Emergency contraception

The three options

Both pills work primarily by delaying or preventing ovulation. Neither ends an established pregnancy; both are ineffective once ovulation has already occurred, which is why timing matters more than the label's outer limit. Take as soon as possible — every hour counts.

Choosing between them

• Copper IUD is the most effective option in every published comparison, and Cleland et al.'s review of 42 studies covering over 7,000 insertions found a pregnancy rate of 0.09 percent. It also gives 5–10 years of ongoing contraception. Requires a clinic appointment; ask about pain relief.
• Ulipristal acetate outperforms levonorgestrel, particularly in the 72–120 hour window and around the time of ovulation. Glasier et al.'s meta-analysis (2010, Lancet) found roughly half the pregnancy rate of levonorgestrel.
• Levonorgestrel is the most accessible. Its efficacy declines steeply across the 72 hours and it is materially less effective after 72 hours.

Two things that change the choice

Body weight. Both pills are less effective at higher body weight. Levonorgestrel efficacy declines from around 70 kg and appears substantially reduced above 80 kg or BMI over 30; ulipristal is affected above roughly 88 kg or BMI over 35. UK guidance advises considering a double dose of levonorgestrel (3 mg) above 70 kg where ulipristal and an IUD are unsuitable, and the copper IUD is unaffected by weight — which makes it the preferred option at higher weights.

Hormonal contraception interaction. Ulipristal is a progesterone receptor modulator, so progestogen blocks it and it blocks hormonal contraception. Do not take ulipristal within 7 days of taking progestogen, and wait 5 days after ulipristal before starting or restarting hormonal contraception, using condoms in the interim. Levonorgestrel has no such interaction and hormonal contraception can be restarted immediately, with 7 days of additional precautions (2 days for the progestogen-only pill, 7 for combined).

Enzyme-inducing drugs — some anti-epileptics, rifampicin, St John's wort — reduce the effectiveness of both pills. A copper IUD is recommended instead.

After taking it

• Vomiting within 3 hours of either pill means the dose should be repeated.
• The next period may be early, late, heavier or lighter. Take a pregnancy test if it is more than 7 days late.
• Emergency contraception does not protect against later acts in the same cycle. Start or resume ongoing contraception.
• Consider STI testing after the appropriate window; consider HIV PEP within 72 hours if there is any HIV exposure risk.

Myths worth correcting

• It is not the abortion pill. Mifepristone and misoprostol are different medicines with a different purpose.
• It does not cause infertility, and it does not accumulate harm with repeated use. Repeated use is less effective than ongoing contraception, which is a reason to arrange ongoing contraception, not a reason to withhold emergency contraception.
• There is no limit on how many times it can be taken. It can be used more than once in a cycle if needed.
• It is not restricted by age in most countries.

Sources

• Glasier AF et al. "Ulipristal acetate versus levonorgestrel for emergency contraception: a randomised non-inferiority trial and meta-analysis." Lancet, 2010.
• Cleland K et al. "The efficacy of intrauterine devices for emergency contraception: a systematic review of 35 years of experience." Human Reproduction, 2012.
• Faculty of Sexual and Reproductive Healthcare. FSRH Clinical Guideline: Emergency Contraception, current edition.
• WHO. Emergency contraception fact sheet and Medical Eligibility Criteria, current editions.
• Festin MPR et al. "Effectiveness of emergency contraceptive pills and body weight." Contraception, 2017.

Last reviewed: August 2026.