Ornidazole: veterinary dose and monograph

At a glance

Drug class

Antibiotic

Also known as

Dazolic, Ornof, Giro

General dose range

10–20 mg/kg PO q12–24h — check the species tables for the indication

Last reviewed

1 October 2026

Indications

• Giardiasis (dogs and cats)
• Anaerobic bacterial infections
• Amoebic / protozoal enteritis
• Acute diarrhoea with suspected anaerobic/protozoal component (often as ofloxacin-ornidazole)

Ornidazole dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• No reference dose for this species: Neither Papich 5e nor Plumb’s has an ornidazole monograph, so the previous 10–20 mg/kg row had no reference source. Metronidazole, tinidazole or fenbendazole have referenced doses.

Cats

• No reference dose for this species: Neither Papich 5e nor Plumb’s has an ornidazole monograph, so the previous cat row had no reference source.

Contraindications

Significant hepatic impairment. Pregnancy (especially early — nitroimidazole mutagenicity concerns). Neurologic disease / high-dose use (neurotoxicity). Known nitroimidazole hypersensitivity. Alcohol-containing vehicles (disulfiram-like reaction in some species).

Species-specific warnings

Cats: Cats are sensitive to nitroimidazole neurotoxicity — keep doses/durations conservative and discontinue at the first neurologic sign.

Adverse effects

• GI upset (often less than metronidazole), metallic taste
• Neurotoxicity (ataxia, nystagmus, seizures) at high/prolonged dose
• Lethargy, anorexia
• Rare hepatic/haematologic effects

Drug interactions

• Alcohol / alcohol vehicles: disulfiram-like reaction (some species)
• Warfarin: potentiated anticoagulation
• CYP inducers/inhibitors: alter levels
• Other neurotoxic drugs: additive CNS risk

Pregnancy and lactation

Avoid in pregnancy (nitroimidazole mutagenicity concerns), particularly early gestation.

Monitoring

• Neurological status on higher/longer dosing (vestibular signs = stop)
• GI tolerance and parasitological/clinical response (Giardia recheck)
• Liver values on prolonged therapy

Toxicity and overdose

As with the nitroimidazole class, the dose-limiting toxicity is NEUROTOXICITY — cumulative high or prolonged dosing causes central vestibular signs (ataxia, nystagmus, head tilt) and seizures; generally better GI tolerated than metronidazole. Hepatic/haematologic effects with chronic use.

Management: Stop the drug at any neurologic sign — nitroimidazole neurotoxicity resolves slowly; supportive care, and diazepam may hasten recovery (as reported for metronidazole). Decontaminate large acute ingestions; monitor liver values.

Clinical pearls

• A once-daily, well-tolerated metronidazole alternative for Giardia and anaerobic infections — cheap and ubiquitous in India
• Respect the nitroimidazole neurotoxicity ceiling — stop at the first ataxia/nystagmus; recovery is slow (diazepam may help)
• Often sold combined with ofloxacin for acute diarrhoea — be deliberate about whether both components are needed
• Avoid in pregnancy and with alcohol-containing products

Mechanism of action

A nitroimidazole pro-drug reduced inside anaerobic bacteria/protozoa to reactive nitro radicals that damage microbial DNA, causing strand breakage and death. Selectivity arises because only low-redox (anaerobic) organisms perform this reduction; aerobic mammalian cells do not. Its longer half-life than metronidazole allows less-frequent dosing.

Formulations and products

• 250 mg tablet
• 500 mg tablet
• Oral suspension / IV infusion
• Dazolic — 250 / 500 mg tablet, Tablet (human-label)
• Ornof / Giro — 500 mg tablet (± ofloxacin), Tablet (human-label)

Storage

Tablets/suspension: 15–30 °C, dry, protect from light.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other antibiotics

• Neomycin
• Nitrofurantoin
• Norfloxacin
• Orbifloxacin
• Oxytetracycline
• Penicillin G
• Pirlimycin
• Polymyxin B

Frequently asked questions