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How effective is the morning after pill?

Picture of Andrew Bellingham

Andrew Bellingham

Pharmacist | 20+ Years | BPharm | Dip Clin Pharm

The morning after pill, also known as emergency contraception or Plan B, is a type of birth control you can take after unprotected sex to prevent pregnancy. It’s not meant for regular use but as a backup option if your usual contraception fails or you have unprotected intercourse.

So, what does it do? The morning after pill works by preventing or delaying ovulation, the release of an egg from the ovary. It may also make it harder for sperm to reach an egg or for a fertilised egg to implant in the uterus. Importantly, it doesn’t terminate an existing pregnancy but prevents one from happening in the first place.

Its effectiveness depends on how soon you take it after unprotected sex. The longer you wait, the less effective it becomes. This is because it mainly works by preventing ovulation, and once ovulation has occurred, the chances of pregnancy rise.

Why does this matter to you? Knowing when to take the morning after pill can help you make the right decisions if you find yourself in a situation where you need emergency contraception.

How long after sex can you take the morning after pill?

When it comes to emergency contraception, timing is everything. The effectiveness of the morning after pill depends largely on how soon it is taken after unprotected sex. So, let’s address the burning questions: how long after sex can you take the morning after pill, and just how effective is it?

The window of opportunity for taking the morning after pill is relatively narrow. Generally, it’s most effective when taken within 24 hours after unprotected sex. However, it can still be taken up to 72 hours (3 days) afterward, but its effectiveness decreases as time passes. 

  • Taken within 24 hours: reduces the risk of pregnancy by up to 95%
  • Taken within 25-48 hours: reduces the risk of pregnancy by up to 85%
  • Taken within 49-72 hours: reduces the risk of pregnancy by up to 58%


It’s important to understand that emergency contraception is not 100% effective. While it significantly reduces the risk of pregnancy, there is still a chance of becoming pregnant even after taking the morning after pill, especially if it’s taken later in the 72-hour window.

Factors affecting effectiveness

Several factors can affect the effectiveness of the morning after pill, including:

  • Weight: the effectiveness of emergency contraception may be reduced in women with a higher body mass index (BMI)

  • Timing: the sooner you take the morning after pill after unprotected sex, the more effective it is

  • Individual response: responses to emergency contraception can vary from person to person

Frequently asked questions about the morning after pill

How long after unprotected sex can you take the morning after pill?

You can take the morning after pill up to 72 hours after unprotected sex, though it’s most effective within 24 hours.

How long do you have to take the morning after pill?

You should take the morning after pill within 72 hours after unprotected sex, however the sooner you take it the more effective it will be. If taken within 24 hours, you can reduce the risk of pregnancy by up to 95%.

What does morning after pill do?

The morning after pill prevents pregnancy by delaying ovulation or making it harder for sperm to fertilise an egg.

A woman sits on the side of her bed with her partner in the background, she is holding the morning after pill in her hand.

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