Women exposed to higher amounts of antibiotics have as much as twice the risk of breast cancer, but it is not known if the treatment is an actual cause of the most common form of cancer afflicting women worldwide, researchers said on Tuesday.

The findings however are consistent with an earlier Finnish study of almost 10,000 women and pending further research they underscore the need to use antibiotics prudently, the report said.

The study from the University of Washington in Seattle was based on a look at 10,219 women in a group health plan, some of whom were being treated for breast cancer. Antibiotic use of the patients studied was determined by the prescription records maintained by the health plan.

It 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. But even women who had between one and 25 prescriptions over an average of 17 years were about one and a half times more likely to be diagnosed with breast cancer than women who didn't take any antibiotics.

The increased risk involved all classes of antibiotics that they studied.

"Increasing cumulative days of antibiotic use was also associated with death due to breast cancer ...," said the study, which was published in the Journal of the American Medical Association.

CANCER RISK

"While the implications for clinical practice will not be clear until additional studies are conducted, the results of this study support the continued need for prudent long-term use of antibiotics and the need for further studies of the association between antibiotic use and cancer risk," the report concluded.

Though more research is needed to determine if antibiotics are a cause of the cancer, the researchers offered some possible theories on how they could be. One is that antibiotics can affect bacteria in the intestine, perhaps impacting how certain foods that might otherwise prevent cancer are broken down in the body.

Another involves the effects of antibiotics 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 underlying medical problems that led to antibiotic use are a factor, the report said.

"This is an important study, as it appears to be the first major work to describe a possible association between antibiotic use and the increased risk of cancer," commented Jeanne Calle, director of analytical epidemiology at 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. Clearly no one should stop using antibiotics in acute situations based on the results of this single study," she said.

"There is no question other researchers will now begin to investigate this potential association. Such additional studies will clarify the role, if any, of antibiotic use and breast cancer risk," she added.

(Agencies   February 20, 2004)