Emergency contraception prevents pregnancy after unprotected sex mainly by delaying or blocking ovulation — the release of an egg — and it has no effect on a pregnancy that has already implanted, per ACOG's 2015 practice bulletin on emergency contraception. Three documented options exist: the copper IUD, the most effective method available for this purpose; ulipristal acetate, a prescription pill usable up to 120 hours after sex; and levonorgestrel, an over-the-counter pill that works best within 72 hours. Speed is the variable you control most.
This article publishes information, not medical advice. It explains what each method does, when each window closes, and what the evidence says about the myths that surround these products. For an actual situation, a pharmacist or clinician can confirm which option fits your timing and history.
What are the options, exactly?
ACOG's practice bulletin and the CDC's contraception guidance describe the following.
- Copper IUD. Inserted by a clinician up to five days after sex, it is the most effective emergency contraceptive, with a failure rate below 0.1 percent in the studies summarized by ACOG. It then provides ongoing contraception for up to a decade. It requires an appointment, which is its main constraint.
- Ulipristal acetate. A single prescription pill, taken up to 120 hours after sex. In clinical data summarized by ACOG, it remains more effective than levonorgestrel in the 72-to-120-hour window and shows less decline in effect over time.
- Levonorgestrel. Available over the counter in US pharmacies without age restriction since a 2013 FDA decision, this single-dose pill works best within 72 hours and loses effectiveness afterward. It is less effective at higher body weights: ACOG's bulletin notes that efficacy was reduced among people with obesity in pooled analyses, without a documented threshold below which it fails — the guidance supports use regardless, with awareness of the limitation.
An older combined-estrogen regimen, sometimes called the Yuzpe method, exists as a fallback where other options are unavailable; per ACOG it is less effective and more associated with nausea, which is why the newer options have displaced it.
How do the pills actually work?
Both ulipristal and levonorgestrel act on ovulation, the narrow window when pregnancy can begin. They delay or inhibit the hormonal surge that releases an egg from the ovary. If ovulation has not yet happened, delaying it past the lifespan of sperm — which survive up to about five days in the reproductive tract — prevents fertilization. If ovulation has already occurred, per ACOG's bulletin, the pills do not prevent pregnancy; this is the biological reason timing matters and why no pill option matches the copper IUD's consistently low failure rate. The copper IUD works differently: copper is toxic to sperm and eggs, preventing fertilization outright, and it keeps working as ordinary contraception afterward.
Does emergency contraception cause abortion?
No, per the medical definition in ACOG's guidance and the FDA labeling for these products. Pregnancy begins with implantation of a fertilized egg; the pills work before fertilization by delaying ovulation and have no documented effect on an implanted pregnancy, and ACOG states plainly that emergency contraception does not interrupt pregnancy. It also does not harm a future pregnancy, does not increase ectopic pregnancy risk, and does not affect long-term fertility, per the same bulletin. These are documented, dated findings, repeated across successive reviews by the agency and professional bodies.
Why can the same method seem more or less effective?
Three documented reasons. Timing: levonorgestrel's effectiveness declines with each passing day after sex, per the ACOG bulletin. Body weight: the pooled analyses noted above found reduced levonorgestrel efficacy at higher BMI, with ulipristal showing a smaller decline. Ongoing risk: a single dose protects against one act of sex, not the rest of the cycle — ovulation is delayed, not canceled, so pregnancy remains possible from sex later in the same cycle, which is why guidance recommends starting or resuming regular contraception immediately and using backup for the first week of a new hormonal method, per the CDC's selected practice recommendations.
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What are the side effects?
Per ACOG and the product labeling, the pills commonly cause nausea, and sometimes headache, dizziness, cramping, or a change in the next period's timing — earlier or later by days in many users. No serious adverse effects are documented from single use, and no contraindication to levonorgestrel exists in guidance, meaning it can be used by people who cannot take regular hormonal contraception. If vomiting occurs within a few hours of taking a pill, guidance suggests contacting a pharmacist or clinician about whether to repeat it. An unexpectedly light or absent next period is a reason to take a pregnancy test.
Does emergency contraception affect future fertility?
No effect is documented, per ACOG's bulletin. Fertility returns with the next cycle — a person can ovulate as early as the same month after taking it, and can become pregnant from sex in that cycle. This is worth stating twice: the same mechanism that delays ovulation once has no carry-over, which is why emergency contraception is a safety net, not a method, and why the guidance recommends moving promptly to a regular method. For someone finishing a worrying week, that transition conversation with a clinician or pharmacist is the logical next step.
When to talk to a clinician
Act within days, not weeks: after unprotected sex or a contraceptive failure, per the windows above — the copper IUD and ulipristal extend the usable window to five days, while levonorgestrel is best inside three. Contact a clinician promptly if your next period is more than a week late or unusually light, if you have severe abdominal or shoulder pain after using emergency contraception — symptoms that require urgent evaluation for ectopic pregnancy — or if you vomit soon after a dose. A pharmacist can counsel on the over-the-counter option without an appointment, under US rules as of the mid-2020s.
The effective window is measured in hours and days. Speed, method choice, and a plan for afterward are the three levers the evidence actually supports.
Frequently asked questions
How late is too late for the morning-after pill? Ulipristal acetate is documented as usable up to 120 hours — five days — after sex, per ACOG, though effectiveness is best the sooner it is taken. Levonorgestrel's window is shorter in practice: its documented effectiveness declines steadily after 72 hours. Past five days, the copper IUD remains an option if a clinician can place it in time. When in doubt, a pharmacist or clinician can time the options against your situation.
Can I take emergency contraception more than once? Yes. Per ACOG, no harm from repeated use is documented, and there is no medical limit on frequency. But repeated use usually signals a method gap rather than a pill problem: each use is less reliable than regular contraception, and guidance treats repeat requests as a natural moment to discuss a longer-lasting method, without judgment.
Does the copper IUD work as emergency contraception if I have never had children? Yes — per ACOG's guidance on long-acting reversible contraception, IUDs are first-line options for most people, including those who have not given birth. As an emergency method it is the most effective option available, and it converts a crisis appointment into a decade of contraception. The insertion requires a clinician, so timing and access are the practical constraints.
Will emergency contraception show up on a pregnancy test? No. The pills contain hormones that delay ovulation; they neither cause nor mask a pregnancy, and they do not affect test results. A pregnancy test measures a hormone produced after implantation. If your next period is late, light, or unusually short after taking emergency contraception, guidance recommends testing — that is the documented follow-up step, not repeated pill doses.
Is emergency contraception the same as the abortion pill? No, and the distinction is documented at the pharmacological level. The abortion pill — mifepristone, used with misoprostol — ends an established pregnancy and works after implantation. Emergency contraception pills act before fertilization by delaying ovulation and have no documented effect on an implanted pregnancy, per ACOG's 2015 practice bulletin. They are different medicines with different mechanisms, labels, and uses.