How Long Can You Wait to Take Plan B and It Still Be Effective?

Plan B One-Step ( levonorgestrel ) used for emergency contraception is most effective when taken as soon as possible after unprotected sex or birth control failure. However, it can be effective for up to three days (72 hours) and sometimes up to five days.

Plan B decreases the chance of unintended pregnancy. It is available over-the-counter (OTC) at pharmacies, family planning clinics, or health department clinics. You do not need a prescription or identification to get Plan B One-Step, and people of any age or gender may buy it.

Plan B is not the same thing as the abortion pill, mifepristone (also known as RU-486), sold under the brand name Mifeprex.

This article will discuss when you should take Plan B to optimize its effectiveness, how it works to prevent pregnancy, the cost, and other important information.

How Long Is Plan B Effective?

Plan B One-Step is a hormonal drug taken by mouth that contains 1.5 milligrams of levonorgestrel. Levonorgestrel, used for many years in birth control pills and hormonal intrauterine devices (IUDs), works by preventing the release of an egg from the ovary (called ovulation).

Plan B One-Step works by blocking the surge of a luteinizing hormone (LH) that triggers ovulation. If you have not yet ovulated, stopping this process can protect against pregnancy.

Peak concentrations of levonorgestrel are reached in the blood within one to two hours of a dose. Thereafter, Plan B can continue to prevent LH surges for four to five days.

This is the same timeframe during which sperm are still viable. Sperm can survive in the female reproductive tract and be capable of fertilizing an egg for four to five days. After six days, the ability of sperm to conceive pregnancy drops to less than 1%.

What Can Reduce How Long Plan B Works?

Plan B One-Step does not work if you have already ovulated.

Despite misconceptions that Plan B can prevent implantation of a fertilized egg, prevent sperm from entering the egg, or change cervical mucus to prevent sperm from reaching the egg, no such benefits have been found.

While Plan B One-Step may be able to prevent ovulation for up to five days, the level of protection decreases as the drug is gradually eliminated from the body.

Levonorgestrel has a relatively long half-life (meaning the time during which half of the drug is eliminated from the body), but it is more effective during the earlier part of treatment when drug concentrations are high than the tail end when drug concentrations are reduced.

At the same time, the half-life of levonorgestrel can vary, ranging from 20 hours to 60 hours. For people at a healthy weight, levonorgestrel has an average half-life of up to 46.4 hours, In contrast, levonorgestrel has a half-life of 26.4 hours in people with obesity.

This doesn't mean that Plan B is less effective if you are overweight. It only highlights the need to take the drug as early as possible to improve your odds of protection.

How Effective Is Plan B?

Plan B One-Step is estimated to be 75% to 89% effective if taken within three days of unprotected sex. The sooner it is taken, the more effective it is.

It may also be effective—just not as effective—when taken four to five days after unprotected sex.

While Plan B One-Step is commonly called the "morning-after pill," you do not have to wait until the next morning to take it. Experts recommend keeping a box on hand so that you can take it if a condom fails or is not used.

After taking emergency contraception, you should use barrier protection (such as a condom) and your regular birth control method for at least seven days if you have sexual intercourse.

Does Plan B Affect Future Pregnancies?

Emergency contraception does not affect future fertility or any future chances of getting pregnant. This includes emergency contraceptive pills like Plan B as well as copper-containing intrauterine devices (IUDs) which can also be used for emergency contraception.

What Medications Interact With Plan B?

Certain drugs may interact can Plan B One-Step, making it less effective. These include:

• Dilantin (phenytoin)
• Phenobarbital
• Rifadin (rifampin)
• Topamax (topiramate)
• Saint John's wort
• Paxlovid (nirmatrelvir/ritonavir)
• Sustiva (efavirenz)
• Tegretol (carbamazepine)
• Trileptal (oxcarbazepine)

If you take any of these drugs, ask your healthcare provider if it is reasonable to interrupt treatment for several days. Do not stop unless your provider gives you the OK. With some of these drugs, stopping can lead to withdrawal symptoms, drug resistance, or treatment failure.

You should also not take Plan B One-Step (or any levonorgestrel-based drug) if you have taken the emergency contraceptive Ella (ulipristal) in the past five days. Taking them this close together can counteract their effects and cause them to fail.

Can Plan B Cause Side Effects?

As with all drugs, Plan B One-Step can cause side effects. Some of the more common include:

• A period that is earlier or later than normal
• A period that is heavier or lighter than normal
• Spotting
• Nausea and vomiting
• Diarrhea
• Stomach pain
• Fatigue
• Headache
• Dizziness
• Breast tenderness

If you vomit within two hours of a dose, contact your healthcare provider to see if you should repeat the dose.

Do not take more than one dose of Plan B. Taking more will not make the drug more effective and may only serve to increase the risk of side effects.

Do I Need a Pregnancy Test After Using Plan B?

While it is common for your period to be temporarily lighter, heavier, or different after taking Plan B, it shouldn't altogether cease. If you don't get your period within three weeks of using Plan B, take a pregnancy test to see if you are, in fact, pregnant.

Who Can Take Plan B?

Plan B One-Step is indicated for any situation where you may be at risk of unintended pregnancy following sex, including:

• When you don't use a condom
• When a condom breaks or slips off
• When a diaphragm, cervical cap, or sponge moves out of place
• When you forget to take your birth control pills or take them incorrectly
• After sexual assault or rape

Plan B One-Step is only used to prevent pregnancy after unprotected sex. it does not protect against sexually transmitted infections (STIs) like HIV.

In addition, Plan B One-Step should not be used if:

• You are allergic to levonorgestrel or any ingredient in it.
• You are pregnant or think you may be pregnant.
• You have undiagnosed vaginal bleeding.
  

Where Can You Get Plan B?

Anyone of any age or gender can buy Plan B One-Step without a prescription. Identification is not required.

You can find Plan B One-Step (or generic versions of the drug) at:

• Pharmacies (often kept behind the counter)
• Online stores (ideally using same-day delivery)
• Family planning clinics
• Health department clinics
• Planned Parenthood centers

How Much Does Plan B One-Step Cost?

You can purchase Plan B One-Step for about $40 to $50. There are generic versions that cost between $11 and $45, including Take Action, My Way, Option 2, Preventeza, AfterPill, My Choice, Aftera, and EContra.

You can also check with your Planned Parenthood center, family planning clinic, or municipal health department to see if you can get Plan B at a lower cost or for free.

Many Medicaid and private insurance plans cover Plan B with no co-pay. However, you will need to get it prescribed by a healthcare provider in order for the drug to be covered.

Summary

Plan B One-Step (levonorgestrel) is an emergency contraceptive used to prevent pregnancy following unprotected sex or birth control failure. It should be taken as soon as possible, ideally within three days (and possibly up to five days).

Plan B works by preventing ovulation. Side effects include irregular periods, spotting, nausea, and breast pain. Plan B can be purchased online, in drugstores, or in family planning clinics without a prescription. You can buy it irrespective of age or gender and do not need identification.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

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7. Kolles S. Sperm‐oviduct interactions: key factors for sperm survival and maintenance of sperm fertilizing capacity. Andrology. 2022 Jul;10(5):837–843. doi: 0.1111/andr.13179
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10. New York State Department of Health. Emergency contraception for preventing pregnancy.
11. Food and Drug Administration. Plan B One-Step (1.5 mg levonorgestrel) information.

By Karen Berger, PharmD
Karen Berger, PharmD, is a community pharmacist and medical writer/reviewer.

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