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Oral contraceptives (birth-control pills) are used to prevent pregnancy. Estrogen and progestin are two female sex hormones. Combinations of estrogen and progestin work by preventing ovulation (the release of eggs from the ovaries). They also change the lining of the uterus (womb) to prevent pregnancy from developing and change the mucus at the cervix (opening of the uterus) to prevent sperm (male reproductive cells) from entering. Oral contraceptives are a very effective method of birth control, but they do not prevent the spread of human immunodeficiency virus (HIV, the virus that causes acquired immunodeficiency syndrome [AIDS]) and other sexually transmitted diseases. Some brands of oral contraceptives are also used to treat acne in certain patients. Oral contraceptives treat acne by decreasing the amounts of certain natural substances that can cause acne. Some oral contraceptives (Beyaz, Yaz) are also used to relieve the symptoms of premenstrual dysphoric disorder (physical and emotional symptoms that occur before the menstrual period each month) in women who have chosen to use an oral contraceptive to prevent pregnancy.