If the antibiotic is formulated to kill gram negative bacteria and the dog is ill with a gram
positive bacteria, the antibiotic will be ineffective. Ditto if the antibiotic is supposed to kill
gram positive bacteria and the dog is sick with a gram negative bacteria. And if the dog has a virus and not a bacteria, the antibiotic will not help ANYTHING and, in fact, could actually serve to throw off the balance between good and bad gut flora and fauna leading to serious problems.