The use of prescription antibiotics for conditions such as respiratory infections and rosacea has been linked to an increased risk of breast cancer in women, according to a new study.
Researchers at the University of Washington, Seattle, and a nonprofit health plan found that women who took common antibiotics were up to twice as likely to develop breast cancer as women who had taken none.
The study, to be published in tomorrow's issue of The Journal of the American Medical Association, does not conclude that antibiotics cause breast cancer. In fact, the researchers note that other factors - such as weakened immune systems or hormonal imbalances - could account for the increased risk.
"It takes a lot more than one study to build a case that there is a causal relationship between any risk factor and cancer," said Christine M. Velicer, a research associate at the Group Health Cooperative in Washington state and the study's lead author.
"It's one line of reasoning, one line of questioning, that may go any of a number of directions," she said. "It could be much broader than antibiotic use; it could be the underlying condition. We haven't even begun to look at these things yet in a systematic way."
Even so, experts said, the study could lead to a better understanding of the biology of breast cancer - and underscore yet another potential hazard of the overuse of antibiotics.
Until now, there has been little research into the possible link between antibiotic use and breast cancer. The previous study to date, conducted in Finland and published in 2000, found a positive association in a group of about 10,000 women.
For the current study, Velicer analyzed pharmacy and breast cancer screening data for more than 10,000 women - age 20 or older - enrolled in the Group Health Cooperative.
Women who took antibiotics for more than 500 days, over an average of 17 years, had more than twice the risk of breast cancer as women who had taken none, the researchers found.
Even those who took antibiotics much less frequently had a greater likelihood of developing the disease: Women who used one to 25 antibiotic prescriptions were about 50 percent more likely to get it.
The type of antibiotic didn't matter. Researchers found an increased risk among all classes of the drugs, which are commonly prescribed for bacterial infections of the bladder, respiratory system and skin, among other conditions.
Dr. John D. Potter, director of public health sciences at the Fred Hutchinson Cancer Research Center in Seattle and another study author, said that the JAMA article comes with caveats and that it shouldn't, at this point, influence either clinical or policy decisions.
"We're trying to be very careful here," he said. "This is not intended to make people worried."
One possible explanation, Potter said, is that women who have never taken antibiotics are a uniquely healthy group. Another is that the drugs affect the body's immune response and its response to inflammation in a way that could be related to cancer growth.
A third is that antibiotics can alter so-called "good" bacteria in the digestive tract, thus affecting the metabolizing of foods that might be useful in fending off the disease.
Said Potter: "This is a piece that tells us we should go look to try and understand this association more, both considering whether there are other, more boring explanations or whether there is some interesting biology in here."
Breast cancer is the most common type of cancer in women worldwide and the second leading cause of cancer deaths among women in the United States.
In an accompanying JAMA editorial, Dr. Roberta B. Ness and Jane A. Cauley of the University of Pittsburgh said the research raises important questions.
"This observation is potentially worrisome in that antibiotic exposure is common and sometimes essential," they wrote.
At the same time, they noted, more research must be done before the study's implications are fully understood. "We are left with many more questions than answers," Cauley said in an interview. "Right now, it's an interesting observation."
Cauley pointed out that the study was not a random, controlled trial - the "gold standard" in clinical research. In that type of trial, a group of healthy volunteers would be given antibiotics or placebos and then monitored to see who developed cancer. But that approach would have been unethical in this case, so researchers instead compared antibiotic use in women who had breast cancer with that of women who did not have it.
Dr. Neil B. Friedman, director of the Hoffberger Breast Center at Mercy in Baltimore, said even if scientists find that antibiotics cause or contribute to breast cancer - which he doubts - the benefits of antibiotics would far outweigh the risks.
"If you need antibiotics you should still be on antibiotics," he said. "If you don't need them, you shouldn't be on them. That's no different than it was before. ... People are going to jump to conclusions that antibiotics cause breast cancer, and I think that that's a bad conclusion."