How effective is the morning after pill?

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.

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 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 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.

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.

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:

Some studies have found that morning after pills are less effective for women with a higher body mass index (overweight or obese).

If you have a BMI over 26 (weight over 70kg), levonorgestrel (Levonelle) has been found to be less effective than the copper coil or ulipristal acetate (ellaOne).

A 2024 clinical study found that a 30mg standard dose can be effective for emergency contraception in women with a BMI of 30, though having a BMI over 30 or a weight over 85kg has been thought to make ellaOne less effective. In most cases, a double dose of levonorgestrel can be used, but not a double dose of ellaOne.

The most effective form of emergency or regular contraceptive if you are overweight or obese is the copper coil (intrauterine device or IUD). This form of contraception is not affected by weight.

If you have a high body mass index (BMI), talk with your doctor or pharmacist. They may suggest increasing your dose.

GLP-1 medications and the morning after pill

Certain glucagon-like peptide-1 (GLP-1) medications for weight loss can stop morning after pills from working properly.

Wegovy (semaglutide) or Nevolat (liraglutide) do not stop emergency contraceptives like ellaOne, levonorgestrel, or Levonelle from working. This means you can safely use them alongside either of these weight loss medications.

Mounjaro can stop emergency contraceptive pills (like Levonelle and levonorgestrel) from working, putting you at risk of unintended pregnancy. If you are currently taking Mounjaro, and you need emergency contraception, we recommend the copper coil, which is:

• a small, t-shaped device inserted by a healthcare provider
• highly effective, and you can insert it 120 hours (5 days) after unprotected sex (or ovulation)
• accessible at some local GP surgeries or at sexual health clinics
• can be kept in place as regular contraception
• not affected by Mounjaro

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

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)

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.