Combined oral contraceptive: Commonly called "the pill," combined oral contraceptives are the most commonly used form of reversible birth control in the United States. This form of birth control suppresses ovulation (the monthly release of an egg from the ovaries) by the combined actions of the hormones estrogen and progestin. If a woman remembers to take the pill every day as directed, she has an extremely low chance of becoming pregnant in a year. But the pill's effectiveness may be reduced if the woman is taking some medications, such as certain antibiotics.
Besides preventing pregnancy, the pill can make periods more regular. It also has a protective effect against pelvic inflammatory disease (PID), an infection of the fallopian tubes or uterus that is a major cause of infertility in women, and against ovarian and endometrial cancers.
Birth control pills are considered safe for most women but they carry some risks. Current low-dose pills have fewer risks associated with them than earlier versions. But women who smoke, especially those over 35, and women with certain medical conditions such as a history of blood clots or breast or endometrial cancer, may be advised against taking the pill.