Used correctly within the right time window, emergency contraception can reduce the risk of pregnancy after unprotected intercourse by 75 to 99 percent depending on the method chosen. 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. The most important practical fact is that taking emergency contraception sooner is always better, and the copper IUD is the most effective option, but most women reach for a pill because access is faster.
How Emergency Contraception Works
Emergency contraception (EC) works primarily by delaying or preventing ovulation. ACOG emphasizes that EC pills do not interrupt an established pregnancy and are not abortifacient. They prevent pregnancy from happening, not from continuing.
The copper IUD works through a sterile inflammatory response and copper toxicity to sperm and eggs, preventing fertilization. Once a pregnancy is established (when a fertilized egg has implanted), no form of EC is effective. This is why timing matters so much.
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 but retains some efficacy up to 120 hours. Effectiveness is approximately 89 percent when used within 72 hours, dropping with each passing day.
Levonorgestrel works mainly by delaying or inhibiting ovulation. It is less effective if ovulation has already occurred, and recent studies suggest reduced effectiveness in women with body weight above 165 pounds (75 kg). Despite this finding, the FDA has not changed labeling, and clinicians often recommend ulipristal or a copper IUD for women in this weight range.
Side effects are usually mild: nausea, headache, fatigue, breast tenderness, and changes in the timing or character of the next period. Vomiting within 2 hours of taking the pill warrants a repeat dose. Plan B and generics are available over the counter without age restriction.
Ulipristal Acetate (ella)
Ulipristal acetate (ella) is a single 30 mg oral dose, available by prescription only in the United States. It is effective up to 120 hours (5 days) after unprotected sex with little decline in efficacy across that window. Ulipristal is approximately 85 to 95 percent effective at preventing pregnancy.
Mechanistically, ulipristal is a selective progesterone receptor modulator. It can delay ovulation even when the LH surge has already started, making it more effective than levonorgestrel close to ovulation. It also performs better in women with higher body weight, though some reduction in effectiveness occurs above 195 pounds (88 kg).
Side effects are similar to levonorgestrel: headache, nausea, abdominal pain, fatigue, and altered next period. Hormonal contraception (including pills, patches, rings, and hormonal IUDs) should not be started for 5 days after ulipristal because it can interfere with the medication’s mechanism. Conversely, hormonal contraception in the prior 7 days can reduce ulipristal’s effectiveness.
Copper IUD as Emergency Contraception
The copper IUD inserted within 5 days of unprotected intercourse is the most effective form of emergency contraception, with a failure rate below 1 percent. CDC selected practice recommendations place the copper IUD ahead of pills for women who are also seeking ongoing contraception.
The advantage extends beyond the moment: a copper IUD provides ongoing contraception for up to 12 years. The trade-off is that placement requires a clinical visit, which is not always feasible within 5 days. Some health systems have developed same-day IUD insertion pathways for emergency contraception.
The 52 mg levonorgestrel hormonal IUD has emerging evidence as effective emergency contraception, supported by a 2021 randomized non-inferiority trial. ACOG and CDC have updated guidance to allow this use, though many clinicians still default to copper for emergency placement. For women considering long-term IUD use, this dual-purpose option is increasingly attractive.
Comparing the Options
Effectiveness ranking (most to least effective): copper IUD (greater than 99 percent), 52 mg levonorgestrel IUD (~99 percent based on emerging data), ulipristal (85 to 95 percent), levonorgestrel pills (around 89 percent within 72 hours).
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.
Access: levonorgestrel is OTC. Ulipristal requires a prescription (now available through telehealth and some pharmacist-prescribed channels in select states). The copper IUD requires a clinical visit.
Body weight considerations: levonorgestrel may be less effective above 165 pounds. Ulipristal performs better at higher weights but loses some efficacy above 195 pounds. The copper IUD is unaffected by weight.
When to Use Emergency Contraception
EC is appropriate after unprotected intercourse, condom failure, missed birth control pills, late or missed depo-provera injection, sexual assault, or any concern about pregnancy risk. Earlier use is always better. There is no harm in taking EC unnecessarily, so erring on the side of using it is reasonable.
EC is not a regular contraceptive method. Repeated use is safe but less effective and more expensive than ongoing contraception. After EC use, transitioning to a reliable ongoing method (a birth control pill, IUD, implant, or other) reduces the need for future emergencies.
What to Expect After EC
The next period may come earlier or later than expected. If the period is more than 1 week late, take a pregnancy test. EC has no long-term effect on fertility, future pregnancies, or hormonal cycles. Some women experience nausea, irregular spotting, or mild cramping in the days after.
If pregnancy occurs despite EC, current evidence does not show increased risk of birth defects or pregnancy complications. Continued pregnancy is normal.
Cost and Access
Levonorgestrel EC retails for $40 to $50 per dose at most pharmacies. Generic versions and online pharmacies (such as AfterPill, Stix) often cost $20 to $30. ACA preventive services coverage means many insured patients can obtain EC pills at no cost with a prescription, even though they are also OTC.
Ulipristal (ella) costs about $50 to $90 with a prescription. Some online services prescribe and ship within 24 to 48 hours, including KwikMed, Wisp, and Nurx. Copper IUD insertion is usually fully covered by insurance under ACA preventive coverage. Without insurance, IUD placement costs $800 to $1,800 total. Title X clinics and Planned Parenthood offer sliding-scale services.
Stocking advance EC (“just in case” supply) is reasonable for women with episodic risk. The shelf life of levonorgestrel EC is typically 4 years. The intersection of contraception with broader women’s health needs often shapes which method makes sense.
When to See a Doctor
Schedule a follow-up if the next period is more than a week late, you have severe abdominal pain (especially one-sided), heavy or unusual bleeding, or symptoms of pregnancy. EC failure is uncommon but occurs, and ectopic pregnancy is a concern with any pregnancy regardless of EC use. Consider a visit to discuss ongoing contraception options if you have used 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 was confirmed or suspected. These symptoms can indicate 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. Sooner is always better regardless of which method.
Can I use emergency contraception multiple times?
Yes, but it is less effective and more expensive than regular contraception. Repeated EC use is medically safe but signals a need for an ongoing contraceptive plan. Levonorgestrel can be taken multiple times in the same cycle if needed.
Will emergency contraception cause an abortion?
No. EC works by preventing or delaying ovulation and preventing fertilization. It does not affect an established pregnancy. Mifepristone (the abortion pill) is a separate medication used for pregnancy termination.
Does ella interact with other birth control?
Yes. Hormonal contraception in the prior 7 days can reduce ulipristal’s effectiveness, and starting hormonal contraception within 5 days after ulipristal can reduce its effectiveness too. Use barrier methods for those windows. Levonorgestrel does not have these interactions.
The Practical Takeaway
Emergency contraception is most effective when used quickly, and the copper IUD is the gold standard if access allows. Levonorgestrel pills are available over the counter and effective within 72 hours, with reduced efficacy at higher body weight and as time passes. Ulipristal works for the full 5-day window with consistent effectiveness and is increasingly accessible through telehealth. None of these methods replaces ongoing contraception, but they are important safety nets when something goes wrong. Knowing your options in advance, including stocking a dose at home, makes the time-sensitive decision much simpler.