How Effective Is Plan B?

In the U.S., about 24% of all women between the ages of 22 and 49 years old reported using a form of emergency contraception, such as Plan B, at some point in their lives. Now that these products are available without a prescription, you might wonder how effective they are and what side effects you can expect should you ever need to use them.

What is Plan B, and how does it work?

Plan B One-Step (levonorgestrel) is the most well-known brand of emergency contraception (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.

The medication doesn’t hang around in your system too long. Within a few days, EC 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).

How safe is Plan B?

Safety is always important when it comes to medication, so you can rest easy knowing that Plan B is very safe. There are no age restrictions; as long as you are getting your period, you can take it. If you have not gotten your first period yet, you should not take Plan B unless a healthcare provider has told you to do so.

Plan B is also safe to use while breastfeeding, although some experts suggest waiting to nurse for about 3 to 4 hours after your dose. EC won’t affect your future chances of getting pregnant either. There’s no maximum to the amount of times you can take Plan B per menstrual cycle. If you do take it more than once within a short amount of time, it’s perfectly safe to do so.

And if you use a regular form of birth control, like 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.

What are the side effects of Plan B?

Plan B’s side effects are typically mild and fast passing. These may include:

• Changes to your next period

• Nausea

• Lower stomach pain similar to menstrual cramps

• Tiredness

• Headache

• Dizziness

• Breast tenderness

The most common side effect you might experience is changes to your next period. While heavier bleeding is more typical, you could also have lighter bleeding or a delayed period by up to a week. If your period is late, don’t panic. Take a home pregnancy test, and as long as it’s negative, this is probably the reason why you haven’t gotten your period yet.

While less common, some people also experience vomiting in the hours after taking Plan B. If you vomit within 3 hours of taking it, contact your healthcare provider. You may need to take another dose.

If any side effects are severe or last longer than a few days, reach out to your healthcare provider.

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, you’ll need to contact your healthcare provider 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. Double check with your pharmacist to make sure what you’re taking doesn’t interfere with EC. If you take any of the following, reach out to your healthcare provider 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)

  • St. John’s wort

  • Some HIV medications, including efavirenz (Sustiva)

• Vomiting within 3 hours of taking it: While vomiting is a possible side effect from Plan B, throwing up too soon after taking it can prevent your body from absorbing all of the medication. If this happens, contact your provider 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.

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 more recent research has 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.

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 less likely to get pregnant later in your menstrual cycle. You’re most likely to conceive if you’ve had sex 3 days before or on the day of ovulation.

Keep in mind that 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 later in your menstrual cycle.

Can my weight affect how well Plan B works?

There’s some worrisome information online about whether Plan B is as effective for women with higher BMIs (body mass index) 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 weight. So, do not let the number on the scale stop you from taking Plan B if you need it.

How to save on Plan B

There are many ways to save on Plan B, which is available OTC as both a brand-name and generic medication. 

• Save with GoodRx. 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 Plan B without a prescription, the manufacturer has a $10 coupon available to download and print from your computer. Just present it at the register during your purchase.

• Save with a rebate. If you've already purchased brand-name Plan B without a prescription, you can still save money. By submitting a copy of your sales receipt to the manufacturer's website, you can receive a rebate in the mail.

Why trust our experts?

Written by:

Christina Aungst, PharmD

Christina Aungst, PharmD, is a pharmacy editor for GoodRx. She began writing for GoodRx Health in 2019, transitioning from freelance writer to editor in 2021.

Reviewed by:

Stacia Woodcock, PharmD

Stacia Woodcock, PharmD, is a pharmacy editor for GoodRx. She earned her Doctor of Pharmacy degree from the University of Kentucky and is licensed in New York and Massachusetts.

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