Birth Control Pills

Birth control pills are oral tablets that are taken to prevent unintentional pregnancy. They are available by prescription and must be taken every day, at the same time each day. There are many types of birth control pills; some contain estrogen and progestin hormones, while others contain only a progestin. Potential side effects include nausea, headaches, and breakthrough bleeding.

What Are Birth Control Pills?

Birth control pills, also known as oral contraceptives, are prescription medications that are taken by mouth to prevent pregnancy. Many different kinds are currently available (see List of Birth Control Pills). While all are approved for preventing pregnancy, some are approved for additional uses.
(Click Birth Control Pill Uses for more information, including possible off-label uses.)

How Do They Work?

Birth control pills contain hormones that act in a few different ways to prevent pregnancy. Most are classified as combined oral contraceptives and contain two different types of hormones: an estrogen and a progestin. The hormones in combined oral contraceptives prevent pregnancy primarily by stopping ovulation (the maturation and release of eggs from the ovaries). However, they also work to prevent pregnancy in two other, less important ways. Birth control pills change the cervical mucus (the fluid of the cervix, which is the lower, narrow part of the uterus that is connected to the vagina), making it more difficult for sperm to enter the uterus. They also alter the lining of the uterus (the endometrium), making it less receptive to an embryo.
Some birth control pills contain just a progestin and no estrogen. These are known as progestin-only contraceptives. These pills rely more heavily on changes in the cervical mucus and uterine lining, since they are less effective at preventing ovulation (only about half of the women who take them stop ovulating). In comparison, combined oral contraceptives stop ovulation in almost all women when taken correctly.
Last reviewed by: Kristi Monson, PharmD
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