A study done at the University of Washington at Seattle found that the greater the total number of days a woman was on antibiotics (cumulatively), the greater was her risk of breast cancer. The study, published in February 2004 in the Journal of the American Medical Association, was done by studying records of more than 10,000 women over the age of 19.
There was a 45% greater risk for women who had been on antibiotics for 1 to 50 days than for women who had never taken antibiotics. There was a 53% increased risk for those who had been on antibiotics for 51 to 100 days compared to those who had never been on them; 68% increased risk for those who had been on antibiotics for 101 to 500 days compared to those who had never been on them and a 107% increased risk for those who had been on antibiotics for 501 to 1000 days compared to those who had never been on them.
In the group of women with the greatest number of days of antibiotic treatment with tetracycline or Erythromycin-class antibiotics strictly for acne or rosacea (a chronic redness of the skin and sometimes pimples, usually on the face), there was no increased risk of breast cancer compared to women using them exclusively for throat and lung infections.