Ovarian and Endometrial Cancers
Many studies have found that using OCs reduces a woman's risk of ovarian cancer by 40 to 50 percent compared with women who have not used OCs. The Centers for Disease Control and Prevention's (CDC) Cancer and Steroid Hormone Study (CASH), along with other research conducted over the past 20 years, shows that the longer a woman uses OCs, the lower her risk of ovarian cancer. Moreover, this lowered risk persists long after OC use ceases. The CASH study found that the reduced risk of ovarian cancer is seen in women who have used OCs for as little as 3 to 6 months, and that it continues for 15 years after use ends. Other studies have confirmed that the reduced risk of ovarian cancer continues for at least 10 to 15 years after a woman has stopped taking OCs. Several hypotheses have been offered to explain how oral contraceptives might protect against ovarian cancer, such as a reduction in the number of ovulations a woman has during her lifetime, but the exact mechanism is still not known.
Researchers have also found that OC use may reduce the risk of endometrial cancer. Findings from the CASH study and other reports show that combination OC use can protect against the development of endometrial cancer. The CASH study found that using combination OCs for at least 1 year reduced the risk of developing endometrial cancer to half of that seen for women who never took birth control pills. In addition, the beneficial effect of OC use persisted for at least 15 years after OC users stopped taking birth control pills. Some researchers have found that the protective effect of OCs against endometrial cancer increases with the length of time combination OCs are used, but results have not been consistent.
The reduction in risk of ovarian and endometrial cancers from OC use does not apply to the sequential type of pill, in which each monthly cycle contains 16 estrogen pills followed by 5 estrogen-plus-progesterone pills. (Sequential OCs were taken off the market in 1976, so few women have been exposed to them.) Researchers believe OCs reduce cancer risk only when the estrogen content of birth control pills is balanced by progestogen in the same pill.
Cancer of the Cervix
There is some evidence that long-term use of OCs may increase the risk of cancer of the cervix (the narrow, lower portion of the uterus). The results of studies conducted by NCI scientists and other researchers support a relationship between extended use of the pill (5 or more years) and a slightly increased risk of cervical cancer. However, the exact nature of the association between OC use and risk of cervical cancer remains unclear.
One reason that the association is unclear is that two of the major risk factors for cervical cancer (early age at first intercourse, especially age 16 or younger, and a history of multiple sex partners) are related to sexual behavior. Because these risk factors may be different between women who use OCs and those who have never used them, it is difficult for researchers to determine the exact role that OCs may play in the development of cervical cancer.
The two major risk factors that contribute to the development of cervical cancer are also risk factors that contribute to the development of human papillomavirus (HPV) infection in the cervix. Of the more than 100 types of HPV, over 30 types can be passed from one person to another through sexual contact. HPV is one of the most common sexually transmitted diseases. Certain HPVs, particularly HPV type 16, are known to cause cervical cancer. Compared to non-OC users, women who use OCs may be less likely to use barrier methods of contraception (such as condoms). Since condoms can prevent the transmission of HPVs, OC users who do not use them may be at increased risk of becoming infected with HPVs. Therefore, the increased risk of cervical cancer that some studies found to be caused by prolonged OC use may actually be the result of HPV infection.
There is evidence that pill users who never use a barrier method of contraception or who have a history of genital infections are at a higher risk for developing cervical cancer. This association supports the theory that OCs may act together with sexually transmitted agents (such as HPVs) in the development of cervical cancer. Researchers continue to investigate the exact nature of the relationship between OC use and cancer of the cervix.
OC product labels have been revised to inform women of the possible risk of cervical cancer. The product labels also warn that birth control pills do not protect against human immunodeficiency virus (HIV) and other sexually transmitted diseases such as HPV, chlamydia, and genital herpes.
Liver Tumors
There is some evidence that OCs may increase the risk of certain malignant (cancerous) liver tumors. However, the risk is difficult to evaluate because of different patterns of OC use and because these tumors are rare in American women (the incidence is approximately 2 cases per 100,000 women). A benign (noncancerous) tumor of the liver called hepatic adenoma has also been found to occur, although rarely, among OC users. These tumors do not spread, but they may rupture and cause internal bleeding.