Vancomycin is a complex glycopeptide that binds to precursors of the peptidoglycan layer in bacterial cell walls. This effect prevents cell wall synthesis and produces a rapid bactericidal effect in dividing bacteria. Its efficacy is time dependent. Vancomycin is active against most gram-positive bacteria but is not effective against gram-negative cells because of their large size and poor penetrability. Resistance to vancomycin does not readily develop. The drug is widely distributed in the body. Excretion (in active form) is via the kidneys; in renal insufficiency, striking accumulations may develop. The plasma half-life in dogs is 2–3 hr. The only indication for use of parenteral vancomycin is serious infection due to methicillin-resistant Staphylococcus aureus. Although poorly absorbed, oral vancomycin is used to treat antibiotic-associated enterocolitis, especially if caused by Clostridium difficile. Febrile reactions and thrombophlebitis (because of tissue irritation) at injection sites may be seen. Hypersensitivity reactions are seen infrequently. Ototoxicity and nephrotoxicity were fairly common in the past but are rare today because of fewer impurities in the final form.