A study published today 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 - from Group Health
Cooperative (GHC) in Seattle; the National Cancer Institute (NCI), a
part of the National Institutes of Health in Bethesda, Md.; the
University of Washington, Seattle; and the Fred Hutchinson Cancer
Center, also in Seattle - 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. "These results only show that there is an association
between the two," explained co-author Stephen H. Taplin, M.D., of NCI's
Division of Cancer Control and Population Sciences and formerly of the
GHC. "More studies must be conducted to determine whether there is
indeed a direct cause-and-effect relationship."
"This trial suggests another piece in the puzzle of factors
that may potentially be involved in the development of breast cancer,"
said NCI Director Andrew C. von Eschenbach, M.D. "The NCI will continue
to support research into underlying mechanisms of cancer risk."
The authors of this JAMA 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 not taken any antibiotics. The risk was
smaller for women who took antibiotics for fewer days. However, 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 women who didn't take
any antibiotics. The authors found an increased risk in all classes of
antibiotics that they studied.
"Breast cancer is the second leading cause of cancer deaths
among women in the United States - with an estimated 40,000 deaths this
year - and is the most common cancer in women worldwide," said first
author Christine Velicer, Ph.D., of GHC's Center for Health Studies.
"Antibiotics are used extensively in this country and in many parts of
the world. The possible association between breast cancer and
antibiotic use was important to examine."
To gather the necessary data, the researchers used computerized
pharmacy and breast cancer screening databases at GHC, 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.
The authors offer 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,
though, for us to know why we see this increased risk and the full
implications of these findings," said Velicer. Studies are also
necessary to clarify whether specific indications for antibiotic use,
such as respiratory infection or urinary tract infection, or times of
use, such as adolescence, pregnancy 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, acne, and urinary tract infections, in addition
to a wide range of other conditions or illnesses. In this JAMA study,
for example, more than 70 percent of women had used between one 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 (CDC), tens of millions of antibiotics are prescribed for
viral infections that are not treatable with antibiotics, contributing
to the troubling growth of antibiotic resistance. 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.
"These study results do not mean that women should stop using
antibiotics to treat bacterial infections," stressed Taplin. "Until we
understand more about the association between antibiotics and cancer,
people should take into account the substantial benefits that
antibiotics can have, but should continue to use these medicines
wisely."
Questions and Answers about this study can be found at http://www.cancer.gov/newscenter/pressreleases/AntibioticsQandA.
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