Birth control pills (also called oral contraceptives or “the pill”) are used by millions of women in the United States to prevent pregnancy. The pill is safe and effective for most women.

This page will explain:


To understand how birth control pills work, you should know what happens during reproduction. A woman has two ovaries, one on each side of the uterus. Each month, one of the ovaries releases an egg into a fallopian tube. This is called ovulation. It typically occurs about 12–14 days before the start of the menstrual period.

A woman can get pregnant if she has sex around the time of ovulation. During sex, the man ejaculates sperm into the vagina. The sperm travel up through the cervix and into the fallopian tubes.

The pill is a good choice for women who may want to get pregnant later. It is easy to use, convenient, and reversible.

When a sperm meets an egg in the fallopian tube, fertilization—union of egg and sperm—can occur. The fertilized egg then moves down the fallopian tube to the uterus. It then attaches to the lining of the uterus and grows into a fetus.

Birth Control Pills

Birth control pills are made of hormones that prevent ovulation. The hormones in the pill cause changes in the cervical mucus and the lining of the uterus. The cervical mucus thickens, which blocks the sperm from entering the cervix. The lining of the uterus thins, making it less likely that a fertilized egg can attach to it. Together, these events make it very unlikely that someone taking the pill will become pregnant.

Each month during ovulation, an egg is released (1) and moves into one of the fallopian tubes (2). If a woman has sex around this time, an egg may meet a sperm in the fallopian tube and the two will join (3). This is called fertilization. The fertilized egg then moves through the fallopian tube into the uterus and becomes attached there to grow during pregnancy (4).

The pill is a very effective form of birth control. When women use the pill correctly, fewer than 1 in 100 will get pregnant over 1 year. But, about 8 in 100 typical users (8%) will become pregnant. This is because one or more pills may be missed or are not absorbed (due to vomiting, for instance). If this occurs, a backup method of birth control, such as a condom or spermicides, should be used. If pills are missed and a backup method is not used, emergency contraception can be used to prevent pregnancy.

The birth control pill is easy to use and convenient. But it does not protect against sexually transmitted diseases (STDs). If you are at risk for STDs, you should use condoms for protection.

Some women, such as those with certain medical conditions, should not take birth control pills at all (see box). These women may have to consider other types of birth control, such as condoms or spermicides. Although rare, some types of pills can cause severe illness in some women. The most serious problems are blood clots in the legs or lungs, heart attack, or stroke. Other rare problems that may occur include high blood pressure and problems with the liver and gall bladder. So, before your doctor or nurse will prescribe birth control pills for you, he or she will ask you some questions about your health and your family history. He or she also may do a physical exam, which may include a pelvic exam and a breast exam.

There are two basic types of birth control pills: combination pills and progestin-only pills. Most women who take the pill take a combination pill.

If you have a 28-day combination pill pack or a 91-day extended-use pack and forget to take one of the last seven “reminder” pills (pills without hormones), do not worry. Throw away the reminder pills you missed. Keep taking one pill a day until the pack is empty. You should start your new pack of pills on the same day of the week that you started your first pack.

If You Miss a Pill

You may forget to take a pill once in a while. If you forget to take one pill, take it as soon as you remember.

Take the next pill at the normal time. It is okay if you have to take two pills in the same day. It is normal to feel a bit queasy if you do this.

If you forget to take two or more pills, use a backup method of birth control. Call your doctor or nurse, and ask what you should do.

If you miss some pills, you may have some spotting or light bleeding even if you make up the missed pills. These side effects are not harmful.

Combination Pills

Combination birth control pills contain the hormones estrogen and progestin (a synthetic form of the hormone progesterone).

There are many different brands with different doses of hormones. This gives a woman a choice in finding a pill that is right for her.

How to Take Them

Based on convenience and advice of their doctors, different women may start taking their pills at different times. Discuss your options with your doctor:

No matter which day you choose to start taking the pill, you will start each new pack of pills on the same day of the week as you started the first pack. It is a good idea to have an extra pack of pills and a backup method on hand in case you miss some pills or lose your pack.

Pills work only if you take them correctly. Do not skip pills for any reason—even if you bleed between periods or feel sick. Call your doctor or nurse if you are concerned. Even if you do not have sex very often, you should keep taking the pill.

Your pills may not work well if your body does not absorb them. This may happen if you are taking certain medications, such as antibiotics, or if you vomit. If you can’t keep your pill down, you should use a backup method for the rest of your cycle.

Pills come in packs of 21, 28, or 91 pills:

Each new pack of pills comes with facts about the pill. Be sure to read this information and ask your doctor about anything that is not clear to you.


The combination birth control pill has benefits in addition to preventing pregnancy. Using this type of birth control pill may reduce the risk of certain health problems:

The pill also helps to keep your periods regular, lighter, and shorter and reduces menstrual cramps. Some women use continuous dosing birth control because they want to have fewer periods. For others, this method can help relieve symptoms of problems that are affected by having periods, such as endometriosis or fibroids. Taking hormones continuously also may help with migraine headaches and depression.

Some women are afraid to take the pill because they have a family history of breast cancer. Most experts agree that women with a family history of breast cancer and no other risk factors do not increase their risk of breast cancer by taking the pill.

Emergency Contraception

Emergency contraception is a type of hormonal birth control. It is used to help prevent pregnancy when a woman had sex without using birth control or the birth control was not used properly. Emergency contraception is very effective in preventing pregnancy. The most commonly used method of emergency contraception is pills (also known as the “morning-after pill”). There are two types of emergency contraception pills—combined birth control pills and progestin-only pills.

Emergency contraception must be taken within 120 hours (5 days) of unprotected sex. If you have had unprotected sex, call your doctor’s office right away. You also can call the Emergency Contraception Hotline (888-NOT-2-LATE) or visit its web site,, to find a doctor who will provide you with a prescription.

Progestin-Only Pills

Another type of birth control pill, called the progestin-only pill or the minipill, contains only progestin. The progestin-only pill is a better choice for women who have certain health problems—such as blood clots—and cannot take pills with estrogen. This method also may be the best choice for women who had side effects with the combination birth control pill or who are breastfeeding.

How to Take Them

The minipill comes in packs of 28 pills. All the pills in the pack contain hormones. It is important not to miss a pill.

It is very important to take the progestin-only pill at the same time each day. So, it may not be a good choice for people who have trouble keeping to a schedule. You should use a backup method of contraception for 48 hours if vomiting occurs after taking a pill or if you are 3 or more hours late in taking a pill.


Progestin-only pills do not offer the same benefits that combination birth control pills offer. Most people who choose the progestin-only pill do so because there are reasons they should not take estrogen. Progestin-only pills offer some protection against:

Side Effects

Depending on the type of pill, some women may have one or more of the following side effects when taking the pill:

For nausea, many women find that taking the pill with a meal or before bed helps. For headaches, taking over-the-counter products for pain relief often helps.

Women who take the progestin-only pill may have more bleeding or spotting days than women who take birth control pills that contain estrogen.

Missing a period also is common. If you miss one period and have taken the pill as prescribed, you should keep taking the pill. If you have forgotten some pills and miss one period, call your doctor. Don’t stop taking the pill. It is not likely that you are pregnant. Using birth control pills during pregnancy does not increase the risk of birth defects.

Most side effects are minor and often go away after a few months of use. There will likely be fewer side effects if the pill is taken at the same time every day.

Who Should Not Use Birth Control Pills

The birth control pill is not a good choice for some women. You should not take combination birth control pills if you:

If you cannot take a combination birth control pill, the progestin-only pill may be a good choice for you. But, the progestin-only pill has its own risks and you should not take it if you:

If you have any of the above problems, you may not be able to take any birth control pills. You may have to use a different, non-hormonal birth control method. Your doctor will help you determine which form is best for you.

Myths About the Combination Pill

Taking the pill is risky.The pill may be risky for women older than 35 years who smoke. For almost all women, though, the benefits of the pill outweigh any possible risks.
Taking the pill causes weight gain.As many women lose weight as gain weight while taking the pill.
Taking a break from the pill now and then is a good idea.There is no health benefit to taking a break from the pill. Taking a break may increase a woman’s chances of an unwanted pregnancy.
The pill causes cancer.Most studies show that the pill does not increase the risk of cervical or breast cancer and it decreases the risk of endometrial cancer.
A woman will not be able to get pregnant until she has stopped taking the pill for a long period of time.The pill is out of a woman’s system and she is able to get pregnant within 24 hours of when she stops taking it.


The pill is a good choice for women who may want to get pregnant later. It is a safe and effective way to prevent pregnancy. It is easy to use, convenient, and reversible. The pill may protect against some cancers. Some benefits of pill use last months or years after you stop taking it. For almost all women, the benefits of pill use outweigh the risks.


Cervix: The lower, narrow end of the uterus, which protrudes into the vagina.

Ectopic Pregnancy: A pregnancy in which the fertilized egg begins to grow in a place other than inside the uterus, usually in the fallopian tubes.

Emergency Contraception: Birth control methods that are used to prevent pregnancy after a woman has had sex without birth control or after the method she used has failed.

Endometriosis: A condition in which tissue similar to that normally lining the uterus is found outside of the uterus, usually on the ovaries, fallopian tubes, and other pelvic structures.

Estrogen: A female hormone produced in the ovaries that stimulates the growth of the lining of the uterus.

Fallopian Tube: One of the two tubes through which an egg travels from the ovary to the uterus.

Fetus: A baby growing in the woman’s uterus.

Fibroids: Benign growths that form in the muscle of the uterus.

Hormones: Substances produced by the body to control the functions of various organs.

Ovaries: Two glands, located on either side of the uterus, that contain the eggs released at ovulation and that produce hormones.

Pelvic Inflammatory Disease: An infection of the uterus, fallopian tubes, and nearby pelvic structures.

Polycystic Ovary Syndrome: A condition in which increased androgen levels occur and eggs are not released from the ovaries.

Progesterone: A female hormone that is produced in the ovaries and makes the lining of the uterus grow. When the level of progesterone decreases, menstruation occurs.

Sexually Transmitted Diseases: Diseases that are spread by sexual contact, including chlamydial infection, gonorrhea, genital warts, herpes, syphilis, and infection with human immunodeficiency virus (HIV, the cause of acquired immunodeficiency syndrome [AIDS]).

Spermicides: Chemicals (creams, gels, foams) that inactivate sperm.