What Is It? The birth control pill (also called "the Pill") is a daily pill that contains hormones to change the way the body works and prevent pregnancy. Hormones are chemical substances that control the functioning of the body's organs. In this case, the hormones in the Pill control the ovaries and the uterus. How Does It Work? Most birth control pills are "combination pills" containing a mix of the hormones estrogen and progesterone to prevent ovulation (the release of an egg during the monthly cycle). A woman cannot get pregnant if she doesn't ovulate because there is no egg to be fertilized. The Pill also works by thickening the mucus around the cervix, which makes it difficult for sperm to enter the uterus and reach any eggs that may have been released. The hormones in the Pill can also sometimes affect the lining of the uterus, making it difficult for an egg to attach to the wall of the uterus. How Is the Pill Taken? Most combination pills come in either a 21-day pack or a 28-day pack. One hormone pill is taken each day at about the same time for 21 days. Depending on your pack, you will either stop taking birth control pills for 7 days (as in the 21-day pack) or you will take a pill that contains no hormones for 7 days (the 28-day pack). A woman has her period when she stops taking the pills that contain hormones. Some women prefer the 28-day pack because it helps them stay in the habit of taking a pill every day. There is also a type of combination pill that decreases the frequency of a woman's period by supplying a hormone pill for 12 weeks and then inactive pills for 7 days. This decreases the number of periods to one every 3 months instead of one every month. Another kind of pill that may change the number of monthly periods is the low-dose progesterone pill, sometimes called the mini-pill. This type of birth control pill differs from the other pills in that it only contains one type of hormone — progesterone — rather than a combination of estrogen and progesterone. It works by changing the cervical mucus and the lining of the uterus, and sometimes by affecting ovulation as well. The mini-pill may be slightly less effective at preventing pregnancy than combination pills. The mini-pill is taken every day without a break. A girl who is taking it might have no period at all or she may have irregular periods. For the minipill to work, it must be taken at the same time every day, without missing any doses. Any type of birth control pill works best when it is taken every single day at the same time of day, regardless of whether a girl is going to have sex. This is especially important with progesterone-only pills. For the first 7 days of taking the Pill, a girl should use an additional form of contraception, such as condoms, to prevent pregnancy. After 7 days, the Pill should work alone to prevent pregnancy. This timing can vary based on the type of Pill and when you start taking it — so be sure to talk about it with your doctor. Also, it's important to continue using condoms to protect against sexually transmitted diseases (STDs). If pills are skipped or forgotten, a girl is not protected against pregnancy and she will need a backup form of birth control, such as condoms. Or she will need to stop having sex for a while. Do not take a friend's or relative's pills. How Well Does It Work? Over the course of 1 year, about 9 out of 100 typical couples who rely on the Pill to prevent pregnancy will have an accidental pregnancy. Of course, this is an average figure and the chance of getting pregnant depends on whether you take your birth control pills every day. The Pill is an effective form of birth control, but even missing 1 day increases the chance of getting pregnant. In general, how well each type of birth control method works depends on a lot of things. These include whether a person has any health conditions or is taking any medicines or herbal supplements that might interfere with its use (for example, an herb like St. John's wort can affect how well the Pill works). Talk to your doctor to check if anything you take could affect how the Pill works for you. How well a particular method of birth control works also depends on whether the method chosen is convenient — and whether the person remembers to use it correctly all the time. Protection Against STDs The birth control pill does not protect against STDs. Couples having sex (or any intimate sexual contact) must always use condoms along with the Pill to protect against STDs. Abstinence (the decision to not have sex or any intimate sexual contact) is the only method that always prevents pregnancy and STDs. Possible Side Effects The birth control pill is a safe and effective method of birth control. Most young women who take the Pill have none to very few side effects. The side effects that some women have while on the Pill include: irregular menstrual bleeding (more common with the mini-pill) nausea, headaches, dizziness, and breast tenderness mood changes blood clots (rare in women under 35 who do not smoke) Some of these side effects improve over the first 3 months on the Pill. When a girl has side effects, a doctor will sometimes prescribe a different brand of the Pill. The Pill also has some side effects that most girls are happy about. It usually makes periods lighter, reduces cramps, and is often prescribed for women who have menstrual problems. Taking the combination Pill often improves acne, and some doctors prescribe it for this purpose. Combination birth control pills have also been found to protect against some forms of breast disease, anemia, ovarian cysts, and ovarian and endometrial cancers. Who Uses It? Young women who can remember to take a pill each day and who want excellent protection from pregnancy use birth control pills. Not all women can — or should — use the Pill. In some cases, medical or other conditions make the use of the Pill less effective or more risky. For example, it is not recommended for women who have had blood clots, certain types of cancers, or some migraine headaches. It also might not be recommended for girls who have had unexplained vaginal bleeding (bleeding that is not during their periods) or who think they might be pregnant. Girls who are interested in learning more about different types of birth control, including the Pill, should talk to their doctors or other health professionals. How Do You Get It? A doctor or a nurse practitioner (NP) must prescribe the Pill. He or she will ask about a girl's health and family medical history, and do a complete physical exam, which might include a pelvic exam. If the doctor or NP prescribes birth control pills, he or she will explain when to begin taking the Pill and what to do if pills are missed. The doctor or NP will usually ask the girl to come back in a few months to have her blood pressure checked and to see if she is having any problems. After that, girls who are having sex should get routine exams every 6 months to a year, or as recommended. How Much Does It Cost? The Pill usually costs between $0 and $50 a month, depending on the type. Many health and family planning clinics (such as Planned Parenthood) sell birth control pills for less. In addition, birth control pills and doctor visits are covered by many health insurance plans. Back to Articles Related Articles About Birth Control Before you consider having sex, you need to know how to protect yourself. Read this article to get the basics on birth control. Read More Birth Control Methods: How Well Do They Work? Some birth control methods work better than others. This chart compares how well different birth control methods work. Read More STDs (Sexually Transmitted Diseases) You've probably heard lots of discouraging news about sexually transmitted diseases. The good news is that STDs can be prevented. Find out how to protect yourself. Read More When Is it Time to Start Seeing a Gynecologist? Find out what the experts have to say. Read More Gyn Checkups Girls should get their first gynecological checkup between ages 13 and 15. Find out what happens during a yearly gyn visit -- and why most girls don't get internal exams. Read More Female Reproductive System Why do girls get periods? What goes on when a woman gets pregnant? What can go wrong with the female reproductive system? Find the answers to these questions and more in this article for teens. Read More Talking to Your Partner About Condoms Some people - even those who are having sex - are embarrassed by the topic of condoms. Here are some tips for talking about condoms with your partner. Read More Do You Need a Pelvic Exam to Get Birth Control? Find out what the experts have to say. Read More How Can I Get on the Pill Without Telling My Parents? Find out what the experts have to say. Read More How Can I Get the Pill if I Don't Have a Family Doctor? Find out what the experts have to say. Read More Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.