the lining of the uterus (preventing implantation). This is because that’s how levonorgestrel works when it’s included in primary birth control methods, like Mirena (levonorgestrel).
But further research found that Plan B has little to no effect on cervical mucus and the lining of the uterus. Plan B’s only effect is its ability to delay ovulation. That means that if you’ve already ovulated, the medication won’t work.
When should you take Plan B? Pharmacists answer this and other common questions about Plan B (levonorgestrel).
OTC birth control pills: Plan B isn’t intended to replace daily contraception. Read about Opill (norgestrel), the first OTC birth control pill that’s meant for daily use.
Possible side effects: Learn about the most common Plan B side effects — from nausea to breakthrough bleeding — and how to manage them.
It’s important to know that your risk of pregnancy is very low if it’s been more than 24 hours since you ovulated. So while Plan B isn’t effective after ovulation, you’re also less likely to get pregnant later in your menstrual cycle.
That being said, most women don’t know exactly when they ovulate. And Plan B won’t harm you if you take it after ovulation. So there’s little risk in taking levonorgestrel EC pills, no matter where you are in your menstrual cycle.
Can your body weight affect how well Plan B works?
There’s some worrisome information online about whether Plan B is as effective for women with higher body mass indexes (BMIs) as it is for women with lower BMIs. This is based on a study that showed higher rates of pregnancy after taking Plan B in women weighing more than 165 lbs. But even though there was a difference, Plan B was still over 90% effective for all the women in that study.
If you are concerned about this, just realize the biggest factor in how well Plan B works is how quickly you take it after having unprotected sex. Plan B works for 7 out of 8 women who take it within the first 72 hours regardless of their body weight. So, do not let the number on the scale stop you from taking Plan B if you need it.
How safe is Plan B?
Safety is always important when it comes to medication, and Plan B is very safe. There are no age restrictions; as long as you are getting your period (menstruating), you can take it. If you have not gotten your first period yet, you should not take Plan B unless a healthcare professional has told you to do so.
Can you take Plan B while nursing?
Yes, Plan B is safe to use while nursing. Levonorgestrel does pass into your breast milk. But studies suggest the medication has no long-term effects on infants who were exposed to it while nursing.
Can Plan B cause infertility?
No, Plan B won’t affect your future chances of getting pregnant. As soon as levonorgestrel leaves your body (a few days after you take it), your body will return to its natural fertility levels.
Keep in mind that it’s common for your next period to come later than usual after taking Plan B. This is because Plan B delays ovulation. And the timing of your period is primarily based on when your body releases an egg.
How many times can you take Plan B per menstrual cycle?
There’s no maximum number of times you can take Plan B per menstrual cycle. And because Plan B doesn’t last long in your body, you should take a dose each time you’ve had unprotected sex.
However, Plan B isn’t intended to replace your primary form of birth control. So if you find yourself taking it frequently, it’s a good idea to discuss other options with your healthcare team. There are birth control methods that are more effective and typically cost less than taking Plan B multiple times.
Is it safe to take Plan B and the pill at the same time?
Yes, if you use a regular form of birth control, such as the pill, you can continue taking it as you normally would. There’s no need to restart your pack or wait until your next period to go back to using it. Plan B doesn’t interfere with hormonal contraception or change its effectiveness.
How to save on Plan B
There are ways to save on Plan B, which is available OTC as both a brand-name and generic medication.
Save with GoodRx. If a healthcare professional writes you a prescription for Plan B, GoodRx can help you save over 70% off the average retail price of the generic version. Generic Plan B's price at certain pharmacies is as low as $12.27 with a free GoodRx discount.
Save with a coupon from the manufacturer. If you’re purchasing brand-name Plan B without a prescription, you may be able to save about $8 with a manufacturer coupon. Just visit their website and request a coupon be sent to your mobile phone. Savings may vary by location.
Frequently asked questions
Yes, Plan B can delay your period as a side effect. This is because Plan B delays ovulation. And ovulation plays the biggest role in determining when your next period will start. Usually, Plan B doesn’t delay periods by more than about 1 week. If your period is more than a week late, you should take a pregnancy test.
Yes, as with all medications, Plan B does expire. Plan B’s expiration date depends on when it was manufactured. So always double-check the package before taking it. Expired Plan B may not work as well.
Plan B’s average retail cost is $. Generic levonorgestrel has an average cost of $.
There are ways to save on both versions. Brand-name Plan B has a manufacturer coupon you can access. And if a healthcare professional writes a prescription, you can apply a GoodRx coupon to help lower costs. With GoodRx, brand-name Plan B may cost as low as $ and generic levonorgestrel may cost $.
Yes, Plan B can delay your period as a side effect. This is because Plan B delays ovulation. And ovulation plays the biggest role in determining when your next period will start. Usually, Plan B doesn’t delay periods by more than about 1 week. If your period is more than a week late, you should take a pregnancy test.
Yes, as with all medications, Plan B does expire. Plan B’s expiration date depends on when it was manufactured. So always double-check the package before taking it. Expired Plan B may not work as well.
Plan B’s average retail cost is $. Generic levonorgestrel has an average cost of $.
There are ways to save on both versions. Brand-name Plan B has a manufacturer coupon you can access. And if a healthcare professional writes a prescription, you can apply a GoodRx coupon to help lower costs. With GoodRx, brand-name Plan B may cost as low as $ and generic levonorgestrel may cost $.
The bottom line
Plan B (levonorgestrel) is up to 89% effective at preventing pregnancy if taken within 72 hours (3 days) of unprotected sex. It’s very safe and won’t affect future fertility. Plan B isn’t effective during or after ovulation. But it can be hard to know when you ovulate, and it’s not harmful to take it later in your menstrual cycle. There are multiple ways to save on Plan B, and it’s available without a prescription. So don’t delay if you need it.
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References
Centers for Disease Control and Prevention. (2021). Quickstats: Percentage* of women who have ever used emergency contraception among women aged 22–49 years who have ever had sexual intercourse, by education — national survey of family growth, United States, 2017–2019.
Centre for Experimental Therapeutics. (n.d.). HIV drug interactions. University of Liverpool.
Cleland, K., et al. (2014). Emergency contraception review: Evidence-based recommendations for clinicians. Clinical Obstetrics and Gynecology.
Drugs and Lactation Database (LactMed). (2025). Oral levonorgestrel. National Library of Medicine.
MedlinePlus. (2024). Levonorgestrel.
MotherToBaby. (2024). Levonorgestrel (plan B one-step).
Pubchem. (n.d.) Norgestrel. National Center for Biotechnology Information.
Shohel, M., et al. (2014). A systematic review of effectiveness and safety of different regimens of levonorgestrel oral tablets for emergency contraception. BMC Women’s Health.
U.S. Food and Drug Administration. (2022). Decisional memorandum: New drug application 21998, supplement 5, levonorgestrel 1.5 mg tablet emergency contraceptive, labeling supplement for update to mechanism of action information.
Vrettakos, C., et al. (2022). Levonorgestrel. StatPearls.
World Health Organization. (2021). Emergency contraception.
