There are many bacteria and micro-organisms that can make your cat sick. Fighting these infections with a proper antibiotic is the way to go. Antibiotics should only be used after being instructed to do so by a veterinarian. The antibiotic you should use to treat your cat will depend on which organism is causing your cat's infection and how the medication works in your cat's body. As with all prescription medication, there are side effects, which your veterinarian will discuss with you. There are different antibiotics, such as Zeniquin, a synthetic broad-spectrum antibacterial, and Clavamox, a broad spectrum antibiotc.