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. The pill may also contribute to cardiovascular disease, including high blood pressure, blood clots, and blockage of the arteries.
One of the biggest questions has been whether the pill increases the risk of breast cancer in past and current pill users. An international study published in the September 1996 journal Contraception concluded that women's risk of breast cancer 10 years after going off birth control pills was no higher than that of women who had never used the pill. During pill use and for the first 10 years after stopping the pill, women's risk of breast cancer was only slightly higher in pill users than non-pill users.