Birth control pills: how they work, how effective they are, when to start taking them, side effects

The birth control pill

When taken correctly, the pill is more than 99% effective in preventing pregnancy. This means that less than 1 in 100 who use the birth control pill as a contraceptive will become pregnant in 1 year. The standard way to take the pill is to take 1 a day for 21 days then take a break for 7 days and during this week you have bleeding like a period. Start taking the pill again after 7 days.

You may be able to take some types of pills without breaks or for shorter periods (a tailored regimen), which can reduce some side effects. Talk to a doctor or nurse about your options. You must take the pill at about the same time each day. You could get pregnant if you don’t, or if you miss a pill, or are vomiting, or have severe diarrhea. Some medicines can make the pill less effective. Check with your doctor if you are taking any other tablets. If you have heavy or painful periods, PMS, or endometriosis, birth control pills can help.

Minor side effects include mood swings, nausea, breast tenderness and headaches – these usually subside within a few months. There is no evidence that the pill makes you fat. There is a very low risk of serious side effects, such as blood clots and cervical cancer. Birth control pills are not suitable if you are over 35 and smoke, or if you have certain medical conditions. The pill does not protect against sexually transmitted infections (STIs), so use condoms as well. There may be a link between the pill and depression, but the evidence is mixed and more research is needed.

How the birth control pill works

The birth control pill is often referred to simply as ;the pill;. It contains man-made versions of the female hormones estrogen and progesterone, which are naturally produced in the ovaries . If the sperm reaches an egg (ovum), pregnancy can occur. Contraception tries to prevent this from happening usually by keeping the egg and sperm separate or by stopping the release of an egg (ovulation).

How does it prevent pregnancy

The pill prevents the ovaries from releasing an egg each month (ovulation). Furthermore:

  • thickens the mucus in the cervix, so it is more difficult for sperm to enter the uterus and reach an egg
  • thins the lining of the uterus, so there is less chance of a fertilized egg implanting in the uterus and being able to grow

The pill is more than 99% effective when used correctly. Other methods of contraception are better for preventing pregnancy, such as the IUD, SUI, implant, and injection.

There are many different brands of pill, consisting of 3 main types:

21 day monophasic pills

This is the most common type. Each pill contains the same amount of hormone. One pill is taken every day for 21 days and then no pills are taken for the next 7 days. Microgynon, Marvelon and Yasmin are examples of this type of pill.

21 day phasic pills

Phasic pills contain 2 or 3 different colored pill sections in one pack. Each section contains a different amount of hormones. One pill is taken every day for 21 days and then no pills are taken for the next 7 days. Phasic pills must be taken in the right order. Logynon is an example of this type of pill.

Daily Pills (ED)

There are 21 active pills and 7 inactive (dummy) pills in one pack. The two types of pill look different. One pill is taken every day for 28 days without a break between pill packs. Every day the pills must be taken in the right order. Microgynon ED is an example of this type of pill.

Follow the instructions that came with the package. If you have any questions, ask a doctor, nurse or pharmacist.

It’s important to take your pills as directed, because missing pills or taking them at the same time as certain medications can make them less effective.

How to take 21 day pills – standard regimen

  • Take your 1st pill from the packet marked with the correct day of the week, or the 1st pill of the 1st color (phasic pills).
  • Continue to take one pill at the same time each day until the pack is finished.
  • Stop taking the pills for 7 days (during these 7 days you will bleed).
  • Start your next pack of pills on day eight, whether or not you’re still bleeding. It should be the same day of the week you took your first pill.

How to take the daily pills?

  • Take the first pill from the section of the packet marked ;start;. This will be an active pill.
  • Continue to take one pill each day, in the correct order and preferably at the same time each day, until the pack is finished (28 days).
  • During the 7 days of taking the inactive pills, you will bleed.
  • Start your next pack of pills after finishing the first pack, whether or not you’re still bleeding.

Tailored regimes

You can also take birth control pills as a tailored regimen. This may include taking the pill for 21 days and not taking it for 4 days, or taking the pill continuously without a break. For more information, speak to a doctor or nurse.

When to take it

You can usually start taking the pill at any time during your menstrual cycle. There is special guidance if you have just had a baby, miscarriage or miscarriage. The guidance may also be different if you have a short period. Ask a doctor or nurse for advice if you need it. You may need to use extra contraception during the first few days on the pill – this depends on when in your menstrual cycle you start taking it.

Starting from the 1st day of the cycle

If you start birth control pills on the first day of your period (day 1 of your period) you will be protected from pregnancy right away. You will not need additional contraceptives.

From the 5th day of the cycle or earlier

If you start the pill on day 5 of your cycle or earlier, you will still be immediately protected against pregnancy.

Starting from the 5th day of the cycle

You will not be protected from pregnancy right away and will need additional contraception until you have been on the pill for 7 days.

If you start the pill after day 5 of your period, make sure you haven’t risked becoming pregnant since your last period. If you’re worried you might be pregnant when you start the pill, take a pregnancy test 3 weeks after the last time you had unprotected sex.

What to do if you miss a pill

If you miss one or more pills, or if you start a pack late, this can make the pill less effective at preventing pregnancy. The chance of getting pregnant after missing a pill or pills depends on:

  • when the pills are missing
  • how many pills are missing

Find out what to do if you miss a birth control pill.

Vomiting and diarrhea

If you vomit within 3 hours of taking the birth control pill, it may not have been fully absorbed into your bloodstream. Take another pill straight away and the next one at the usual time.

If you continue to be ill, continue using another form of contraception until you have been on the pill for 7 days without throwing up.

Very severe diarrhea (6 to 8 watery stools in 24 hours) can also mean that the pill isn’t working properly. Continue taking your pill as normal, but use additional contraceptives, such as condoms, while you have diarrhea and for 2 days after recovery.

Talk to a GP or nurse, or call NHS 111 for more information, if your illness or diarrhea continues.

Who can use the birth control pill

If there are no medical reasons why you cannot take the pill and you do not smoke, you can take the pill until menopause. However, the pill is not suitable for everyone. To find out if the pill is right for you, talk to a GP, nurse or pharmacist.

The pill may not be right for you if:

  • I’m pregnant?
  • smoke and are 35 or older
  • stopped smoking less than a year ago and are 35 years of age or older
  • i am very overweight
  • take certain medicines

The pill may also not be right for you if you have (or have had):

  • blood clots in a vein, for example in the leg or lungs
  • stroke or any other disease that narrows the arteries
  • anyone in your immediate family who has a blood clot under the age of 45
  • a heart abnormality or heart disease, including high blood pressure
  • severe migraines , especially with aura (warning symptoms)
  • breast cancer
  • gallbladder or liver disease
  • diabetes with complications or diabetes in the last 20 years

After having a baby

If you have just had a baby and are not breastfeeding, you can most likely start the pill on day 21 after delivery, but you will need to consult a doctor. You will be protected from pregnancy right away.

If you start the pill 21 days after giving birth, you will need additional contraception (such as condoms) for the next 7 days.

If you are breastfeeding, we do not recommend taking birth control pills until 6 weeks after delivery.

After an abortion or miscarriage

If you have had a miscarriage you can start the pill up to 5 days later and you will be immediately protected from pregnancy. If you start the pill more than 5 days after the miscarriage or miscarriage, you will need to use additional contraception until you have been on the pill for 7 days.

Advantages and disadvantages of the birth control pill

Some benefits of the pill include:

  • does not interrupt sex
  • it usually makes your bleeding regular, lighter, and less painful
  • reduces the risk of ovarian, uterine and colon cancer
  • it can reduce the symptoms of PMS
  • it can sometimes reduce acne
  • may protect against pelvic inflammatory disease
  • it can reduce the risk of fibroids, ovarian cysts, and non-cancerous breast disease

Some disadvantages of the pill include:

  • can cause temporary side effects at first, such as headaches, nausea, breast tenderness and mood swings – if these don’t go away after a few months, switching to a different pill may help
  • can raise blood pressure
  • it does not protect you from sexually transmitted infections
  • breakthrough bleeding and spotting are common in the first few months of using the pill
  • it has been linked to an increased risk of some serious health conditions, such as blood clots and breast cancer

The birth control pill with other medicines

Some medicines interact with birth control pills and do not work properly. Some interactions are listed on this page, but it’s not a complete list. If you want to check that your medicines are safe to take with birth control pills, you can:

  • ask a family doctor, nurse or pharmacist
  • read the patient information leaflet that comes with the medicine


The antibiotics rifampin and rifabutin (which can be used to treat diseases including tuberculosis and meningitis) can make birth control pills less effective. Other antibiotics do not have this effect.

If you are prescribed rifampicin or rifabutin, you may be advised to switch to an alternative contraceptive. If not, you will need to use extra contraception (such as a condom) while taking the antibiotic and for a short time afterwards. Talk to a doctor or nurse for advice.

Epilepsy and HIV medications and St. John’s Wort

Birth control pills can interact with medicines called enzyme inducers. These speed up the breakdown of hormones by the liver, reducing the effectiveness of the pill.

Examples of enzyme inducers are:

  • the epilepsy drugs carbamazepine, oxcarbazepine, phenytoin, phenobarbital, primidone, and topiramate
  • St. John’s Wort (an herbal remedy)
  • antiretroviral medicines used to treat HIV (research suggests that interactions between these medicines and the progestogen-only pill can affect the safety and effectiveness of both)

A GP or nurse may advise you to use an alternative or additional form of contraception while taking any of these medicines.

Risks of taking the birth control pill

There are some risks associated with using birth control pills. However, these risks are small.

Blood clots

The estrogen in the pill can make blood clot more easily. If a blood clot develops, it could cause:

  • deep vein thrombosis (clot in the leg)
  • pulmonary embolism (clot in the lung)
  • stroke
  • heart attack

The risk of getting a blood clot is very low, but your doctor will check if you have certain risk factors before prescribing the pill.


The pill may slightly increase your risk of developing breast cancer and cervical cancer. It can also reduce the risk of developing uterine (womb) cancer, ovarian cancer, and bowel cancer.

However, 10 years after you stop taking the pill, your risk of breast and cervical cancer returns to normal.

Where can you find birth control pills

Contraception is free for everyone through the NHS. Places where you can get contraception include:

  • community contraception clinics
  • some genitourinary medicine (GUM) clinics
  • sexual health clinics – they also offer contraception and STD testing services
  • GP interventions: Talk to a GP or nurse
  • some youth services (call the Sexual Health Line on 0300 123 7123 for more information)

How do I switch to a different pill?

If you want advice on changing the contraceptive pill, you can visit a GP, contraceptive nurse (sometimes called a family planning nurse), or sexual health clinic.

You shouldn’t have to take a break between different packs, so you will usually be advised to start the new pill immediately or wait until the day after you take the last of your old pills.

You may also be advised to use alternative contraceptive methods when switching, as the new pill may take a short time to take effect.

If you are under 16

Contraceptive services are free and confidential, even for children under 16.

If you are under 16 and want contraception, your doctor, nurse or pharmacist will not tell your parents (or guardian) as long as they think you fully understand the information they are giving you and your decisions .

Doctors and nurses work under strict guidelines when dealing with people under 16. They’ll encourage you to consider telling your parents, but they won’t force you.

The only time a professional might want to tell someone else is if they believe you are at risk of harm, such as abuse. The risk should be serious and they would usually discuss it with you first.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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