How effective is the emergency contraception pill?
How effective is the emergency contraception pill?
Emergency contraception can be an important safety net when something unexpected happens, whether a condom breaks, you miss a pill, or you have unprotected sex. One of the most common questions people ask is: How effective is the emergency contraception pill? Understanding how it works, what affects its success rate, and when to use it can help you make informed decisions about your reproductive health.
Emergency contraception pills (ECPs) are highly effective at preventing pregnancy when taken promptly, but their effectiveness varies based on the type of pill and how quickly you take it. While they offer reassurance in urgent situations, they are not intended to replace regular, long-term contraception.
In this guide, we explain how effective each type of emergency contraception is, how it works, what factors influence its success, and when to seek further medical advice.
Types of emergency contraception pills and their effectiveness
There are two main types of emergency contraception pills available in the UK: Levonorgestrel (LNG) and Ulipristal acetate (UPA). They work slightly differently and have different time windows for use.
Levonorgestrel (e.g., Levonelle, generic Plan B)
- Must be taken within 72 hours (3 days) after unprotected sex.
- Most effective within the first 24 hours, preventing up to 94% of expected pregnancies.
- By 72 hours, effectiveness drops to around 58%.
- Works mainly by delaying ovulation.
Ulipristal acetate (e.g., ellaOne)
- Effective up to 120 hours (5 days) after unprotected sex.
- Up to 98% effective within the first 24 hours.
- Around 85% effective when taken within 120 hours.
- More effective than levonorgestrel for people close to ovulation.
The copper IUD
While not a pill, the copper IUD is the most effective form of emergency contraception overall.
- Can be inserted up to five days after unprotected sex.
- Over 99% effective and maintains the same effectiveness regardless of timing.
- Also offers long-term contraception for 5-10 years.
How emergency contraception pills work
Emergency contraception pills do not cause an abortion and do not affect an existing pregnancy. Instead, they work by:
- Delaying ovulation so the egg is not released
- Preventing fertilisation if intercourse occurred before ovulation
- Potentially affecting sperm movement
They will not work if you have already ovulated and fertilisation has taken place.
Factors that influence effectiveness
- Timing
The sooner you take the pill, the more effective it will be. Every hour matters. - Your menstrual cycle
If ovulation has already occurred, ECPs may not work. - Body weight/BMI
Some research suggests levonorgestrel may be less effective for individuals with higher BMI, making ellaOne or a copper IUD a better option. - Vomiting after taking the pill
If you vomit within three hours, you may need another dose. - Drug interactions
Certain medications (e.g., epilepsy medicines, some HIV treatments, herbal remedies like St. John's Wort) may reduce the pill's effectiveness.
When to take a pregnancy test
Take a test if:
- Your period is more than seven days late
- Your period is much lighter or shorter than usual
- You develop pregnancy symptoms (tender breasts, nausea, fatigue)
FAQs
What is emergency contraception?
Emergency contraception is a method used to prevent pregnancy after unprotected sex or contraceptive failure. It includes pills (levonorgestrel or ulipristal acetate) and the copper IUD.
How effective is the emergency contraception pill?
Effectiveness ranges from 58% to 98%, depending on timing and type. Ulipristal acetate is generally more effective than levonorgestrel, especially later in the five-day window.
How does emergency contraception work?
It works primarily by delaying ovulation so the egg is not released. If ovulation has already happened, the pill is unlikely to work.
How late can you take emergency contraception?
- Levonorgestrel: Within 72 hours
- Ulipristal acetate: Within 120 hours
- Copper IUD: Within five days
Does emergency contraception protect against STIs?
No. It does not protect against sexually transmitted infections.
What if I vomit after taking the pill?
If you vomit within three hours, its effectiveness is reduced. You may need to take another dose.
Can I use emergency contraception regularly?
No. ECPs are safe but not recommended as an ongoing contraception. They are less effective than regular birth control and may cause cycle irregularities.
What happens if I take emergency contraception too often?
Frequent use is not harmful but can lead to irregular bleeding, cycle changes, and reduced reliability. It is better to use a consistent method of contraception.
How does emergency contraception affect your period?
Your next period may come earlier or later than usual. It may also be lighter, heavier, or more irregular for one cycle.
When should I seek medical advice?
Seek help if:
- Your period is more than one week late
- You have severe abdominal pain
- You suspect pregnancy despite taking ECPs
- You need advice on long-term contraception options
Why emergency contraception is not a replacement for regular contraception
Although effective, emergency contraception is a back-up, not a primary method. If you're sexually active and do not want to get pregnant, consider reliable options such as:
- Birth control pills
- Hormonal IUD
- Copper IUD
- Implant
- Injection
- Patch or vaginal ring
- Condoms
These methods are far more effective for ongoing protection.
How My Local Surgery can help
At My Local Surgery, we make professional healthcare easy, fast, and accessible, with friendly health care professionals who provide judgement-free advice. If you need emergency contraception, want guidance on choosing the right method, or prefer discreet online support, we can help.
Visit our website to book your consultation today. We're here to support your reproductive health with compassionate, confidential care.
My Local Surgery
February 16, 2026
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6 min read
