Your contraception guideHow effective is emergency contraception?
It depends which type of emergency contraception you use and how soon you use it.
There are two methods of emergency contraception:
- the emergency contraceptive pill – or morning-after pill – known as Levonelle or ellaOne
- the copper IUD (intrauterine device, or coil)
Emergency contraception prevents pregnancy if you haven't used contraception or think your usual method of contraception has failed.
Both types of emergency contraception are effective at preventing pregnancy if they are used soon after unprotected sex.
Less than 1% of women who use the IUD get pregnant, whereas pregnancies after the emergency contraceptive pill are not as rare. It's thought ellaOne is more effective than Levonelle.
Emergency contraception should not be used instead of your usual method of contraception. See Where can I get emergency contraception? if you need emergency contraception now.
The emergency contraceptive pill
It can be difficult to know how many pregnancies the emergency pill or IUD prevent, as there is no way to know for sure how many women would have got pregnant if they didn't take it.
Levonelle contains levonorgestrel and has to be taken within 72 hours (three days) of sex. EllaOne contains ulipristal acetate and is licensed to be taken within 120 hours (five days) of sex.
Both Levonelle and ellaOne are effective only if taken before the release of an egg from the ovary (ovulation). The sooner you take Levonelle or ellaOne, the more effective it will be.
Ovulation is triggered by rising levels of a hormone called luteinising hormone (LH).
Levonelle appears not to be effective after levels of LH start to rise. EllaOne continues to be effective a little later in the cycle.
This may be one reason why ellaOne has been slightly more effective than Levonelle in clinical trials.
A paper published in 2010 combined the results of two clinical trials, and showed:
- of 1,714 women who received ulipristal acetate, 22 (1.3%) became pregnant
- of 1,731 women who received levonorgestrel, 38 (2.2%) became pregnant
If you vomit within two hours of taking Levonelle or three hours of taking ellaOne, seek medical advice as you will need to take another dose or have an IUD fitted.
The IUD (coil)
Trials suggest the failure rate for the IUD as emergency contraception is lower than 1%. This means less than 1 woman in 100 using the IUD as emergency contraception will get pregnant.
The IUD is more effective than the emergency pill at preventing pregnancy after unprotected sex.
The IUD must be fitted by a healthcare professional within five days (120 hours) of having unprotected sex or, if it's possible to estimate when you ovulate, up to five days after you ovulate.
Find out more about emergency contraception, including how it works and possible side effects.
Page last reviewed: 29/01/2018
Next review due: 29/01/2021