Combination birth control pills, also known as the pill, are oral contraceptives that contain estrogen and a progestin.

Combination birth control pills keep your ovaries from releasing an egg. They also cause changes in the cervical mucus and the lining of the uterus (endometrium) to keep sperm from joining the egg.

Different types of combination birth control pills contain different doses of estrogen and progestin. Continuous-dosing or extended-cycle pills allow you to reduce the number of periods you have each year.

If you want to use combination birth control pills, your health care provider can help you decide which type is right for you.

Combination birth control pills are a reliable form of contraception that's easily reversed. Fertility can return to normal almost immediately after stopping the pills. Other, noncontraceptive benefits of these pills include:

• Decreased risk of ovarian and endometrial cancers, ectopic pregnancy, ovarian cysts, benign breast disease
• Improvement in acne
• Less-severe menstrual cramps (dysmenorrhea)
• Reduction in androgen production caused by polycystic ovary syndrome
• Reduction in heavy menstrual bleeding due to uterine fibroids and other causes, as well as a reduction in related iron iron-deficiency anemia
• Relief from premenstrual syndrome (PMS)
• Shorter, lighter and more-predictable periods or, for some types of combination pills, fewer periods yearly
• Better control of monthly cycle and a reduction in hot flashes for women nearing menopause (perimenopause)

Combination birth control pills come in different mixtures of active and inactive pills, including:

• Conventional pack. The most common type contains 21 active pills and seven inactive pills. Formulations containing 24 active pills and four inactive pills, known as a shortened pill-free interval, also are available.

  You take a pill every day and start a new pack when you finish the old one (every 28 days). Bleeding occurs every month during the week when you take the last four to seven inactive pills.

• Continuous dosing or extended cycle. These typically contain 84 active pills and seven inactive pills. Bleeding generally occurs only four times a year during the seven days you take the inactive pills.

  A 365-day pill also is available. You take this pill every day at the same time. For some women, periods stop altogether. For others, periods become significantly lighter.

Continuous-dosing and extended-cycle pills might provide additional benefits of suppressing menstruation, such as:

• Prevention and treatment of excessive bleeding related to uterine fibroids
• Prevention of menstrual migraines
• Reduction in menstruation-associated worsening of certain conditions, including seizures
• Relief from pain related to endometriosis

Combination birth control pills aren't appropriate for everyone, however. Your health care provider might suggest you take another form of birth control if you:

• Are in the first month of breast-feeding
• Are older than 35 and smoke
• Have poorly controlled high blood pressure
• Have a history of or current deep vein thrombosis or pulmonary embolism
• Have a history of stroke or heart disease
• Have a history of breast cancer
• Have migraines with aura
• Have diabetes-related complications, such as nephropathy, retinopathy or neuropathy
• Have liver disease
• Have unexplained uterine bleeding
• Will be immobilized for a prolonged period due to major surgery

An estimated 9 out of 100 women taking combination birth control pills will get pregnant in the first year of use. With perfect use as directed, the pregnancy rate is less than 1 in 100 women every year.

Although taking combination birth control pills during early pregnancy doesn't increase the risk of birth defects, it's best to stop them as soon as you suspect you're pregnant. Combination birth control pills won't protect you from sexually transmitted infections.

Combination birth control pills can cause side effects such as:

• Breakthrough bleeding or spotting — more common with continuous-dosing or extended-cycle pills
• Breast tenderness
• Elevated blood pressure
• Headaches
• Nausea
• Bloating

Some side effects — including nausea, headaches, breast tenderness and breakthrough bleeding — might decrease with continued use.

Combination birth control pills increase the risk of certain conditions, which can be serious. They include:

• Blood clots in the legs
• Heart attacks and stroke, especially if you smoke
• Liver disorders
• Gallbladder disease

Consult your health care provider as soon as possible if you're taking combination birth control pills and have:

• Abdominal pain
• Breast lump
• Chest pain
• Depression
• Difficulty speaking
• Eye problems, such as blurred or double vision or loss of vision
• Fainting
• Jaundice — yellowish discoloration of the skin
• New or worsening headaches
• Seizures
• Severe allergic skin rash
• Severe leg pain or swelling
• Severe mood swings
• Two missed periods or signs of pregnancy

You'll need to request a prescription for combination birth control pills from your health care provider. Your health care provider will check your blood pressure, check your weight and review your medical history, including medications you're taking.

He or she will also ask about your concerns and preferences to help determine which combination birth control pill is right for you. Health care providers generally recommend pills with the lowest dose of hormones that will help prevent pregnancy, give you important noncontraceptive benefits and minimize side effects.

Although the amount of estrogen in combination pills can be as low as 10 micrograms (mcg) of ethinyl estradiol, most pills contain about 35 mcg. Low-dose pills can result in more breakthrough bleeding than pills with more estrogen.

Combination pills are categorized based on whether the dose of hormones stays the same or varies:

• Monophasic. Each active pill contains the same amount of estrogen and progestin.
• Biphasic. Active pills contain two combinations of estrogen and progestin.
• Triphasic. Active pills contain three combinations of estrogen and progestin. In some types, the progestin content increases; in others the progestin dose remains steady, and the estrogen content increases.

To use combination birth control pills:

• Consult your health care provider about a starting date. If you use the quick-start method, you can take the first pill in the pack immediately. If you use the Sunday start, you'll take your first pill on the first Sunday after your period starts.

  With either method, use a backup contraception method for the first seven days you take combination birth control pills. If you use the first-day start, you'll take your first pill on the first day of your next period. No backup method of contraception is needed.

• Pick a time to regularly take the pill. Following a routine might keep you from missing a pill and help you take the pill at the same time every day. For example, consider taking your pill when you brush your teeth in the morning.

• Follow your health care provider's instructions carefully. Birth control pills only work if you use them correctly, so make sure you understand the instructions. If you're using conventional combination birth control pills and want to have regular periods, you will take all of the pills in your pack — the active and the inactive ones — and start a new pack the day after you finish your current one.

  If you want to avoid monthly periods, ask your health care provider about how to take the pills and how many active pill packs you can take in a row.

• Be cautious with missed pills. If you miss an active pill, take it as soon as you remember — even if it means taking two active pills in the same day. Take the rest of the pack as usual, and use a backup method of contraception for seven days if you missed your pill by more than 12 hours.

  If you miss more than one active pill, take the last pill you missed right away. Take the rest of the pack as usual, and use a backup method of contraception for seven days. If you've had unprotected sex, consult your health care provider about emergency contraception.

• Don't take breaks between packs. Always have your next pack ready before you finish your current pack.

If you vomit within two hours after taking a combination birth control pill or have severe vomiting and diarrhea for two or more days, proceed as if you've missed a pill.