How Effective is the Morning After Pill?

dr zoe miller medical editor

Medically reviewed by

Dr Zoe Miller

Last reviewed: 01 May 2025

The morning after pill is a type of emergency contraception that you can take after unprotected sex to prevent an unwanted pregnancy. Types of morning after pills available in the UK include – ellaOne, Levonelle, and its generic equivalent, levonorgestrel.

Both types of emergency contraception can prevent an unwanted pregnancy if taken correctly. Both morning after pills are over 95% effective if taken within 24 hours of unprotected sex. However, this does not mean they will always prevent pregnancy, there are a few things that can make them less effective.

If you’ve had unprotected sex and would like to take emergency contraception, it’s important to know how these medications work and when to take them for the best chance of them working. In this guide we’ll provide more information on how effective the morning after pill is and the difference between the ellaOne and Levonelle.

Contents
How effective is the morning after pill?
 

Key takeaways

  • There are 2 types of morning after pills available, ellaOne and Levonelle. ellaOne and Levonelle work to delay ovulation.

  • Both types of morning after pills need to be taken before ovulation.

  • ellaOne’s effectiveness is 98% in preventing pregnancy when taken 1 to 5 days after having unprotected sex.

  • Levonelle’s effectiveness is around 95% in preventing pregnancy if you take it within 24 hours of having unprotected sex.

How do emergency contraceptive pills work?

Emergency contraceptive pills work to delay ovulation which prevents the fertilisation of the egg by sperm.

Step-by-step overview on how emergency contraceptives prevent pregnancy:

  1. To become pregnant, an egg must be released from one of your ovaries. This is called ovulation.
  2. To begin ovulating, your body triggers a process called the LH surge (LH stands for luteinising hormone). An increase in LH leads to the release of an egg from one of your ovaries. The LH surge begins (roughly) a day before you ovulate.
  3. The egg is viable for 12 to 24 hours after ovulation, and needs to ‘meet’ a sperm cell within that time. If it doesn’t meet a sperm, then you won’t become pregnant.
  4. Although the ‘window’ for ovulation is fairly small, sperm can live in your body for up to 5 days, so you don’t have to have sex on the same day as you ovulate to become pregnant.
  5. Both ellaOne and Levonelle work by delaying ovulation for a few days (they may also delay your next period). This means it’s less likely the egg will be released and meet a sperm.

ellaOne is generally seen as the most effective morning after pill because it can delay ovulation even after the LH surge has begun. On the other hand, Levonelle isn’t effective once the LH surge has started.

Dr Zoe Miller, medical editor

Taking the morning after pill as soon after unprotected sex as possible is recommended for the best chance of success. However, at certain points in your cycle (ovulation), the morning after pill may not work as intended. When you request emergency contraception from a clinic, pharmacy, or online doctor, they’ll ask you when you had your last period to ensure that the morning after pill is the right treatment option for you, and can help you to explore alternatives.” – Dr Zoe Miller, Medical Editor.

Both morning after pills will only work for one instance of unprotected sex. This means you can still get pregnant if you have unprotected sex again after you’ve taken the morning after pill.

➤ While it’s better to use conventional contraceptive methods (such as ‘the pill’ or condoms), it is still possible to take the morning after pill more than once each month, though it’s not recommended to do this often. Morning after pills generally have limited side effects, and these are usually mild.

ellaOne effectiveness

ellaOne 30mg table box jpg

ellaOne is 98% effective in preventing pregnancy when taken within 24 hours of having unprotected sex. ellaOne appears to remain 98% effective if you take it up to 120 hours (5 days) after unprotected sex. The contraceptive effect is reduced after the first 24 hours, so it’s best to take the pill as soon as possible.

ellaOne contains the active ingredient ulipristal acetate. Ulipristal acetate delays ovulation by binding to progesterone receptors in your body. This stops ovulation (and pregnancy) from happening by preventing an increase in luteinising hormone (LH). The LH surge triggers ovulation (the release of an egg) in your normal menstrual cycle.

ellaOne’s effectiveness may be lower if you’re taking any other medications, including:

  • epilepsy treatment, such as carbamazepine, phenytoin or phenobarbital
  • treatment for tuberculosis, such as rifampicin
  • HIV treatment, including ritonavir
  • antifungal medications, such as griseofulvin
  • St John’s wort, a herbal remedy used to treat low moods

Speak to a pharmacist or doctor before taking ellaOne if you’re taking any of these medications. They may advise an alternative method of emergency contraception.

Levonelle effectiveness

Levonelle 1500mg table box jpg

Levonelle is around 95% effective in preventing pregnancy if you take it within 24 hours of having unprotected sex. You can take Levonelle up to 72 hours (3 days) after unprotected sex, but its effectiveness will decrease the longer you delay taking it.

Levonelle also works by preventing ovulation. Levonelle contains levonorgestrel, a synthetic progestogen hormone. Synthetic progestins mimic how natural progesterone works in your body. Levonelle contains 1500 micrograms of levonorgestrel, which is a high dose of progestogen. Taking a high dose suppresses your ovaries from ovulating and releasing an egg.

Like with ellaOne, certain medications can change the effectiveness of Levonelle, including some medications for:

  • TB
  • epilepsy
  • HIV

Antifungal treatments and herbal remedies like St John’s wort can also change the effectiveness of Levonelle.

Speak to a pharmacist or doctor if you are taking any regular medications or over-the-counter products.

Levonorgestrel effectiveness

Levonorgestrel 1.5mg table box jpg

Generic levonorgestrel is the unbranded version of Levonelle. This means it works in the exact same way as Levonelle, it just has a different name, price, packaging and non-active ingredients. It is as effective as branded Levonelle as it contains the same amount of active ingredient (1500 micrograms of levonorgestrel) in each tablet.

You can request generic levonorgestrel, Levonelle, or ellaOne from ZAVA.

from £18.95

from £29.95

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How long is the morning after pill effective?

The morning after pill becomes less effective the longer you wait before taking it:

  • ellaOne is 98% effective for up to 5 days after taking it, but won’t be effective after day 6
  • Levonelle is most effective in the first 24 hours, but its effectiveness declines more quickly, becoming only 58% effective by day 3

The following graph shows how the effectiveness of the pills reduces over time:

morning after pill effectiveness bar chart comparing ellaOne to Levonelle/levonorgestrel

BMI and morning after pill effectiveness

Some studies have found that morning after pills are less effective for women with a higher body mass index (being overweight or obese). If you have a high body mass index (BMI), talk with your doctor or pharmacist. They may suggest increasing your dose.

How effective is the morning after pill during ovulation?

The morning after pill is not effective when you’re ovulating. ellaOne and Levonelle (levonorgestrel) work to delay ovulation. Once ovulation has begun, they will no longer work.

Ovulation usually occurs around 2 weeks after your period, but everyone’s body is different. You can track your menstrual cycle using an app or calendar to get a better idea of when you are ovulating.

It can be difficult to tell when you are ovulating, but here are some signs to look out for:

  • cervical mucus becomes slippery or wetter
  • a slight increase in your body temperature
  • mild pelvic pain that happens on one side

If you believe you had unprotected sex around the time you were ovulating, then you may wish to use another form of emergency contraception: the ‘coil’ or copper IUD (intrauterine device). The copper IUD is 99% effective when it’s fitted as soon as possible. You’ll need to get it fitted within 5 days of having unprotected sex. The coil can prevent a pregnancy, even if you have already ovulated.

You may be able to get the copper IUD fitted up to 5 days after the earliest point you could have ovulated. You can also use the copper IUD as regular contraception once it’s been fitted. Speak to a doctor or sexual health clinic for more information.

When is the morning after pill not effective?

Signs the morning after pill may not be effective include:

  • throwing up shortly after taking the pill, meaning it doesn’t absorb into your body properly
  • you’ve already ovulated
  • the LH surge has already begun and you took Levonelle (which is ineffective if the LH surge has started)
  • taking the morning after pill too long after having unprotected sex
  • taking other medication that can make the morning after pill less effective
  • having a high BMI (being significantly overweight or obese)

If you’ve taken the morning after pill and are worried that it hasn’t worked properly or that you’ve taken it incorrectly, get in touch with your doctor to discuss your options.

Is the morning after pill 100% effective?

No, neither Levonelle, levonorgestrel, or ellaOne are 100% effective. Even if you take them within 24 hours of having unprotected sex, sometimes they still don’t work.

For every 100 women who take Levonelle within 24 hours of unprotected sex, 5 might still become pregnant. For those taking ellaOne, 2 in 100 will still get pregnant.

Sometimes, the morning after pill can delay your period or even cause a missed period, even though you’re not pregnant. However, if your period is more than a week, or if you’re experiencing any pregnancy symptoms including nausea (feeling sick), breast tenderness, or needing to pee more frequently, we recommend taking a pregnancy test.

Remember that although it’s unlikely you’ll get pregnant after taking the morning after pill, there is always help available to advise and explore options and next steps.

dr-zoe-miller.png
Medically reviewed by:
Dr Zoe Miller Medical Editor
Accreditations: MBChB, BSc

After completing her first degree, she went on to study graduate-entry medicine at Warwick University. After graduating as a doctor, she worked within the West Midlands in Urology, Respiratory Medicine, Infectious Disease, and Psychiatry before transitioning into a full-time medical communications role.

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Last reviewed: 01 May 2025


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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