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How Effective Is The Pill?

When considering your contraception options, it’s important to consider which method would suit you best given your personal situation and circumstances. There are 15 forms of contraception available in the UK, so it’s very likely that you’ll find something that suits you well.

Of all the kinds of contraceptives for women that are available, the oral contraceptive pill is one of the most commonly used. There are two main categories of pill (the progestogen only pill (the mini pill) and the combined pill) and there are a number of different brands and varieties within each category.

What happens if I'm not consistent when taking my pill?

Two terms are generally used when talking about the effectiveness of different methods of contraception. These are:

  1. Perfect use: This is when a method of contraception is used exactly as it should be 100% of the time. With contraceptive pills this means you never forget to take a pill and you always take your pill on time each day (if applicable).  

  2. Typical use: This describes how people are more likely to take their oral contraceptive pills. This means they might not take it at the same time each day, and there may be days when they forget to take it altogether. Not taking or using contraceptives correctly lowers their effectiveness and increases your risk of becoming pregnant.

With perfect use, the pill is more than 99% effective. This means that if the pill is taken correctly, fewer than 1 woman in 100 will become pregnant each year. With typical use, oral contraceptives are around 91% effective, which means roughly 9 women out of 100 will become pregnant each year.

What makes the pill less effective?

Other medications including certain antibiotics

Until fairly recently it was thought that all antibiotics interact negatively with oral contraceptives. It’s now thought that the only antibiotics that affect the pill are rifampicin and rifabutin. These are enzyme-inducing antibiotics, which means they increase the levels of enzymes in the body and can affect hormonal contraception. You should use additional contraception if you are taking these antibiotics.

Other medication that can affect hormone-based female contraception such as the pill includes some epilepsy medications such as lamotrigine, an immunosuppressant called ciclosporin and bile acid sequestrant drugs like cholestyramine. If you are taking any of these medications, you should make sure your doctor has explained to you how they may affect your oral contraceptive so you know if you need to take extra precautions when having sex.

A herbal remedy called St John’s Wort can also affect how oral contraceptives work.  

Sickness / diarrhoea

If you vomit within 2 hours of taking the pill, your body will not have absorbed all the hormones. In this situation you should take another pill immediately, and as long as you aren’t sick again, you’ll be protected from pregnancy as usual.

If you are sick again or you have severe diarrhoea (6-8 times over 24 hours), your protection against pregnancy may be compromised, so you should use a barrier method of contraception such as a condom.

If you have sickness or severe diarrhoea for longer than 24 hours, you should take the same precautions as you would if you had missed a pill. Check the patient information leaflet that came with your medication for more advice on this.

Missed pills

Missing a pill or starting a new pack late can affect the contraceptive effect of your pill, and you may need to take extra precautions. The action you should take depends on the type of contraceptive pill you use, how many pills you’ve missed and where you are in your cycle.

For more advice on this, check the patient information leaflet in the box for your contraceptives. If you don’t have a leaflet or you’ve lost yours, you can find an electronic version on the Electronic Medicines Compendium (eMC) website.