Study links antibiotics, breast cancer
The use of antibiotics has increased dramatically in the past decade, government figures show.
According to a new study, heavy use of antibiotics may increase a woman's risk of developing breast cancer.
(CNN) -- Increased use of antibiotics may heighten women's risk of breast cancer, a study looking at possible connections between the two suggests.
Researchers found that women who took antibiotics for more than 500 days or who had more than 25 prescriptions in the course of a 17-year period more than doubled their risk of breast cancer compared with women who had not taken any antibiotics.
The fewer the days on antibiotics resulted in a smaller risk, the authors wrote in the study appearing in Wednesday's Journal of the American Medical Association.
"It's as strong as any of the risk factors that we know," said Dr. Roberta Ness of the University of Pittsburgh Graduate School of Public Health, who is author of an editorial accompanying the study.
"To put it into perspective, the risk for developing breast cancer from hormone replacement use is about a 30 [percent] to 40 percent increase in risk. And here we're talking about a doubling in risk of those women who are using chronic antibiotics."
But researchers caution that the findings do not mean antibiotics cause breast cancer.
"These results only show that there is an association between the two," co-author Dr. Stephen Taplin of the National Cancer Institute said in a statement. "More studies must be conducted to determine whether there is indeed a direct cause-and-effect relationship."
Taking computerized pharmacy and breast cancer screening data in Washington state, researchers compared the use of some of the most frequently prescribed antibiotics by 2,266 women with breast cancer and almost 8,000 without the disease.
Why antibiotics may possibly increase breast cancer risk is still a mystery, researchers said. The conditions that necessitated the antibiotics in the first place may have put the women at higher risk. Or, researchers said, the women in the study who had never taken antibiotics might have been generally healthier overall.
Another theory suggested in the study involves the way antibiotics affect bacteria in the intestine, which may disable possible cancer-fighting properties of some foods. Other explanations involve the effect of antibiotics on the body's immune system.
The research is not the first to show an association between antibiotics and a higher risk of breast cancer, the second most deadly cancer in women. In 1999, a Finnish study of almost 10,000 women found similar results.
Authors of the new study included members from the Fred Hutchinson Cancer Research Center, Group Health Cooperative and University of Washington -- all in Seattle -- and the National Cancer Institute, based in Bethesda, Maryland.
According to the Centers for Disease Control and Prevention, the overuse of antibiotics has exploded in the past 10 years, with the public mistakenly taking the medicine for colds, flu and coughs. These types of illnesses are caused by viruses and cannot be helped with antibiotics, which fight bacterial infections.
Lead study author Dr. Christine Velicer of the Group Health Cooperative said that more research needs to be done to understand any link between breast cancer and antibiotics and that the drugs remain an important health tool.
"At this point, continued, prudent use of antibiotics and recognizing the substantial benefits that antibiotics have is an important way to go," Velicer said.