Does Antibiotic Use Cause Breast Cancer?
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Does Antibiotic Use Cause Breast Cancer?

Doctors and scientists say it's too early to tell whether the two are linked, but that patients should be careful about over-using antibiotics.

Antibiotic Use and Breast Cancer Risk

You may have seen or heard the recent news reports on a possible link between antibiotic use and breast cancer risk. These reports are based on a study published in the February 18, 2004 issue of the Journal of the American Medical Association, which found that women who had more than 25 antibiotic prescriptions over an average period of 17 years were significantly more likely to develop breast cancer than women who never took them.

If you take or have taken antibiotics, you may be concerned that you have put yourself at greater risk for breast cancer. But experts say that it is too early for such concerns. Study coauthor Stephen Taplin, MD, of the National Cancer Institute's Division of Cancer Control and Population Sciences, stressed that the results do not necessarily mean that antibiotics cause breast cancer. Rather, they simply mean that researchers have found an important association worthy of further investigation.

"This is not a reason to feel scared or to stop taking antibiotics," Dr. Taplin said. "None of us believes that it is anything more than a very interesting finding that needs to be looked at more carefully. It's a clue to where we might look to find more answers."

The study's lead author, epidemiologist Christine Velicer, PhD, Dr. Taplin, and a team of other researchers compared nearly 3,000 women with breast cancer who were enrolled in the Group Health Cooperative health plan in Seattle with nearly 8,000 plan members who did not have breast cancer. They 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 not taken any antibiotics. The authors found an increased risk in all classes of antibiotics that they studied. "One of the main questions now is whether the antibiotics themselves, or the underlying conditions that lead to antibiotic use, are increasing breast cancer risk," says Dr. Taplin.

It may be that women with a tendency to develop bacterial infections treated by antibiotics are also more susceptible to the development of cancer. On the other hand, the antibiotics themselves are known to affect certain bodily processes -- such as the immune response, and the absorption of cancer-fighting chemicals in food -- and one or more of these effects may raise breast cancer risk. Future studies will be needed to explore these possibilities.

When Are Antibiotics Necessary?

Coincidentally, the results of this study come at a time when the U.S. Centers for Disease Control and Prevention (CDC) is working to make the public more aware of the dangers of overusing antibiotics, mainly through its National Campaign for Appropriate Antibiotic Use.

Antibiotics are regularly prescribed for conditions such as respiratory and ear infections, acne, and urinary tract infections. While such uses are appropriate, these medications are more commonly being misused -- to treat a mild infection that would likely clear up on its own, symptoms that are not infection-related, or conditions that are caused by viruses, such as a cold or flu. These viral conditions are not cured with antibiotics.

As more and more patients ask for antibiotics and more and more doctors comply, the very bacteria these medicines are designed to fight are developing the ability to resist their effects.

"People who have what is clearly a bacterial infection, with a clear need for antibiotics, should continue to take them," Dr. Taplin said. "There's no reason not to."

"But I think there's a message here for people who are pushing doctors for antibiotics, and taking them unnecessarily," he added.

To be smart about your own antibiotic use, consider discussing the following questions with your healthcare provider:

  1. Is an antibiotic likely to be beneficial for my illness? Is there anything else I can do to feel better sooner?
  2. Are there any alternatives to taking an antibiotic?
  3. Could my symptoms be due to a viral infection that won't be helped by an antibiotic?
  4. Are there tests that should be done to confirm whether or not I have a bacterial infection?
  5. For how long and how often should I take the antibiotic? Would there be any reason for me to stop taking it sooner?
  6. Is there a chance that I will be resistant to this antibiotic's effects? If so, how will I know, and what can I do about it?


From the National Women's Health Resource Center. Copyright 2003-2004 by the National Women's Health Resource Center, Inc. (NWHRC). All rights reserved. Reproducing this content in any form is prohibited without written permission. For more information, please contact


American Cancer Society. "Too Soon to Worry about Antibiotic/Breast Cancer Link." February 18, 2004. Available online at


Centers for Disease Control and Prevention. "Get Smart: Know When Antibiotics Work."


Warren King. "Study Links Antibiotics, Breast Cancer." Seattle Times, February 17, 2004. National Cancer Institute. "Study Shows Link Between Antibiotic Use and Increased Risk of Breast Cancer" and "Q&A: Understanding the Results." February 18, 2004.


Roberta B. Ness and Jane A. Cauley. "Antibiotics and Breast Cancer: What's the Meaning of This?" JAMA. 2004; 291: 880-81.

Christine M. Velicer et al. "Antibiotic Use in Relation to Risk of Breast Cancer." JAMA. 2204; 291: 827-35.