Emergency Contraception: The Three Options and Their Timing

What emergency contraception actually does

It is not an abortion pill. It works before a pregnancy is ever established. The single most important idea — clinically and ethically — is the mechanism. Hormonal emergency contraception works mainly by delaying or inhibiting ovulation: no egg is released, so there is nothing for sperm to fertilise. The copper intrauterine device (IUD) works differently, by releasing copper ions that are toxic to sperm and ova and by preventing implantation. None of these methods disrupts an established pregnancy: they have no effect once implantation has occurred, which is why they are not abortifacients and why they simply fail if ovulation has already happened. This distinguishes them sharply from medical abortion (mifepristone followed by misoprostol) — different drugs, a different purpose, and a later point in the process. Understanding emergency contraception as an ovulation-timing intervention explains almost everything that follows, including why "sooner is better" is not a slogan but pharmacology.

Option 1: the copper IUD — the most effective by far

The copper intrauterine device is the most effective form of emergency contraception, and it is under-offered largely because people ask for a pill. Copper ions are directly toxic to sperm and ova, impairing motility and fertilisation, and they provoke a local inflammatory reaction in the endometrium that prevents implantation. Because it does not depend on catching the ovulation window, its efficacy is far higher and far more consistent than any tablet — the failure rate is well under 1%. It can be inserted up to 5 days after unprotected sex, or up to 5 days after the earliest estimated date of ovulation, which sometimes buys more time than the strict 120-hour rule. And it carries a decisive bonus: once in place it provides ongoing, highly effective contraception for years, turning an emergency into a long-term solution in a single visit. Its efficacy is not reduced by body weight or by enzyme-inducing drugs — which is exactly why it is the preferred choice when those factors are in play. The overlap with the intrauterine methods taught in the Progestogen contraception chapter is deliberate: the same device family, used here for a different indication.

Option 2: ulipristal acetate — the more effective pill

A selective progesterone-receptor modulator that delays ovulation even close to the LH surge. Ulipristal acetate is a selective progesterone-receptor modulator (SPRM): it binds the progesterone receptor and blunts the hormonal trigger for ovulation. Its edge over levonorgestrel is timing — it can still delay ovulation even when the luteinising-hormone (LH) surge has already begun, the exact mid-cycle moment when the risk of pregnancy peaks and levonorgestrel is already losing its grip. It is licensed and effective up to 120 hours (5 days) after unprotected sex, and is the more effective oral option, particularly mid-cycle. The mechanism connects to the Endocrine section, where progesterone-receptor signalling and the LH surge of the menstrual cycle are taught in full, and to the Menstrual and gynaecology chapter, where ulipristal reappears in a different role — shrinking uterine fibroids. But its receptor pharmacology creates a signature interaction: because it competes at the progesterone receptor, ulipristal and progestogens blunt each other. Taking a progestogen too soon can reduce ulipristal's efficacy, and ulipristal can reduce the effect of progestogen contraception — so ongoing progestogen contraception should not be started (or resumed) until 5 days after ulipristal. Its efficacy is also reduced by enzyme-inducing drugs, and it should be used with caution in severe asthma poorly controlled by oral steroids.

Option 3: levonorgestrel — the familiar tablet, and its limits

Levonorgestrel is a progestogen, and the one most people picture when they say "morning-after pill." It also works by delaying ovulation, but it does so less reliably once the LH surge is underway, which is why it is the least effective of the three and why its efficacy falls the later it is taken. It is licensed up to 72 hours (3 days) after unprotected sex — a shorter window than ulipristal's five days. Two adjustments matter: for higher body weight (or a high BMI) and for people taking enzyme-inducing drugs, the efficacy of the standard dose is reduced, and a double dose is used to compensate — though even then the copper IUD is the better answer if it is acceptable. Levonorgestrel has one practical advantage over ulipristal: because it is a progestogen rather than a receptor antagonist, ongoing progestogen or combined hormonal contraception can be quick-started immediately after it, without the 5-day wait ulipristal demands.

The visit isn't over when the method is given. Good emergency-contraception counselling always looks past the crisis to the next cycle. Sooner is better — take or insert it as early as possible. Warn that a hormonal method can fail if ovulation has already happened, and that it does not protect against later acts in the same cycle. Advise a pregnancy test if the next period is more than about a week late, or is unusually light or short. And use the moment to quick-start ongoing contraception so the same emergency doesn't recur — remembering the ulipristal caveat: wait 5 days before a progestogen or combined method after ulipristal, but start immediately after levonorgestrel, with condom cover until the new method is reliable. The combined and progestogen options themselves — how they are chosen and the UKMEC contraindications — belong to the Combined hormonal contraception and Progestogen contraception chapters.