Emergency contraception pills: timing, body weight, interactions and follow-up

How the pills work and why timing matters

Levonorgestrel and ulipristal both act before ovulation, delaying or inhibiting release of an egg. They do not work if the person is already pregnant and do not end an existing pregnancy. Because they act on ovulation, effectiveness depends on how quickly they are taken after unprotected sex or contraceptive failure.

Levonorgestrel is labelled for use within 72 hours and ulipristal within 120 hours, but both work best as early as possible. A copper intrauterine device fitted within five days is the most effective form of emergency contraception and can continue as long-term contraception, so it belongs in the conversation even when someone asks for a pill.

Nurses are often the first contact, so the priority is to establish timing quickly: when unprotected sex occurred, whether there were other episodes in the same cycle, and the date of the last period. Delay reduces effectiveness, so a request should not wait for a routine appointment. Many settings, including pharmacies and sexual health clinics, can supply it promptly.

Body weight and what to tell people

Evidence on body weight is mixed, and US product labelling does not set a weight limit for oral emergency contraception. UK sexual health guidance advises telling people that higher weight or body mass index may reduce oral emergency contraception effectiveness, particularly levonorgestrel, while stressing that a pill is better than no method.

That guidance suggests offering a copper IUD where possible, since its effectiveness is not thought to depend on weight, then considering ulipristal, with any levonorgestrel option decided by the prescriber or pharmacist under local protocol. Nurses present this as information for the person's choice under local protocol, without discouraging use of the pill when it is the available option.

Interactions and restarting contraception

Enzyme-inducing medicines, such as rifampin, carbamazepine and other anti-seizure drugs, efavirenz and St John's wort, can reduce the effectiveness of both pills, so take a medication history. For these patients the copper IUD avoids the interaction entirely, and the prescriber or pharmacist advises on oral options.

Ulipristal and progestins can work against each other. The ulipristal label advises waiting at least five days before starting hormonal contraception, using a barrier method until the next period, and not taking levonorgestrel emergency contraception within five days. Any repeat use within the same cycle is a prescriber or pharmacist decision. Clarify exactly when ongoing contraception will start.

Side effects, vomiting and follow-up

Nausea, headache, breast tenderness, fatigue and changes in the timing of the next period are common. If vomiting occurs within a few hours of a dose, a repeat dose may be needed, so teach the person to seek advice promptly. Emergency contraception does not protect against sexually transmitted infections, so testing and condoms are part of the discussion.

Teach that a period more than a week late needs a pregnancy test, and that severe lower abdominal pain in the weeks afterwards needs urgent assessment because it may signal ectopic pregnancy. Emergency contraception is not intended for routine use. Discuss a regular method, and where sex was not consensual, follow safeguarding and support pathways.

Worked scenario: a request on day four

A hypothetical 22-year-old asks for levonorgestrel four days after a condom broke. She takes carbamazepine for epilepsy. Options are to give levonorgestrel because it is the pill she asked for, to tell her it is too late for any emergency contraception, or to explain the options and refer for a copper IUD or prescriber review.

Explaining and referring is best. Day four is outside the levonorgestrel window but within the window for ulipristal and a copper IUD, and carbamazepine may reduce the effect of both pills, which makes the IUD particularly suitable. Saying it is too late is the dangerous distractor. Document the timing and medication history.

Sources and further reading

DailyMed: ella (ulipristal acetate) prescribing information. 120-hour window, ovulation delay, CYP3A4 inducers, progestin interaction, five-day wait, vomiting, ectopic warning and late period.

MedlinePlus: Levonorgestrel. 72-hour timing, no STI protection, enzyme-inducer interactions, side effects, ectopic pain and late period testing.

NHS: Emergency contraception. Copper IUD as most effective option within five days and earlier use being more effective.

The Pharmaceutical Journal: FSRH guidance on body weight and emergency contraception. UK guidance that higher weight may reduce oral EC effectiveness, copper IUD unaffected, and the order of alternatives.