NCI Cancer Bulletin: A Trusted Source for Cancer Research NewsNCI Cancer Bulletin: A Trusted Source for Cancer Research News
February 17, 2004 • Volume 1 / Number 7 E-Mail This Document  |  Download PDF  |  Bulletin Archive/Search  |  Subscribe

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

A study published this week in the Journal of the American Medical Association (JAMA) provides evidence that use of antibiotics is associated with an increased risk of breast cancer. The authors concluded that the more antibiotics the women in the study used, the higher their risk of breast cancer.

The results of this study do not mean that antibiotics cause breast cancer. They only show that there is an association between the two. Additional studies must be conducted to determine whether there is a direct cause-and-effect relationship.

Antibiotic Drugs The authors of the study 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 more than twice the risk of breast cancer as women who had taken no antibiotics. The risk was smaller for women who took antibiotics for fewer days.

However, even women who had between 1 and 25 prescriptions had an increased risk; they were about 1.5 times more likely to be diagnosed with breast cancer than women who didn't take any antibiotics. The authors found an increased risk across all classes of antibiotics that they studied.

To gather the necessary data, the researchers used computerized pharmacy and breast cancer screening databases at Group Health Cooperative (GHC), a large, nonprofit 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 aged 20 and older, and the researchers examined a wide variety of the most frequently prescribed antibiotic medications.

The authors offered a few possible explanations for the observed association between antibiotic use and increased breast cancer risk. Antibiotics can affect bacteria in the intestine, which may impact how certain foods that might prevent cancer are broken down in the body. Another hypothesis focuses on antibiotics' effects on the body's immune response and response to inflammation, which could also be related to the development of cancer. It is also possible that the underlying conditions that led to the antibiotics prescriptions caused the increased risk, or that a weakened immune system - either alone, or in combination with the use of antibiotics - is the cause of this association.

The results of the study are consistent with an earlier Finnish study of almost 10,000 women. Further studies must be conducted, however, to understand why the researchers saw this increased risk with antibiotic use. Studies are also necessary to clarify whether specific indications for antibiotic use, such as respiratory or urinary tract infection, or times of use, such as adolescence or menopause, are associated with increased breast cancer risk. Additionally, breast cancer risks could differ between women who take low-dose antibiotics for a long period of time and women who take high-dose antibiotics only once in a while.

Antibiotics are regularly prescribed for conditions such as respiratory infections and acne, in addition to a wide range of other conditions or illnesses. In the JAMA study, for example, more than 70 percent of women had used between 1 and 25 prescriptions for antibiotics to treat various conditions over an average 17-year period, and only 18 percent of women in the study had not filled any antibiotic prescriptions during their enrollment in the health plan.

Over the past decade, overuse of antibiotics has become a serious problem. According to the Centers for Disease Control and Prevention, tens of millions of antibiotics are prescribed for viral infections that are not treatable with antibiotics, contributing to the alarming growth of antibiotic resistance.

"Until we understand more about the association between antibiotics and cancer," said co-author Dr. Stephen H. Taplin of NCI's Division of Cancer Control and Population Sciences and formerly of GHC, "people should take into account the substantial benefits that antibiotics can have, but should continue to use these medicines wisely."