Birth control pills and cancer

ROC STATUS: Most birth control pills contain two active ingredients, a form of estrogen and a form of progesterone. Collectively, estrogens have been classified as a known human carcinogen since 1985. Progesterone has been listed as a reasonably anticipated carcinogen since 1985.

ALSO KNOWN AS: Oral contraceptives

RELATED CANCERS: Breast cancer, cervical cancer

DEFINITION: A birth control pill is a tablet containing a synthetic estrogen, such as ethinylestradiol; a synthetic progesterone, such as norethindrone; or both. These compounds mimic the action of naturally occurring sex hormones and are administered to alter the function of tissues that respond to these hormones. For example, a moderate, consistent dose of estrogen will prevent the typical spike in estrogen that occurs naturally during the middle of the menstrual cycle. This, in turn, prevents the ovary from releasing an egg.

Estrogen and progesterone have different effects on reproductive tissues that, in combination, are highly effective in preventing pregnancy. Although the majority of birth control pills contain a dose of each hormone, minipills that include only progesterone are also available. Although contraception is the most common reason for using birth control pills, the pill is also commonly prescribed for other purposes, such as menstrual conditions and acne.

Exposure routes: Birth control pills are tablets that are ingested orally. Several alternative delivery systems, such as transdermal patches, contain similar active ingredients that are taken into the body through other routes.

Where found: Birth control pills are manufactured by drug makers and prescribed by health providers. In 2024, the US Centers for Disease Control and Prevention estimated that 14 percent of American females ages 15–49 use birth control pills. Globally, in 2023, the World Health Organization estimated that 874 million women employed family planning methods, including birth control pills.

At risk: At highest risk are women of reproductive age, particularly in developed countries where the pill is more widely available, and pill use is more prevalent. A small minority of pill users are women who take the pills to manage menopausal symptoms.

Etiology and symptoms of associated cancers: The majority of breast cancers, though not all, involve the abnormal growth and proliferation of cells that are responsive to estrogen. Studies in mice and rats indicate that exposure to either synthetic estrogen alone or synthetic progesterone alone caused malignant tumors to develop in mammary tissue. Likewise, combinations of synthetic estrogen and progesterone appear to cause the development of both benign and malignant mammary tumors in animals. In humans, breast cancer risk increases with prolonged exposure to estrogen from both internal and external sources. However, not all studies of birth control pill users have demonstrated an increased risk of breast cancer. A 2014 Cancer Research case-control study conducted by Elisabeth F. Beaber et al. found that usage of high-dose estrogen pills was associated with a higher risk of breast cancer as compared to lower-dose pills and non-use. After long-term cessation of pill use (ten years or more, according to the National Cancer Institute), the risk of breast cancer appears to decrease to a level comparable to that of women who never used the pill. In 2024, the Susan B. Komen Foundation published data collected from seven studies that indicated higher blood estrogen levels were linked to increased risk of breast cancer in premenopausal women.

The risk of cervical cancer is also increased with birth control pill use and appears to be related to the duration of pill use, although the risk seems to decline over time after cessation of use. Some researchers have argued that pill users differ from nonusers in risk factors for human papillomavirus (HPV) infection, which is responsible for the development of many cervical cancers.

Conversely, the use of birth control pills appears to reduce the risk of some types of cancer. There is abundant evidence suggesting that birth control pill use reduces a woman’s risk of developing ovarian cancer, possibly due to fewer cycles of ovulation over her life span. Birth control pill use also appears to decrease the risk of cancer of the endometrium (the uterine lining), with protective effects lingering long after cessation of pill use.

History: The first birth control pill became available in the United States in 1960, and birth control pills were widely adopted in the decades that followed. Early concern about the safety of birth control pills linked the high doses of estrogen in birth control pills with an increased risk of serious cardiovascular events, including heart attack and stroke. Over time, drug manufacturers lowered the dosages of estrogen in birth control pills.

By the 1980s, researchers had observed associations between birth control pill use and cancer risk. However, studies were inconsistent in confirming this relationship, and several large population-based studies were undertaken to clarify the effect of pill use on cancer risk. After a review of multiple such studies, the International Agency for Research on Cancer, a division of the World Health Organization, classified combined estrogen-progesterone birth control pills as carcinogenic to humans.

In 2024, the MD Anderson Center published information that showed both the positive aspects of birth control use as well as the risk factors. In citing figures from the National Cancer Institute, MD Anderson postulated that these contraceptives do impact risk. In some cases, these risks were slightly elevated, but in others, birth control use decreased the threat of cancers such as ovarian and endometrial (uterine) cancer. This was particularly true for younger women who decide to utilize them.  

Overall, the report indicated that other factors, including those of lifestyle, were more likely to lead to breast and other cancers than birth control pills. For example, women who drank more than two alcoholic drinks daily were 13 percent more likely to contract breast cancer. MD Anderson emphasized that decisions concerning birth control should be considered more than increased cancer risk.

Bibliography

Beaber, Elisabeth F., et al. "Recent Oral Contraceptive Use by Formulation and Breast Cancer Risk among Women 20 to 49 Years of Age." Cancer Research, vol. 74, no. 15, 2014, pp. 4078–89.

"Breast Cancer Risk Factors: Birth Control Pills." Susan B. Komen Foundation, 24 Apr. 2024, www.komen.org/breast-cancer/risk-factor/birth-control-pills. Accessed 6 July 2024.

Cerhan, James R. “Oral Contraceptive Use and Breast Cancer Risk: Current Status.” Mayo Clinic Proceedings 81.10 (2006): 1287–89. Print.

Christin-Maitre, Sophie. "History of Oral Contraceptive Drugs and Their Use Worldwide." Best Practice & Research Clinical Endocrinology & Metabolism, vol. 27, no. 1, 2013.

Collaborative Group on Epidemiological Studies of Ovarian Cancer. "Ovarian Cancer and Oral Contraceptives: Collaborative Reanalysis of Data from 45 Epidemiological Studies including 23 257 Women with Ovarian Cancer and 87 303 Controls." Lancet, vol 371, 2008, pp.303–14.

"Contraceptive Use." US Centers for Disease Control and Prevention, 15 Dec. 2023, www.cdc.gov/nchs/fastats/contraceptive.htm. Accessed 6 July 2024.

"Family Planning/Contraception Methods." World Health Organization, 5 Sept. 2023, www.who.int/news-room/fact-sheets/detail/family-planning-contraception. Accessed 6 July 2024.

"Oral Contraceptives and Cancer Risk." National Institute of Cancer, 22 Feb. 2018, www.cancer.gov/about-cancer/causes-prevention/risk/hormones/oral-contraceptives-fact-sheet. Accessed 6 July 2024.

Parker, James M., and Philip M. Parker, Birth Control Pills: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego, Icon Health, 2003.

US Department of Health and Human Services, 12th Report on Carcinogens. Research Triangle Park, 2011.

Zonderman, Jon, and Laurel Shader. Birth Control Pills. New York, Chelsea, 2006.