Oral Contraceptives and Cancer

by National Cancer Institute

Key Points

Some cancers depend on naturally occurring sex hormones for their development and growth. Researchers are interested in learning whether the hormones in oral contraceptives affect cancer risk in women.

Oral contraceptive use has been shown in multiple studies to decrease the risk of ovarian and endometrial cancer.

Oral contraceptives have been shown to increase the risk of cervical cancer; however, human papillomavirus is the major risk factor for this disease.

The risk of liver cancer is increased in women who take oral contraceptives and are otherwise considered low risk for the disease.

Introduction

Oral contraceptives (OCs) first became available to American women in the early 1960s. The convenience, effectiveness, and reversibility of action of birth control pills (popularly known as "the pill") have made them the most popular form of birth control in the United States. However, concerns have been raised about the role that the hormones in OCs might play in a number of cancers, and how hormone-based OCs contribute to their development. Sufficient time has elapsed since the introduction of OCs to allow investigators to study large numbers of women who took birth control pills for many years.

This fact sheet addresses only what is known about OC use and the risk of developing cancer. It does not deal with other serious side effects of OC use, such as the increased risk of cardiovascular disease for certain groups of women. Recently, alternative methods of delivering hormones for contraception have been developed, including a topical patch, vaginal ring, and intrauterine delivery system, but these products are too new to have been tested in clinical trials (research studies) for long-term safety and other effects. They also are not covered in this fact sheet.

1. What types of oral contraceptives are available in the United States ? Why do researchers believe that oral contraceptives may influence cancer risk?

Currently, two types of OCs are available in the United States . The most commonly prescribed OC contains two man-made versions of natural female hormones (estrogen and progesterone) that are similar to the hormones the ovaries normally produce. This type of pill is often called a "combined oral contraceptive." The second type of OC available in the United States is called the minipill. It contains only a type of progesterone.

Estrogen stimulates the growth and development of the uterus at puberty, causes the endometrium (the inner lining of the uterus) to thicken during the first half of the menstrual cycle, and influences breast tissue throughout life, but particularly from puberty to menopause.

Progesterone, which is produced during the last half of the menstrual cycle, prepares the endometrium to receive the egg. If the egg is fertilized, progesterone secretion continues, preventing release of additional eggs from the ovaries. For this reason, progesterone is called the "pregnancy-supporting" hormone, and scientists believe that it has valuable contraceptive effects. The man-made progesterone used in OCs is called progestogen or progestin.

Because medical research suggests that some cancers depend on naturally occurring sex hormones for their development and growth, scientists have been investigating a possible link between OC use and cancer risk. Researchers have focused a great deal of attention on OC users over the past 40 years. This scrutiny has produced a wealth of data on OC use and the development of certain cancers, although results of these studies have not always been consistent. The risk of endometrial and ovarian cancers is reduced with the use of OCs, while the risk of breast and cervical cancers is increased. A summary of research results for each type of cancer is given in Questions 2-5.

2. How do oral contraceptives affect breast cancer risk?

A woman's risk of developing breast cancer depends on several factors, some of which are related to her natural hormones. Hormonal factors that increase the risk of breast cancer include conditions that may allow high levels of hormones to persist for long periods of time, such as beginning menstruation at an early age (before age 12), experiencing menopause at a late age (after age 55), having a first child after age 30, and not having children at all.

A 1996 analysis of worldwide epidemiologic data conducted by the Collaborative Group on Hormonal Factors in Breast Cancer found that women who were current or recent users of birth control pills had a slightly elevated risk of developing breast cancer. The risk was highest for women who started using OCs as teenagers. However, 10 or more years after women stopped using OCs, their risk of developing breast cancer returned to the same level as if they had never used birth control pills, regardless of family history of breast cancer, reproductive history, geographic area of residence, ethnic background, differences in study design, dose and type of hormone, or duration of use. In addition, breast cancers diagnosed in women after 10 or more years of not using OCs were less advanced than breast cancers diagnosed in women who had never used OCs. To conduct this analysis, the researchers examined the results of 54 studies. The analysis involved 53,297 women with breast cancer and 100,239 women without breast cancer. More than 200 researchers participated in this combined analysis of their original studies, which represented about 90 percent of the epidemiological studies throughout the world that had investigated the possible relationship between OCs and breast cancer.

The findings of the Women's Contraceptive and Reproductive Experiences (Women's CARE) study were in contrast to those described above. The Women's CARE study examined the use of OCs as a risk factor for breast cancer in women ages 35 to 64. Researchers interviewed 4,575 women who were diagnosed with breast cancer between 1994 and 1998, and 4,682 women who did not have breast cancer. Investigators collected detailed information about the participants' use of OCs, reproductive history, health, and family history. The results, which were published in 2002, indicated that current or former use of OCs did not significantly increase the risk of breast cancer. The findings were similar for white and black women. Factors such as longer periods of use, higher doses of estrogen, initiation of OC use before age 20, and OC use by women with a family history of breast cancer were not associated with an increased risk of the disease.

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