Questions and answers: Antibiotics and breast cancer study in JAMA
- What was the purpose of the study?
- What did the study find?
- How much of an increased risk did the antibiotic users in the study face?
- What are the medical and public health implications of these findings?
- What recommendations do the researchers have for women who have taken or are taking antibiotics?
- How was the study conducted?
- Why did the authors conduct this study?
- Why hasn't this issue been studied more thoroughly before?
- What reason is there to suspect that antibiotic use could lead to breast cancer?
- Are there factors other than antibiotic use that may account for the increased risk of breast cancer among women in this study?
- What conditions are antibiotics used to treat?
- What should people know in general about antibiotics?
- Who conducted the study?
- Who funded the study?
Q. What was the purpose of the study?
The researchers wanted to find out whether use of antibiotics increases women's risk of breast cancer.
To do this, they analyzed computerized pharmacy and breast cancer screening data for more than 10,000 women enrolled in Group Health Cooperative over an average of 17 years.
Antibiotic use is associated with an increased risk of breast cancer; the more antibiotics the women used, the higher their risk of breast cancer.
Q. How much of an increased risk did the antibiotic users in the study face?
The study showed that women who took antibiotics for more than 500 days (or more than 25 individual 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 one and a half times more likely to get breast cancer than those who took no antibiotics.
Keep in mind that the study compared the risks of women who took antibiotics to women who never took antibiotics. Women who never take antibiotics may be a uniquely healthy group in terms of general well-being or lifestyle characteristics, and this possibly could account for some of the increased breast cancer risk the researchers saw among antibiotic users.
Q. What are the medical and public health implications of these findings?
- Medical researchers won't know all the implications of these findings until further studies are conducted.
- There is an increasing awareness that antibiotics need to be used wisely. This study provides further motivation to do so.
- Existing guidelines for antibiotic use should continue to be followed. Guidelines are available on the CDC Web site: www.cdc.gov/drugresistance/community/faqs.htm
Q. What recommendations do the researchers have for women who have taken or are taking antibiotics?
- Talk to your doctor about the risks and benefits of antibiotic use for your condition.
- Use antibiotics as recommended only when you and your physician have determined that they are your best course of treatment. Antibiotics are used to treat bacterial infections and should not be used to treat the common cold, which is caused by a virus.
- Follow closely your doctor's recommendations regarding breast cancer screening.
Q. How was the study conducted?
The researchers used computerized pharmacy and breast cancer screening databases to study antibiotic use in a population of 2,266 women prior to their diagnosis of primary, invasive breast cancer. By comparing these data with similar information from a population of 7,953 women who did not get breast cancer, they were able to determine how antibiotic use was related to women's risk for the disease. All the women in the study were age 20 years and older. The drugs studied included a wide variety of the most frequently prescribed antibiotic medications. Topical medications and mouth rinses were not included in the study.
Q. 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 extensively throughout the world to treat many conditions, illnesses, and diseases. For example, more than 70 percent of women in the Group Health study had used between one and 25 prescriptions for antibiotics in the past 17 years. Only 18 percent of women in the study had filled no 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 period of 17 years.
The hypothesis that antibiotics might increase cancer risk was first proposed in 1981, but prior to this current study in the Journal of the American Medical Association, there had only been one study to examine the association between antibiotics and cancer risk. That study, which also focused on breast cancer, was conducted in Finland and published in 2000. It found a positive association in a group of almost 10,000 women. But it was less thorough in its evaluation of antibiotic use than the current Group Health study.
The Group Health study is only the second study of the issue and it is the first study of its kind in the United States.
Q. Why hasn't this issue been studied more thoroughly before?
The link between antibiotics and cancer is not an easy question to study. A randomized controlled trial—where researchers would intentionally give one group antibiotics over the long-term and withhold antibiotics from the rest—would not be ethical. Therefore, researchers must undertake an observational study like the one undertaken at Group Health.
Also, it's difficult for people to remember accurately what medications they have taken over a long period of time. This study used computerized pharmacy and breast cancer screening databases that allow researchers to examine breast cancer occurrence alongside a comprehensive list of all antibiotics filled by a large population of women over an extended period of time. This kind of information only became available in recent years through research institutions like Group Health Center for Health Studies.
Finally, antibiotic use is a common behavior; breast cancer, unfortunately, is a common disease. It is far harder to see an association between a common exposure and a common outcome than between a rare exposure and a rare outcome.
Q. What reason is there to suspect that antibiotic use could lead to breast cancer?
Based on current understanding, there are a few possible explanations:
- Antibiotics can affect bacteria in the intestine, which may impact the metabolism of certain foods that are hypothesized to protect us from cancer.
- Antibiotics can affect the body's immune response and response to inflammation, both of which could be related to the development of cancer.
- There may also be other ways that antibiotics could lead to cancer development. However, the research in this area has been limited and further study is needed.
Q. Are there factors other than antibiotic use that may account for the increased risk of breast cancer among women in this study?
Possibly, but more research needs to be done to find out. Some of these factors could be the conditions that required the women to take the antibiotics in the first place. For example, a weakened immune function might lead both to infections that require antibiotics and to the development of cancer. A hormone imbalance that causes acne might do the same. This study did not address that question, but these are important questions for researchers to continue to pursue.
Q. 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, cause redness, pimples and, in advanced stages, thickened skin.
Q. What should people know in general about antibiotics?
Antibiotics can show substantial results when used to treat bacterial infections and should be taken exactly as prescribed by a health care 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. More information about this campaign is available at http://www.cdc.gov/drugresistance/community/default.htm.
Christine M. Velicer, PhD, of Group Health Center for Health Studies is the study's lead author. Co-authors are:
- Susan R. Heckbert, MD, PhD, of the University of Washington Department of Epidemiology
- Johanna W. Lampe, PhD, RD, and John D. Potter, MD, PhD, of Fred Hutchinson Cancer Research Center's Public Health Sciences Division
- Carol A. Robertson, RPh, Group Health Cooperative's Northgate Medical Center
- Stephen H. Taplin, MD, MPH, of the National Cancer Institute and formerly of Group Health Cooperative.
It was supported by grants from the National Cancer Institute and the Gustavus and Louise Pfeiffer Research Foundation.
Group Health Center for Health Studies
Founded in 1947, Group Health Cooperative is a Seattle-based, consumer-governed, nonprofit health care system that coordinates care and coverage. Group Health Cooperative is ranked "Highest in Member Satisfaction among Commercial Health Plans in the Northwest Region" by J.D. Power and Associates. For 25 years, the Group Health Center for Health Studies has conducted research on preventing, diagnosing, and treating major health problems. Government and private research grants provide its main funding.