MONDAY, Feb. 16, 2004 (HealthDayNews) -- A provocative new study provides the first real indication that antibiotic use may be tied to a greater risk of breast cancer.
Experts are quick to point out, however, that the study, appearing in the Feb. 18 issue of the Journal of the American Medical Association, does not prove a definitive cause-and-effect relationship.
"There are three possibilities in terms of interpreting this," says Dr. Roberta B. Ness, chairwoman of the department of epidemiology at the University of Pittsburgh School of Public Health and author of a related editorial in the journal. "One is that chronic antibiotic use may increase the risk for breast cancer -- that is to say, the antibiotics themselves may have some carcinogenic effect."
"Second is that the antibiotics are not harmful but they mark something that's harmful," Ness continues. "The third possibility is that the authors have observed something in this study that won't be replicated."
The study authors looked at medical data from 10,219 women enrolled at Group Health Cooperative (GHC), a nonprofit health plan in western Washington state, with an eye to seeing if there was any association between breast cancer and antibiotic use.
In the group, 2,266 women had had breast cancer while 7,953 had not. The GHC pharmacy database provided information on cumulative number of days of antibiotic use and total number of antibiotic prescriptions for each woman.
As it turned out, as the total number of days of antibiotic use and total number of antibiotic prescriptions increased, so did the apparent risk of breast cancer and death from breast cancer. This was true for all classes of antibiotics.
Women who had used antibiotics for a total of one to 500 days had an increased risk of breast cancer of about one-and-a-half times that of women who had never taken antibiotics. Women who used antibiotics for 501 to 1,000 days had an increased risk of about one-and-two-thirds, while those who used them for 1,001 or more days had more than double the risk. These numbers refer to relative risk, not absolute risk, which is much lower.
Among women who used large amounts of macrolide and tetracycline antibiotics, there was no difference in the incidence in breast cancer between those who used the drugs for acne and/or rosacea or for respiratory tract infections. There has been concern that women with acne or rosacea might be at higher risk for breast cancer because of altered hormone levels.
Whatever the underlying reason turns out to be, the current study does not mean women should forgo antibiotics if they truly need them, experts quickly point out.
"This study does not say that antibiotic use causes breast cancer," says senior study author Christine M. Velicer, a research associate at the Center for Health Studies at Group Health Cooperative in Seattle. "Any public health implications would be very premature. . . Our bottom-line message is that we recommend continued prudent use of antibiotics. If a woman goes to the doctor and they decide that antibiotics are the best course of action, she should be comfortable with taking them."
Her comments were echoed by Jeanne Calle, director of analytical epidemiology for the American Cancer Society. "It is critical to realize we cannot say with any certainty how valid these results are until and unless they are replicated in additional studies," Calle says in a statement. "Clearly no one should stop using antibiotics in acute situations based on the results of this single study."
The research is important and essentially the first in its class. "Frankly, this study is really the first large and well-conducted report that makes this assertion," Ness says.
But even biologically, the results do not necessarily make sense.
"All of the antibiotic classes had a similar effect, and these antibiotics work by very different mechanisms and have very different effects on the body," Ness adds. "Therefore, it's difficult to understand biologically how that could be the case."
For that reason, Ness and her co-author, Jane Cauley, propose that this might be a "marker effect" or an epiphenomenon. "It's not really the antibiotic, but the reason why people use the antibiotics," Ness explains. That reason is always the same, she adds: an infection of some kind that involves inflammation.
"Because there is a rich literature that suggests that chronic inflammation is associated with cancer, our guess was that perhaps that was what we were seeing here," Ness says.
It's also possible that women who use antibiotics have weakened immune systems that make them more susceptible to both infections and cancer, Velicer adds.
There are just as many biological reasons why antibiotic use might be associated with a decreased risk of breast cancer, Velicer notes.
Whatever the case, it's not a time for panic.
"It's a first report, and first reports can be meaningful or not meaningful," Ness says. "It may be something that's behind the antibiotics. This is not a time for panic."
On the other hand, anything that reduces antibiotic use is good. "We know that the overuse of antibiotics is harmful," Ness says. "We know that overuse causes resistant bugs. And so, if this study serves to encourage women to question the use of unnecessary antibiotics, that would not be a bad thing."