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  • Posted: 02/16/2004

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Study Shows Link Between Antibiotic Use and Increased Risk of Breast Cancer


Key Points
  • The authors of the study found that antibiotic use was associated with an increased risk of breast cancer. The more antibiotics the women in the study had used, the higher their risk of breast cancer. (Question 1)
  • These results do not mean that antibiotics cause cancer, only that there is an association between the two. (Question 2)
  • Breast cancer is the second leading cause of cancer deaths among women in the United States. An estimated 40,110 women will die of the disease this year. (Question 9)
  • Antibiotics are used extensively in this country and in many parts of the world. They can have substantial benefits when used to treat bacterial infections. Antibiotics also are overused in many parts of the world, contributing to the growth of antibiotic resistance. This study provides more reason to use antibiotics wisely. (Questions 9, 12 and 13)
  • More studies must be done to understand what causes the increased risk of breast cancer with antibiotic use. (Questions 2, 7, 8, 14)

UNDERSTANDING THE RESULTS

  1. What were the basic findings of this study?

    Antibiotic use is associated with an increased risk of breast cancer. The authors found that the more antibiotics the women in the study had used, the higher their risk of breast cancer.

  2. What do these results mean?

    These study results do not mean that antibiotics cause cancer. These results only show that there is an association between the two. More research is necessary to learn if there is a direct cause-and-effect relationship.

  3. What specifically did the authors find?

    The authors found that women who took antibiotics for more than 500 days—or had more than 25 prescriptions—over an average period of 17 years had twice the risk of breast cancer as women who had taken no antibiotics. The risk was smaller for women who took fewer antibiotics, but even women who had between one and 25 prescriptions over an average period of 17 years had an increased risk; they were about 1.5 times more likely to be diagnosed with breast cancer than those women who didn't take any antibiotics. The authors found the increase in risk across all classes of antibiotics that they studied.

  4. Should women not take antibiotics, even if their doctor tells them that they have a bacterial infection?

    No. There is no reason to avoid antibiotics for bacterial infections; antibiotics can have a significant health benefit for such infections. However, this study provides more reason for people to use antibiotics wisely—to take antibiotics only for bacterial infections and not to use them to try to treat common colds and other illnesses caused by viruses.

  5. What should women do who take antibiotics long-term for conditions such as acne or recurrent bladder infections?

    Women should talk to their healthcare providers about the risks and benefits of antibiotic use for their condition(s). Antibiotics should be used as recommended only when patients and their physicians have determined that they are the best treatment. In addition, women should follow their doctor's recommendations regarding breast cancer screening.

  6. Is it possible that other risk factors for breast cancer could explain the increased risk seen in this study?

    Factors such as age, reproductive and menstrual history, lifestyle factors, use of hormone therapy, and high breast density, among other factors, can affect breast cancer risk. The authors designed the JAMA study in such a way as to minimize the chance that these and other factors would affect the results. They matched the “cases”—women who were diagnosed with breast cancer—and “controls”—women who did not have breast cancer—with regard to these factors. For example, 23.7 percent of cases had a body mass index of more than 30.0 kg/m2 and almost the same number (21.8 percent) of controls had the same body mass index, lowering the chance that the higher risk of breast cancer seen in the case group was due to obesity.

    Additional information about breast cancer and risk factors is available at http://www.cancer.gov/cancertopics/types/breast or by calling the NCI's Cancer Information Service at 1–800–4–CANCER (1–800–422–6237).

  7. What factors, other than antibiotic use, might explain why the women in this study have an increased risk of breast cancer?

    It is possible that the conditions that required the women to take the antibiotics might have increased their risk of breast cancer. For example, a weakened immune system might lead both to infections that require antibiotics and to cancer development.

    It is also possible that women who have not taken any antibiotics may be a uniquely healthy group in terms of general well-being or lifestyle factors, possibly accounting for some of the increased breast cancer risk that the authors observed among antibiotic users.

  8. Alternatively, if there is a cause-and-effect relationship, how do researchers think that antibiotic use might lead to breast cancer?

    Based on current understanding, there are a couple of possible explanations for why antibiotics might lead to breast cancer. One theory is that antibiotics can affect bacteria in the intestine, which may impact the way certain foods that protect against cancer are broken down in the body. Another possibility is that antibiotics can affect the body's immune response and response to inflammation, both of which could be related to the development of cancer. Many more studies are necessary to better understand how or why antibiotics could lead to cancer.

    ABOUT THE STUDY

  9. Why did the authors conduct this study?

    Breast cancer is the second leading cause of cancer deaths among women in the United States. It is estimated that 40,110 women will die of the disease in 2004. In addition, breast cancer is the most common cancer in women worldwide.

    Antibiotics are used—and overused—extensively throughout the world to treat many conditions, illnesses, and diseases. For example, more than 70 percent of women in the study had used between one and 25 prescriptions for antibiotics in the past 17 years. Only 18 percent of women in the study had not filled any antibiotic prescriptions during their enrollment in the health plan. About 2 percent to 3 percent of the women in the study had used more than 50 prescriptions over an average time period of 17 years. For more information about antibiotics, please see Questions 12 and 13.

    The idea that antibiotics might increase cancer risk was first proposed decades ago. Before this study in the Journal of the American Medical Association, there had been only one study to examine the association between antibiotics and cancer risk, and it also focused on breast cancer. That study, of almost 10,000 women in Finland, was less thorough in its evaluation of antibiotic use, but still found that increased antibiotic use was associated with an increased risk of breast cancer.

  10. How was this JAMA study conducted?

    The authors used computerized pharmacy and breast cancer screening databases at Group Health Cooperative in Seattle, a large, non-profit health plan in Washington state. They compared the antibiotic use of 2,266 women with breast cancer to similar information from 7,953 women without breast cancer. All the women in the study were age 20 and older, and the researchers examined a wide variety of the most frequently prescribed antibiotic medications.

  11. Who funded this study?

    The study was funded by the National Cancer Institute and by the Gustavus and Louise Pfeiffer Research Foundation.

    ABOUT ANTIBIOTICS

  12. What conditions are antibiotics used to treat?

    Antibiotics are used to fight bacterial infections. They are commonly prescribed to treat bladder and respiratory infections, acne, and rosacea, among other conditions. Rosacea is a disease that affects the skin and sometimes the eyes, causing redness, pimples and, in advanced stages, thickened skin.

  13. What should people know about antibiotics?

    Antibiotics can show substantial results when used to treat bacterial infections and should be taken exactly as prescribed by a healthcare provider. It is important to note, however, that antibiotics won't help at all against the flu or the common cold, which are caused by viruses, not bacteria. Over the past decade, overuse of antibiotics has become a serious problem. According to the Centers for Disease Control and Prevention (CDC), tens of millions of antibiotics are prescribed for viral infections that are not treatable with antibiotics, contributing to the alarming growth of antibiotic resistance.

    To address this growing health concern, efforts are underway such as the “Get Smart: Know When Antibiotics Work” campaign—unveiled last year by the Department of Health and Human Services’ CDC and the Food and Drug Administration (FDA) and other partners—to lower the rate of antibiotic overuse. For more information about the “Get Smart: Know When Antibiotics Work” campaign, please visit http://www.cdc.gov/drugresistance/community.

    FUTURE WORK

  14. What studies must be done to understand more about the link between antibiotic use and breast cancer?

    Scientists must conduct additional studies to determine what causes the increased risk of breast cancer with antibiotic use. In addition, more studies are necessary to determine whether the risk of breast cancer is higher for women who used antibiotics at a certain period of their life, such as during adolescence or during pregnancy, or whether women who used antibiotics for specific conditions have a higher risk than women who used the drugs to treat other conditions.

Resources

To view the NCI press release on this JAMA study go to http://www.cancer.gov/newscenter/pressreleases/antibiotics.

Additional information about breast cancer is available at http://www.cancer.gov/cancertopics/types/breast.

For information about cancer, please visit the NCI home page at http://www.cancer.gov or call the NCI's Cancer Information Service at 1–800–4–CANCER (1–800–422–6237).

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