How effective is emergency contraception?

How reliable is the morning after pill?


Emergency contraception is used to prevent pregnancy following sexual intercourse and contrary to some beliefs, they do not induce termination. The morning after pill works by preventing or delaying ovulation so there is no egg present to be fertilised by the sperm. The copper intrauterine device (IUD) prevents fertilisation by causing a chemical change in the sperm and egg before they meet. In general, emergency contraception has a 95% success rate but this includes the most effective form which is the copper intrauterine device.


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A woman may seek help after having unprotected sex for a number of reasons:

  • No contraception has been used

  • A sexual assault has occurred and there has been no contraception used

  • Contraception may have been used but there is concern that it may have failed as a result of improper or incorrect use. This can include:

    1. A condom has broken or slipped

    2. Three or more oral contraceptive pills have been missed 

    3. The progesterone only pill has been taken more than three hours late

    4. Contraception injections have been given too late

Emergency contraception can be used by women of any age and there are no ‘absolute’ contraindications to its use. There are however some reasons why it may not be suitable for some women and they include:

  • If you are allergic to any ingredients in the pill
  • If you have severe asthma
  • If you use the herbal remedy ‘St John’s Wort’
  • If you take some medication to treat epilepsy
  • If you are taking some medication to treat Tuberculosis
  • If you are taking some medications designed to treat stomach acid such as omeprazole
  • If you are taking some less common antibiotics such as rifampicin and rifabutin


There are two types of ‘morning after pill’ and they are:

  • Levonelle - the active ingredient found in Levonelle is levonorgestrel; this is a synthetic hormone and is a man made a version of progesterone, a natural hormone produced by the ovaries. The action of levonorgestrel is to delay or prevent ovulation and so by doing this, the egg is not released and so cannot be fertilised by the sperm. In order to be effective, Levonelle must be taken within 72 hours (three days) of having sexual intercourse

  • ellaOne - the active ingredient which is found in ellaOne is ulipristal acetate which interferes with the normal action of progesterone. In the process of doing this, it prevents or delays the release of an egg during ovulation.

The main advantage of this type of morning after pill is that it has a longer period during which it is effective; it can be taken within 120 hours (five days) of having sexual intercourse.

It is important to remember that neither Levonelle nor ellaOne continue to protect you from pregnancy and if you have unprotected sex again, then you will need to take another dose of the morning after pill or you may become pregnant. These pills are exactly as described, emergency contraception and should not be used as a regular form of contraception.  However, should you have unprotected sex more than once in a cycle it is possible to use the morning after pill again.

ellaOne cannot be used at all with any of this medication as it will not work; Levonelle can still be effective but require an increased dose. If you are unsure it is advisable to consult your GP or pharmacist who will be able to advise you.

Similarly, if you are breastfeeding, a small amount of Levonelle may enter breast milk but is not believed to be harmful to your child. In the case of ellaOne however, there have been no studies to show if there are any risks to infants and so the manufacturer recommends that it is not used within one week of breastfeeding.


Side effects of taking the morning after pill

Side effects are not lasting or serious and they include:

  • Headache

  • Abdominal pain

  • Your next period may be more painful and/or longer than usual

  • It may make you sick.  If this happens within two hours of taking Levonelle or three hours of taking ellaOne, a repeat dose will be necessary


How reliable is the morning after pill?

A review of studies carried out by the NHS into how reliable copper IUD’s are in preventing pregnancy after a person has had unprotected sex, suggests that they are 99.9% effective but while we are looking at how reliable the morning after pill is we need to remember that taking all methods into account, emergency contraception is 95%. This would suggest that the morning after pill is a less effective form of emergency contraception.

According to the World Health Organisation:

“A meta-analysis of two studies showed that women who used emergency contraceptive pills with ulipristal acetate had a pregnancy rate of 1.2%. Studies have shown that emergency contraceptive pills with levonorgestrel had a pregnancy rate of 1.2% to 2.1%.

Ideally, emergency contraceptive pills with ulipristal acetate or emergency contraceptive pills with levonorgestrel should be taken as early as possible after ulipristal acetate are more effective between 72–120 hours after unprotected intercourse than other emergency contraceptive pills.”

This is a single study and would suggest that the morning after pill is more effective than suggested when we consider that the overall reliability figure for all forms of emergency contraception is 95% and that for IUDs is 99.9% effective.

It is certainly true that the morning after pill will be more effective the sooner you can take it following unprotected sex. According to the NHS, a 2017 review showed the following:

  • ellaOne was 98% - 99% effective

  • Levonelle was 97% - 99% effective

These figures would suggest that the figure of 95% effective over all forms of emergency contraception is lower than it should be.

However, according to the Levonelle patient information leaflet, the pill will only prevent approximately 84% of pregnancies when taken within 72 hours of having unprotected sex although that level goes up if the pill is taken within 24 hours. In the case of ellaOne, if taken within twenty four hours it is suggested by the manufacturer that it is 98% effective although this level of effectiveness will fall the longer it is taken after unprotected sex.


Availability of emergency contraception

Emergency contraception is available free of charge in the following places:

  • Contraceptive or family planning clinics

  • Sexual health or genitourinary medicine clinics

  • Some GP surgeries

  • Most NHS walk-in clinics and minor injury units

  • Most pharmacies

  • Some A&E departments

Provided you are sixteen years and over you can buy the emergency pill over the counter from most pharmacies, online and from some other organisations such as British Pregnancy Advisory Service (BPAS).



Preventing unwanted pregnancy is an important issue and it is extremely important to consider using some sort of regular protection. It is easy to be carried away in the heat of the moment but at least if you are protected from becoming pregnant, it is one less worry.

It is not necessary any longer to take a pill every day and the following methods of contraception are very popular:

  • The implant

  • The injection

  • The patch

  • The IUD

If you are unsure which will be most suitable for you, your GP or any of the centres listed above will be able to help you to decide.

If you are under sixteen contraceptive services are free and confidential.  

At the end of the day, if you really want to prevent pregnancy but have unprotected sex, the most reliable emergency contraception is the copper IUD. The added advantage with this method is that the IUD can remain in place and provide good, permanent and reliable contraception in the future.

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