Thomas W. Hale does LOTS of research on medications and breastfeeding. His book "Medications and Mothers' Milk" is probably at your peditricians office. I took atibiotics for a vaginal infection after birth which would have complicated the healing of my sutures. The medication labels always caution. If you have told your OB you are breastfeeding then just call you peditrician office also and tell them the name of the medication and they can check Hale's book for you. Most antibiotics are OK and have minimal if any transfer in your milk. They usually just caution to watch for diarrhea in your infant as a precaution. You have to be cautious about your sources on the internet. Scientifically, you can not prove that something DOESN'T happen. The scientific method doesn't work that way. However, if you have a prolonged untreated UTI the infection can move to your kidneys and that can cause problems. I was nervous also but checked with the peditrician and actually purchased Hale's book myself to check everything..