Emergency Contraception: How It Works and How Soon It Is Needed

A condom split, pills were missed, or there was sex without contraception. There are three options: a copper IUD, an ulipristal pill and a levonorgestrel pill. Each has its own time window, and all work better the sooner they are used. The pills work by delaying ovulation; they do not end a pregnancy. The copper IUD is the most effective of the three.

When to see a doctor

Right away: after sexual assault. If you are in danger, call 999 in the UK, 911 in the US, 112 in the EU. Emergency contraception and HIV prevention depend on hours; an examination and protection against other infections belong to the same visit — see STI testing: when and what. In the UK, sexual assault referral centres (SARCs), A&E and sexual health clinics help whether or not you decide to go to the police (NHS); Rape Crisis (England and Wales): 0808 500 2222. In the US, the National Sexual Assault Hotline: 800-656-4673. Also right away: sudden severe pain low in the abdomen, dizziness, fainting or pain in the tip of the shoulder after emergency contraception or with a late period — possible ectopic pregnancy (NHS).

The same day: emergency contraception itself — as early as you can. Pain on one side low in the abdomen with spotting or brown discharge, even after a negative early test: do not wait for a test weeks later; in the UK, call 111 (NHS). Possible exposure to HIV: post-exposure prophylaxis (PEP) must start within hours to days.

Routinely: a pregnancy test if your next period is more than 7 days late or lighter or shorter than usual; a test 3 weeks after the last unprotected sex rules pregnancy out reliably (FSRH, UK). STI tests at the right time after the contact. A regular method from now on.

What is happening

Sperm survive in the body for about 5 days, so sex days before ovulation can lead to pregnancy. The pills delay the release of an egg until the sperm have died. Levonorgestrel works only if taken before the hormone surge that triggers ovulation has begun. Ulipristal can still delay ovulation after the surge has started, though not at its peak (FSRH). Once ovulation has happened, the pills no longer work.

They do not stop a fertilised egg from implanting and do not end or harm an existing pregnancy (WHO); in 2022 the FDA removed implantation wording from the US Plan B label. This is not an abortion pill.

The copper IUD works differently: copper is toxic to sperm and eggs, and if fertilisation happens anyway, the reaction in the womb lining prevents implantation (FSRH) — worth knowing if you have religious or ethical questions.

	Copper IUD	Ulipristal	Levonorgestrel
Window (FSRH, UK)	up to 5 days after the first unprotected sex in the cycle, or up to 5 days after the earliest likely ovulation	up to 120 hours	licensed up to 72 hours; evidence suggests it does not work after 96 hours
Effectiveness	highest: under 1 in 1000 get pregnant	higher than levonorgestrel	lower, especially near ovulation
Weight	no known effect	may be less effective at higher weight	more affected by weight

WHO's illustration: of 100 women with one unprotected act in the second or third week of the cycle, about 8 would become pregnant without emergency contraception, about 1 with levonorgestrel and fewer than 1 with ulipristal. That is an average; close to ovulation your own risk is higher. Emergency contraception lowers the risk, not to zero.

Weight. Data conflict: one analysis of UK and US trials found more pregnancies after levonorgestrel at higher weight, a pooled WHO analysis found no effect, and the European Medicines Agency (2014) called the data insufficient. In the UK, at over 70 kg or a BMI over 26, a pharmacist or doctor may suggest a different option — a copper IUD first, or ulipristal (FSRH). Do not change the dose yourself.

What to do

Act now: a pharmacy for pills, a sexual health clinic or GP for an IUD. Take pills exactly as your pack's leaflet says: the number of tablets differs between products.
Vomiting within 2 hours of levonorgestrel or 3 hours of ulipristal means another dose is needed (WHO); ask the pharmacist.
Afterwards you still need contraception: the pills protect only against sex before you took them, and ovulation may come later. A hormonal method can start straight after levonorgestrel, but only 5 days after ulipristal, which it can weaken. In both cases use condoms until the new method takes effect — usually 7 days, 2 for traditional or desogestrel progestin-only pills (FSRH, UK; CDC SPR, US).
It can be used again, even in the same cycle, with no known health risks, but it is less reliable than a regular method (WHO).
Breastfeeding: UK guidance (FSRH, 2026) no longer advises a pause after ulipristal; limited data on levonorgestrel show no harm. Some leaflets still advise a pause — ask the pharmacist.
Where: in the US, levonorgestrel (Plan B One-Step and generics) is sold without a prescription or age limit since 2013 (FDA); ulipristal (ella) needs a prescription. In the UK both pills are sold in pharmacies and are free from sexual health clinics and most GPs; in England, most pharmacies have provided them free since October 2025 (NHS England).

When this is not the case

Some medicines, now or in the last 28 days — rifampicin, rifabutin, several epilepsy medicines, St John's wort — can make both pills less effective; the copper IUD is preferred (FSRH). See never mix supplements and medications on autopilot.
A progestin (your usual pill, for example) in the previous 7 days may weaken ulipristal (FSRH).
Severe asthma treated with oral steroids: ulipristal is not advised (FSRH).
Already pregnant: the pills will not work and will not harm the pregnancy (WHO).
Choosing a regular method: how effective each contraceptive method is.

Sources

Checked: 24 September 2026