How Effective is the Pill?

Reliability of 'the pill' for contraception
Woman stood in the street checking how effective the pill is on her phone

Key takeaways

  • The pill is most effective when you take it properly - once a day at the right time every day

  • When used correctly the combined pill is over 99% effective and the mini pill is slightly less effective, but still over 99%

  • The pill is effective straight away if you take it on the first day of your period, otherwise it takes 7 days

  • The pill is one of the most effective contraceptives when taken properly, more than condoms or diaphragms

  • Some antibiotics, some herbal remedies, and being ill can all reduce the effectiveness of your pill

Contents of this article

What is ‘the pill’?

‘The pill’ is one of the most common forms of contraception available to women today. The pill is taken orally, and comes in two forms: the combined oral contraceptive pill (COC) and the progesterone-only pill (POP) or ‘mini-pill’.

Combined pills contain synthetic versions of the two hormones produced naturally by the female body: oestrogen and progesterone. The mini-pill contains progesterone only.

How should I take it?

The pill is most effective when taken properly. Different pills will have different instructions for taking them, which you should always follow when possible. If you forget to take the pill or take your usual dose late it can reduce the effectiveness of the pill and put you at risk of getting pregnant.

Both types of pill must be taken regularly in order to protect you properly against pregnancy. The combined pill is taken once a day, at roughly the same time, for 21 days in a row. This is followed by a 7-day ‘break’ period when you take no pills. The mini-pill should be taken once, every day, with no break period.

How effective is it?

If taken correctly, the combined pill (COC) is effective in over 99% of cases. This means that less than 1 in 100 women taking it in one year will get pregnant.

The progestogen-only pill (POP) is slightly less effective because it contains less hormones. However, it is still effective in preventing pregnancy in 99% of cases per year, if taken correctly.

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How soon after I start taking the pill will it be effective?

In most cases, you should be able to start the pill at any point during your menstrual cycle. However, if you have just had a baby, miscarriage or an abortion, you may need to use an additional barrier method (like condoms) for the first 7 days of taking the pill. Always discuss your situation with a doctor beforehand to make sure that you are taking the pill in the correct way.

Different pills will be effective from different points from when you start taking it:

With the combined pill (COC), you will be protected against pregnancy immediately if you start taking it on the first day of your period. You will not need to use additional types of contraception. If you start your pill on any other day during your menstrual cycle, however, you will need to use condoms or another form of contraception for the first 7 days of taking it before it will be an effective protection against pregnancy.

With the mini-pill or progestogen-only pill (POP), you will also be protected from pregnancy straight away if you start taking it on the first day of your menstrual cycle. If you start it any later than the 5th day, you will need to use an additional method of contraception until the 3rd day of taking the pill.

Is the pill more effective than other forms of contraception?

The contraceptive pill is one of the most effective forms of contraception. No one contraceptive is 100% effective in preventing pregnancy, and most forms will have some side effects, so it is worth shopping around to work out what will be best for you.

  • Male condoms are 98% effective when used correctly, and female condoms are 95% effective in preventing pregnancy in every 100 cases per year.
  • Latex diaphragms and latex caps (when used with spermicide) are 92-96% effective in preventing pregnancy in 100 cases per year.
  • Longer-lasting but reversible contraceptive methods are about as reliable as the contraceptive pill. The injection, implant, IUS (intrauterine system) or IUD (intrauterine device) are all more than 99% effective in every 100 cases.
  • Other forms, such as the contraceptive patch and the vaginal ring are also more than 99% effective when used correctly, meaning that less than 1 in 100 women will get pregnant per year when using them.

The main advantages of the pill are that it is a fully reversible form of contraception, easy and convenient to take daily.

Can anything reduce the effectiveness of the pill?

Certain things can make the contraceptive pill less effective. These include:

  • Antibiotics: some antibiotics (rifampicin and rifabutin) have been proven to make the pill less effective, and you will need to use an additional method of contraception for at least 7 days after taking your last antibiotic tablet in order to prevent pregnancy. Always check with your doctor whether medicine you are being prescribed could reduce the effectiveness of your contraceptive pill.
  • Herbal remedies: some herbal supplements and remedies (like St. John’s Wort and high dose Vitamin C tablets) can reduce the effectiveness of the pill. Always check the packet for drug interactions before taking them if you are on the contraceptive pill.
  • Illnesses: some infections or fevers can make the pill less effective. If you have severe diarrhoea or vomit soon after taking your usual pill, you may also be at risk of pregnancy.

If you are ever in doubt or unsure whether the pill has worked, it is always better to be extra cautious and use an additional method of contraception alongside the pill for at least 7 days. If you think you have been unprotected against pregnancy and you have been sexually active over the past 5 days, you may need to take the morning after pill.

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