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July 2017

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Antibiotic, Antiprotozoal (Systemic Drug)

Prescriber Highlights

  • Injectable and oral antibacterial and antiprotozoal agent
  • Main side effects include neurologic disorders, lethargy, weakness, hepatotoxicity
  • Bitter tasting; may be more palatable when compounded

Uses, Indications

  • Used for treatment of Giardia spp in both dogs and cats, in addition to several other parasitic infections (eg, trichomonads, Balantidium coli) and anaerobic enteric and systemic infections 
  • Commonly used for perioperative surgical prophylaxis when anaerobic infections (eg, colon, periodontal) are likely
  • Has some inhibitory actions on cell-mediated immunity that may play a role in its use for treatment of inflammatory bowel disease

Contraindications, Precautions, Warnings

  • Contraindications

Patients with a history of hypersensitivity to metronidazole or nitroimidazole derivatives

  • Precautions

Not recommended for use in severely debilitated, pregnant, or nursing patients; use with caution in patients with hepatic dysfunction

  • Warnings

Total daily dose should not exceed 65 mg/kg because of risk for neurotoxicity

Side Effects

  • Neurologic disorders, lethargy, weakness, neutropenia, hepatotoxicity, hematuria, nausea, anorexia, vomiting, and diarrhea have been reported in dogs
  • Neurologic toxicity in dogs may occur with acute high doses but is more common with chronic moderate- to high-dose therapy
  • Vomiting, inappetence, and hepatotoxicity can occur in cats undergoing metronidazole therapy 
  • Central nervous toxicity, although rare, can also occur in cats

Drug Interactions

  • Cimetidine may decrease the metabolism of metronidazole, increasing the possibility of dose-related side effects
  • Cyclosporine levels may increase when combined with metronidazole therapy
  • Fluorouracil (5-FU) may result in increased 5-FU levels and possible toxicity
  • Phenobarbital or phenytoin may increase the metabolism of metronidazole, decreasing blood levels
  • Metronidazole may prolong prothrombin time in patients receiving warfarin or other coumarin anticoagulants


  • Clinical efficacy, side effects

Client Information

  • Clients should pay close attention to any neurologic abnormalities

Dosage Forms

  • Metronidazole oral tablets: 250 mg, 500 mg
  • Metronidazole oral solution: 5 mg/mL

Compiled and summarized from Plumb’s Veterinary Drugs by Shannon Palermo, VMD

Information about this drug was adapted from Plumb’s Veterinary Drugs. Further details and more therapeutics can be found with a subscription at

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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