Metronidazole is a nitroimidazole antibiotic with an anaerobic antibacterial spectrum with antiprotozoal activity against Giardia spp. In an anaerobic environment, metronidazole is converted to unstable intermediates (nitroso free radicals) that disrupt bacterial DNA synthesis. Immunomodulatory properties capable of inhibiting cell-mediated immunity have been described for metronidazole, although its immunomodulatory properties are reported at dosages well beyond what is recommended for clinical use, raising questions about the clinical use of metronidazole as an adjunctive therapy for treating inflammatory bowel disease.
Resistance to metronidazole is considered rare.The most common adverse reaction is gastrointestinal upset, including inappetence, anorexia, nausea, and vomiting. Profuse salivation can occur in cats after oral administration of metronidazole base (formulation used in standard tablets), which has lead to the use of metronidazole benzoate (a compounded formulation not approved by the Food and Drug Administration) in some cats because of its better oral palatability.At high doses (>200 mg/kg/day) benzoic acid is reported to be neurotoxic in cats, but with appropriate clinical dosing of metronidazole benzoate benzoic acid toxicity is unlikely. Dose-related metronidazole toxicity in cats results in cerebellovestibular ataxia secondary to gamma-aminobutyric acid (GABA) inhibition at dosages greater than or equal to 58 mg/kg/day; clinical signs include nystagmus, head tilt, ataxia, seizures, and obtundation.