# Ornidazole: veterinary dose and monograph

> Ornidazole is a veterinary antibiotic. A nitroimidazole closely related to metronidazole/tinidazole, with a long half-life (allowing once–twice-daily dosing) and generally good GI tolerance — active against anaerobic bacteria and protozoa (Giardia, amoebae, Trichomonas). Indications include giardiasis (dogs and cats); anaerobic bacterial infections; amoebic / protozoal enteritis. The references give no recommended dose for the species on file; see the species notes below before considering it.

Source: https://vetmasterji.com/drugs/ornidazole · Last reviewed: 2026-10-01

## 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

*Ornidazole doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| No reference dose for this species | Not established in Papich 5e or Plumb’s 10e — do not extrapolate from another species | — | — | — |

- 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

*Ornidazole doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| No reference dose for this species | Not established in Papich 5e or Plumb’s 10e — do not extrapolate from another species | — | — | — |

- 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

> **Caution:** 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

> **Caution:** 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](https://vetmasterji.com/drugs/neomycin)
- [Nitrofurantoin](https://vetmasterji.com/drugs/nitrofurantoin)
- [Norfloxacin](https://vetmasterji.com/drugs/norfloxacin)
- [Orbifloxacin](https://vetmasterji.com/drugs/orbifloxacin)
- [Oxytetracycline](https://vetmasterji.com/drugs/oxytetracycline)
- [Penicillin G](https://vetmasterji.com/drugs/penicillin-g)
- [Pirlimycin](https://vetmasterji.com/drugs/pirlimycin)
- [Polymyxin B](https://vetmasterji.com/drugs/polymyxin-b)

## Frequently asked questions

### Can Ornidazole be given to dogs?

Not established in Papich 5e or Plumb’s 10e — do not extrapolate from another species (No reference dose for this species).

### Can Ornidazole be given to cats?

Not established in Papich 5e or Plumb’s 10e — do not extrapolate from another species (No reference dose for this species).

### When should Ornidazole not be used?

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).

### What are the side effects of Ornidazole in animals?

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

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.