How Effective is Emergency Contraception?

Dr Kathryn Basford

Medically reviewed by

Dr Kathryn Basford

Last reviewed: 15 Jul 2019

Effectiveness and time limits of the morning after pill and IUD

How effective is the morning after pill?

Key takeaways

  • In most cases, you need to use emergency contraception within 5 days of unprotected sex

  • The IUD (intrauterine device; copper coil) is the most effective method of emergency contraception

  • ellaOne is the most effective morning after pill

No method of contraception works 100% of the time. When it does not work, using an emergency contraceptive can be a very good way to stop an unwanted pregnancy. Emergency contraception should never be used in place of regular contraception.

There are 2 main types of emergency contraception: the morning after pill and the IUD (intrauterine device; copper coil). The morning after pill is one tablet that you take as soon as possible after unprotected sex. The IUD is a plastic, T-shaped device fitted in the vagina by a doctor or nurse.

How effective emergency contraception is depends on a number of things, like how much time there is between you having sex and using it. For this reason, it’s hard to say exactly how effective each method is, but the IUD is generally thought to be the most effective, followed by ellaOne morning after pill, and then Levonelle morning after pill.

Morning after pill

What are the different types of morning after pill?

There are two main types of morning after pill available in the UK. They are:

Levonelle is the most commonly prescribed emergency contraceptive pill. It goes by the name Plan B in the US and Canada.

If you’ve had unprotected sex, you can take:

  • Levonelle within 3 days (72 hours)
  • ellaOne within 5 days (120 hours)
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Which morning after pill is more effective?

Studies have shown that:

  • if one of them is taken within the first 12 hours after having sex, Levonelle and ellaOne are as good as one another
  • the effectiveness of Levonelle starts to go down the longer you wait to take it after you’ve had unprotected sex
  • ellaOne will work better for longer – up to 5 days (120 hours) after you’ve had unprotected sex

What can make the morning after pill less effective?

Vomiting – the morning after pill can make you feel sick (nausea) and be sick (vomit). If you’re sick before your body has taken in the active ingredients of the pill, it will not work. If this happens, talk to your doctor as you may need to take another dose.

If you’re sick within 3 hours of taking either Levonelle or ellaOne, the dose definitely needs to be taken again.

High body mass index (BMI) – according to some studies, the effectiveness of emergency contraceptive pills can be affected by a high body mass index (BMI).

  • ellaOne is more effective than Levonelle at preventing pregnancy among women with a high BMI
  • Having a BMI over 30kg/m2 or weight over 85kg can also make ellaOne less effective
  • An IUD is more effective than emergency contraceptive pills for women with a very high BMI

Other medication – the morning after pill should not be used if you’re also taking:

  • St John's Wort
  • HIV drugs such as ritonavir, or immune system suppressants
  • drugs for tuberculosis like rifabutin
  • barbiturates and other medication for preventing fits (seizures)

Tell your doctor or pharmacist about any other medications you’re currently taking, before using a morning after pill.

Other health problems – the morning after pill is not suitable for anyone who has:

  • severe liver problems
  • stomach problems like Crohn's disease
  • certain allergies or hormone problems

Regular contraception – if you’re taking a contraceptive pill or using any contraception that contains hormones, like the implant or the injection, it can make ellaOne less effective, but Levonelle will not be affected. If you’re taking the pill and use ellaOne, you should not restart your regular pill for 5 days.


If you’ve had unprotected sex, the most effective way of preventing pregnancy is to have an IUD fitted by a doctor or nurse.

You can use an IUD as emergency contraception up to 5 days after having sex. Once it’s inserted, you do not have to have the IUD taken out and it can be left in place and used as your regular contraception.

Medically reviewed by:
Dr Kathryn Basford

Dr Kathryn Basford is a qualified GP who works as a GP in London, as well as with Zava. She graduated from the University of Manchester and completed her GP training through Whipps Cross Hospital in London.

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Last reviewed: 15 Jul 2019

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Emergency Contraception

You might need emergency contraception if you’ve recently had unprotected sex and want to reduce your risk of getting pregnant. Zava offers a morning after pill service, which includes a variety of options.

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