According to PetMD, penicillin is a safe antibiotic for cats. It's typically given before operations, but it's also used to treat dental and skin abscesses. Penicillin comes in capsule, liquid and injectable forms.
PetMD indicates that, if given by mouth, penicillin is most effective when the dose is administered one hour before mealtime or two hours after feeding. However, if the drug causes stomach upset, it should be administered with food. Because stomach acid neutralizes penicillin, an injection is often a better choice. It's placed directly into muscle, not the bloodstream. Side effects include nausea, diarrhea, appetite loss, fever, rash and joint pain. Occasionally, a cat has an allergic reaction that causes breathing problems or shock.