Ornidazole is a 5-nitroimidazole derivative antimicrobial agent, structurally and pharmacologically similar to metronidazole and tinidazole. It is a prodrug that is selectively activated by anaerobic and microaerophilic bacteria and protozoa, leading to cytotoxic effects. It is widely used in India for the treatment of amoebiasis, giardiasis, trichomoniasis, and anaerobic bacterial infections, including those following surgical procedures. Its longer half-life allows for less frequent dosing compared to metronidazole.
Adult: Amoebiasis: 1.5 g once daily for 3 days (or 500 mg twice daily for 5-10 days). Giardiasis: 1.5 g single dose or 500 mg once daily for 1-2 days. Trichomoniasis: 1.5 g single dose or 500 mg twice daily for 5 days. Anaerobic infections: 500 mg twice daily for 5-10 days. Surgical prophylaxis: 1 g pre-operatively.
Note: Tablets should be swallowed whole with a full glass of water, during or after meals to minimize gastric upset. For IV infusion, reconstitute and dilute as per manufacturer instructions; infuse over 30-60 minutes.
Ornidazole is a prodrug. Its nitro group is reduced intracellularly by low-redox potential electron transport proteins (ferredoxin or flavodoxin) found in anaerobic microorganisms. This reduction forms short-lived, cytotoxic nitro radical anions. These radicals cause strand breaks in microbial DNA, inhibiting nucleic acid synthesis and leading to cell death.
Pregnancy: Category B (as per some regulatory bodies, but often considered Category C). Contraindicated in the first trimester. Should be used in second/third trimesters only if the potential benefit justifies the potential risk to the fetus. Crosses the placenta.
Driving: May cause dizziness, drowsiness, and visual disturbances. Patients should be cautioned about operating machinery or driving until their response to the drug is known.
| Alcohol | Risk of disulfiram-like reaction (flushing, tachycardia, nausea). Severity is debated but caution advised. | Major |
| Warfarin and other Coumarin anticoagulants | Ornidazole may potentiate anticoagulant effect by inhibiting metabolism, increasing INR and bleeding risk. | Major |
| Phenobarbital, Phenytoin, Rifampicin | Enzyme inducers may increase metabolism of ornidazole, reducing its plasma concentration and efficacy. | Moderate |
| Cimetidine, Ketoconazole | CYP450 inhibitors may decrease metabolism of ornidazole, potentially increasing its plasma levels and risk of toxicity. | Moderate |
| Lithium | May increase lithium levels and risk of toxicity; monitor serum lithium. | Moderate |
| Cyclosporine, Tacrolimus | Potential increase in calcineurin inhibitor levels; monitor drug levels and renal function. | Moderate |
Same composition (Ornidazole (NA)), different brands: