How Does Emergency Contraception Work, and How Long After Sex Can You Use It?

Emergency contraception is thought to work mainly by delaying or preventing ovulation, so there is no egg for sperm to meet. It is not expected to end an existing pregnancy. Product labeling describes levonorgestrel pills for use up to three days and ulipristal pills for up to five days after unprotected sex. A copper IUD can also be placed within five days, and sooner generally works better. This article explains how each option works, how timing, weight and medicines can matter, and which symptoms need urgent care.

How emergency contraception works

Emergency contraception is a way to lower the chance of pregnancy after sex without a condom, after a method failed (a broken condom, missed pills), or after sex that was not consensual. MedlinePlus, from the NIH, includes emergency contraception among the birth control options it describes. It is meant as a backup, not as a regular method.

There are three main options in the United States:

• Levonorgestrel pills (for example Plan B One-Step and generic versions). These are sold over the counter.
• Ulipristal acetate (Ella). This is prescription-only in the United States.
• A copper IUD, placed by a clinician.

What the pills are thought to do

Pregnancy needs an egg to be released, sperm to reach it, and a fertilized egg to implant in the uterus. Emergency contraception pills are understood to act mainly on the first step. They can delay or prevent ovulation, which means sperm that are already present have no egg to fertilize. The exact mechanism is not fully understood, and it may differ a little between the two pills.

These pills are not abortion pills. Research and clinical practice do not support the idea that they end a pregnancy that has already implanted. If you are already pregnant, they are not expected to harm it, but they also will not end it.

What the copper IUD does

A copper IUD releases copper ions that are thought to impair sperm function and fertilization. Whether it also affects implantation is not firmly established. A systematic review of 35 years of experience with IUDs for emergency contraception, published in PubMed Central (NIH), found pregnancy was uncommon after insertion. Because the IUD stays in place, it can then serve as ongoing birth control for years if you choose to keep it.

How long after sex can you use emergency contraception?

Sooner is generally better, because these methods work best when used before ovulation has occurred. Product labels and clinical guidance commonly describe the following windows:

• Levonorgestrel pill: labeled for use up to three days (72 hours) after unprotected sex. Effectiveness appears to decline the longer you wait. It may be less effective at higher body weight (a BMI of about 30 or more), although it can still be taken as soon as possible, and ulipristal or a copper IUD may be preferred in that situation.
• Ulipristal pill: labeled for use up to five days (120 hours) after unprotected sex, and it is generally thought to keep its effect better across that period than levonorgestrel. In the United States it is available by prescription only. It may be less effective if you took a progestin-containing product (such as birth control pills or levonorgestrel) in the previous five days, so mention that to the prescriber.
• Copper IUD: can be inserted up to five days after unprotected sex in many clinical protocols. It is considered the most effective emergency option and is not expected to be affected by body weight or by the enzyme-inducing medicines that reduce pill effectiveness. A clinician needs to confirm it is suitable for you, and it needs an appointment.

These windows come from product labeling and clinical practice, so check the package or ask a pharmacist about your specific product. If you are past the usual window, a pill may still be worth discussing with a clinician, because some protection may remain, but it is less certain. A copper IUD is often the option clinicians consider in that situation.

A systematic review of different levonorgestrel regimens, available in PubMed Central (NIH), looked at how well and how safely the pill performs. Its findings support taking it as soon as possible rather than waiting.

Three cautions apply to the pills and are explained in the next section: higher body weight can reduce how well they work, some medicines can lower their effect, and ulipristal can interact with hormonal birth control started soon afterward. Tell the pharmacist or clinician about each of these so they can help you choose between a pill and a copper IUD.

Which option fits your situation

Body weight

Pills may be less effective in people with a higher body weight or BMI, and levonorgestrel appears to be affected more than ulipristal. A copper IUD is not affected by body weight and is considered the most effective option. When a pill is chosen, ulipristal may work better than levonorgestrel at higher body weight. If this applies to you, tell the pharmacist or clinician so they can help you weigh the options. Do not skip emergency contraception for this reason, since a pill taken promptly can still be better than none.

Other medicines you take

Some medicines that speed up how the liver breaks down other drugs can lower how well both pills work. Examples include certain seizure medicines (such as carbamazepine and phenytoin), rifampin and some other tuberculosis and HIV medicines (efavirenz, for example), and the herbal product St John's wort. Tell the pharmacist everything you take, and ask them whether it affects your choice. A copper IUD is not expected to be affected by these medicines, so clinicians often prefer it in this situation, and a clinician can also discuss whether other adjustments to a pill regimen are appropriate. Please do not change or stop a prescription yourself. Ask your prescriber.

Starting regular birth control afterward

Ulipristal can interact with hormonal birth control, and the two may make each other less effective. Ulipristal labeling and clinical guidance generally advise waiting five days after a dose before starting or restarting a pill, patch, ring, implant or shot. After that, use condoms or avoid sex for about 7 days with most methods. The backup period can differ for the progestin-only pill, so ask the pharmacist or prescriber to confirm the timing for yours.

After levonorgestrel, no wait is generally needed. Hormonal birth control can usually be started right away, again with condoms or abstinence as backup for about 7 days with most methods. Ask the pharmacist or prescriber to confirm the exact period for your method.

Breastfeeding, age and health conditions

Levonorgestrel is generally considered compatible with breastfeeding, and breastfeeding does not usually need to be interrupted. US labeling for ulipristal (Ella) advises pumping and discarding breast milk for 24 hours after a dose, so ask a pharmacist or clinician about feeding your baby during that time. In the United States, levonorgestrel is sold over the counter to buyers of any age. If you have a health condition, are unsure whether you could already be pregnant, or have had an ectopic pregnancy before, a pharmacist or clinician can help you choose. One observational study of adolescents and young women at a Greek clinic, found in PubMed Central (NIH), describes which emergency contraception methods that group chose. It cannot show how common these needs are in the United States.

What to expect after taking it

Many people have few or no side effects. Some notice:

• Nausea, headache, tiredness, dizziness or breast tenderness
• Lower abdominal cramping
• Spotting or irregular bleeding
• A period that arrives earlier or later than expected

If you vomit soon after taking a pill, call a pharmacist or clinician, because you may need another dose. Do not guess at this yourself. They can tell you based on the product label and the timing.

Your next period may come a few days early or late. Many things can shift a cycle, and stress is one of them, as covered in our article on stress and period delays. Take a pregnancy test if your period is more than a week late, or about three weeks after the unprotected sex, and sooner if you have symptoms or your usual flow is very different. Early signs of pregnancy are discussed in our related article.

What emergency contraception does not do

• It does not protect against sexually transmitted infections. If you are concerned about exposure, ask a clinician about testing and prevention.
• It does not provide ongoing protection. Sex after taking a pill can still lead to pregnancy, so use a condom or another method.
• It is not a guarantee against pregnancy. It lowers the chance, and outcomes vary with timing, body weight, medicines and where you are in your cycle.

Pills can be used more than once, including more than once in a cycle, and a review of repeated use of pre- and post-sex hormonal contraception in PubMed Central (NIH) examined this. Frequent use is a sign to look at a more dependable regular method. Our comparison of IUDs, implants, pills and rings can help you start that conversation.

Ectopic pregnancy: the uncommon risk to know about

Emergency contraception is not known to cause ectopic pregnancy, which is a pregnancy that grows outside the uterus, usually in a fallopian tube. When a pregnancy does occur after a method fails, a small share may be ectopic. MedlinePlus (NIH) notes that ectopic pregnancy can be dangerous because the tube may rupture and bleed internally.

Symptoms can include one-sided pelvic or abdominal pain, vaginal bleeding that is different from your usual, shoulder pain, and dizziness or fainting. These can also have other causes, and one symptom alone does not establish ectopic pregnancy. Sudden severe pain, or any pelvic pain with dizziness, fainting, shoulder pain or heavy bleeding, needs emergency care. Milder pelvic pain together with a missed period or a positive pregnancy test needs evaluation the same day.

Where to get it and what to ask

Levonorgestrel is available at most pharmacies, usually on the shelf, and does not require an ID or prescription. Ulipristal needs a prescription in the United States, which can come from a clinic, telehealth service or primary care office, and some pharmacies can arrange it. Copper IUD placement is done by a clinician such as a primary care doctor, gynecologist or family planning clinic. Call ahead and say it is for emergency contraception, since this is often prioritized.

Cost and coverage vary. Many insurance plans and some clinics cover emergency contraception, and a clinician or pharmacist can tell you what applies to you. A systematic review in PubMed Central (NIH) on improving access to emergency contraception pills notes that availability and awareness remain uneven, though it covers low- and middle-income countries rather than the United States. Asking directly is reasonable.

If the sex was not consensual, you can get emergency contraception, and medical care is available whenever you decide you want it. A clinician or hospital can also discuss infection prevention and support services.

When to Seek Medical Care