Used correctly within the right time window, emergency contraception can substantially reduce the risk of pregnancy after unprotected intercourse – the exact reduction depends on the method, how quickly it is used, and where you are in your cycle. Three primary options are available in the United States: levonorgestrel pills (Plan B and generics), ulipristal acetate (ella), and the copper IUD. Each has a different mechanism, time window, and effectiveness profile. Two facts matter most in practice: taking emergency contraception sooner is always better, and the copper IUD is the most effective option, even though most people reach for a pill because access is faster. This article is general education, not medical advice – if you can, talk to a clinician or pharmacist quickly.
How Emergency Contraception Works
Emergency contraception (EC) works primarily by delaying or preventing ovulation. The American College of Obstetricians and Gynecologists (ACOG) and the MedlinePlus resource from the U.S. National Library of Medicine both emphasize that EC pills prevent pregnancy from starting – they do not interrupt an established pregnancy and are not abortion pills. In other words, EC prevents pregnancy from happening; it does not end one that has already implanted.
The copper IUD works differently, through a sterile inflammatory response and a copper effect that is toxic to sperm and eggs, preventing fertilization (and, if placed for EC, it can also affect implantation). Once a pregnancy is established – when a fertilized egg has implanted – no form of EC will end it. This is why timing matters so much, and why sooner is always better.
Importantly, no form of emergency contraception protects against sexually transmitted infections (STIs). If there is any STI risk, ask a clinician about testing and, where appropriate, preventive treatment.
Levonorgestrel (Plan B and Generics)
Levonorgestrel-based EC (Plan B One-Step, Take Action, My Way, AfterPill, and many other generics) is a single 1.5 mg oral dose. It is most effective within 72 hours of unprotected sex and retains some effectiveness up to 120 hours, but effectiveness declines with each passing day, so take it as soon as possible. Effectiveness estimates vary across studies; treat any single percentage as an approximation and verify current figures.
Levonorgestrel works mainly by delaying or inhibiting ovulation. It is less effective if ovulation has already occurred, and evidence indicates it becomes less effective at higher body weight or BMI – studies have suggested reduced effectiveness above roughly 165 pounds (about 75 kg) or a BMI in the overweight-to-obese range. The FDA has not changed the Plan B label over this, but ACOG and many clinicians suggest ulipristal (ella) or a copper IUD for people at higher body weight, when access allows.
Side effects are usually mild and short-lived: nausea, headache, fatigue, breast tenderness, and changes in the timing or character of the next period. If you vomit within about 2 hours of taking the pill, contact a pharmacist or clinician, as a repeat dose may be needed. Plan B and its generics are available over the counter without an age restriction.
Ulipristal Acetate (ella)
Ulipristal acetate (ella) is a single 30 mg oral dose that is available by prescription only in the United States (including through many telehealth services and, in some states, pharmacist prescribing). It is effective up to 120 hours (5 days) after unprotected sex with relatively little decline across that window, and it is generally considered more effective than levonorgestrel, particularly in the days closest to ovulation.
Mechanistically, ulipristal is a selective progesterone receptor modulator. It can delay ovulation even after the LH surge has begun, which is why it tends to outperform levonorgestrel near ovulation. It also works better than levonorgestrel at higher body weight/BMI, although its effectiveness still declines at the highest weights (some data suggest reduced effect above roughly 195 pounds / about 88 kg). At very high body weight, a copper IUD remains the most reliable choice.
Side effects resemble levonorgestrel: headache, nausea, abdominal pain, fatigue, and an altered next period. Because ulipristal acts on progesterone receptors, hormonal contraception (pills, patches, rings, hormonal IUD) should generally not be started for about 5 days after taking ella, since it can blunt ella’s effect; conversely, hormonal contraception taken in the days just before ella can reduce ella’s effectiveness. Use a barrier method in the interim and confirm timing with a clinician or pharmacist.
Copper IUD as Emergency Contraception
A copper IUD inserted within 5 days of unprotected intercourse is the most effective form of emergency contraception, with a failure rate below 1 percent. The CDC U.S. Selected Practice Recommendations support the copper IUD as the most effective EC method, and it is especially attractive for anyone who also wants to start ongoing contraception.
The advantage extends beyond the moment: a copper IUD can provide ongoing, highly effective contraception for many years (commonly up to 10-12 years, depending on the device). Its effectiveness as EC is also not reduced by body weight. The trade-off is that placement requires a clinical visit, which is not always feasible within the 5-day window; some health systems now offer same-day insertion pathways for EC.
The 52 mg levonorgestrel hormonal IUD has emerging evidence as effective emergency contraception, supported by a randomized non-inferiority trial. ACOG and CDC guidance now recognize this option in appropriate circumstances, though many clinicians still default to the copper IUD for emergency placement. For people considering long-term IUD use, a dual-purpose device is increasingly appealing – discuss which fits your situation with a clinician.
Comparing the Options
Effectiveness ranking (most to least effective, approximate): copper IUD (failure rate under 1 percent), 52 mg levonorgestrel IUD (highly effective based on emerging data), ulipristal (ella), then levonorgestrel pills – with levonorgestrel dropping off most sharply after 72 hours. Exact percentages vary by study and by how soon EC is used, so treat these as estimates.
Time window: copper IUD up to 5 days, ulipristal up to 5 days, levonorgestrel up to 5 days but with sharply declining effectiveness after 72 hours. In every case, earlier is better.
Access: levonorgestrel is OTC for all ages. Ulipristal requires a prescription (increasingly available through telehealth and pharmacist prescribing in some states). The copper IUD requires a clinical visit for placement.
Body weight/BMI considerations: levonorgestrel may be less effective at higher body weight or BMI; ulipristal performs better at higher weights but still loses some efficacy at the highest weights; the copper IUD is unaffected by weight and is the most reliable option across the board.
When to Use Emergency Contraception
EC is appropriate after unprotected intercourse, condom failure, several missed birth control pills, a late or missed injectable (such as depo-medroxyprogesterone), sexual assault, or any real concern about pregnancy risk. Earlier use is always better. There is no meaningful harm in using EC when it later turns out you did not need it, so erring on the side of using it is reasonable. If you have experienced sexual assault, consider seeking care that can also address STI prevention and support.
EC is not a regular contraceptive method. Repeated use is medically safe but less effective and more expensive than ongoing contraception. After using EC, transitioning to a reliable ongoing method (a birth control pill, IUD, implant, injection, or other option) reduces the chance of future emergencies.
What to Expect After EC
Your next period may come earlier or later than expected. If it is more than about a week late, take a pregnancy test. EC has no known long-term effect on fertility, future pregnancies, or hormonal cycles. Some people experience nausea, irregular spotting, or mild cramping in the days afterward.
If pregnancy occurs despite EC, current evidence does not show an increased risk of birth defects or pregnancy complications from the EC itself, and a continuing pregnancy typically develops normally. If you have concerns, speak with a clinician.
Cost and Access
Levonorgestrel EC commonly retails for about $40 to $50 per dose at pharmacies, while generic and online options (such as AfterPill or Stix) often cost roughly $20 to $30 – verify current prices. Under ACA preventive-services rules, many insured patients can obtain EC pills at no cost with a prescription, even though levonorgestrel is also sold OTC.
Ulipristal (ella) generally costs about $50 to $90 with a prescription; some telehealth services prescribe and ship it within a day or two (for example, Wisp and Nurx), which matters given the time-sensitive window. Copper IUD insertion is usually fully covered by insurance under ACA preventive coverage; without insurance, IUD placement can cost several hundred to a couple thousand dollars all-in. Title X clinics and Planned Parenthood offer sliding-scale services. Prices change, so confirm current costs directly.
Keeping an advance supply of levonorgestrel EC (“just in case”) is reasonable for people with episodic risk; check the product’s expiration date periodically. The intersection of contraception with broader women’s health needs often shapes which method makes the most sense over time.
When to See a Doctor
Follow up with a clinician if your next period is more than a week late, or if you have severe abdominal pain (especially one-sided), heavy or unusual bleeding, or symptoms of pregnancy. EC failure is uncommon but possible, and ectopic pregnancy is a concern with any pregnancy regardless of EC use. Also consider a visit to discuss ongoing contraception if you have needed EC recently.
When to seek emergency care: Call 911 or go to the nearest emergency room if you experience severe one-sided pelvic or abdominal pain, dizziness or fainting, shoulder-tip pain, or heavy vaginal bleeding after EC – particularly if pregnancy is confirmed or suspected. These can be signs of an ectopic pregnancy, which is a medical emergency.
Frequently Asked Questions
How soon after sex should I take emergency contraception?
As soon as possible. Levonorgestrel is most effective within 72 hours and loses effectiveness rapidly after that. Ulipristal and the copper IUD remain effective up to 120 hours, but earlier is still better. Whichever method you choose, sooner is always better.
Can I use emergency contraception more than once?
Yes. Repeated EC use is medically safe, but it is less effective and more costly than regular contraception, and it signals a need for an ongoing plan. Levonorgestrel can be used more than once in the same cycle if needed – a clinician or pharmacist can help you choose an ongoing method.
Will emergency contraception cause an abortion?
No. EC works by preventing or delaying ovulation and preventing fertilization; it does not end an established pregnancy and is not the abortion pill. Mifepristone (used with misoprostol for medication abortion) is an entirely separate medication and a different process.
Does ella interact with other birth control?
Yes. Hormonal contraception taken in the days before ella can reduce its effectiveness, and starting hormonal contraception within about 5 days after ella can reduce ella’s effectiveness too. Use a barrier method during those windows. Levonorgestrel (Plan B) does not have these particular interactions.
Does emergency contraception protect against STIs?
No. No form of EC protects against sexually transmitted infections. If there is STI risk, ask a clinician about testing and any recommended preventive treatment.
The Practical Takeaway
Emergency contraception is most effective when used quickly, and the copper IUD is the gold standard when access allows. Levonorgestrel pills (Plan B) are available over the counter and work best within 72 hours, with reduced effectiveness at higher body weight and as time passes. Ulipristal (ella) is prescription-only, covers the full 5-day window more consistently, performs better at higher weights, and is increasingly accessible through telehealth. None of these replaces ongoing contraception, and none protects against STIs – but they are important safety nets. Knowing your options in advance, including keeping a dose at home, makes a time-sensitive decision much simpler.
This article is general education and is not medical advice. Emergency contraception is time-sensitive: use it or place an IUD as soon as possible after unprotected sex. The copper IUD is the most effective option, ella (ulipristal) is prescription-only, and no EC method ends an existing pregnancy or protects against STIs. Effectiveness, weight/BMI effects, and access change – verify current guidance and talk to a clinician or pharmacist about your situation.
Sources
- American College of Obstetricians and Gynecologists (ACOG) — emergency contraception FAQ and practice guidance
- CDC — U.S. Medical Eligibility Criteria (US MEC) and U.S. Selected Practice Recommendations (US SPR) for Contraceptive Use
- U.S. Food and Drug Administration (FDA) — Plan B One-Step and ella (ulipristal acetate) labeling
- MedlinePlus (U.S. National Library of Medicine) — emergency contraception