Ornidazole is a second-generation 5-nitroimidazole derivative antimicrobial and antiprotozoal agent. It is structurally related to metronidazole but has a longer half-life, allowing for less frequent dosing. It is primarily used in the treatment of anaerobic bacterial infections and protozoal infections, including amoebiasis, giardiasis, trichomoniasis, and bacterial vaginosis. In the Indian context, it is widely used for gastrointestinal and genitourinary infections, often in combination therapies.
Adult: Amoebiasis: 1.5g once daily for 5-10 days, OR 500mg twice daily for 5-10 days. Trichomoniasis: Single 1.5g dose OR 500mg twice daily for 5 days. Anaerobic infections: 500mg twice daily for 5-10 days. Prophylaxis: 1g single dose pre-surgery.
Note: Should be taken with or after food to minimize gastrointestinal upset. Tablets should be swallowed whole with a full glass of water. For the 10mg strength (if found in combination), follow the specific product's instructions. Complete the full prescribed course even if symptoms improve.
Ornidazole is a prodrug. Its nitro group is reduced intracellularly by low-redox potential electron transport proteins (ferredoxin or flavodoxin) found in anaerobic bacteria and protozoa. This reduction forms cytotoxic, short-lived nitro radical anions. These radicals cause strand breaks in microbial DNA, inhibiting nucleic acid synthesis and leading to cell death.
Pregnancy: Category C (US FDA). Contraindicated in the first trimester. Animal studies have shown fetotoxicity. Use in second and third trimesters only if the potential benefit justifies the potential risk to the fetus. Should not be used for trichomoniasis in pregnant women (use topical agents).
Driving: May cause dizziness, drowsiness, visual disturbances, or confusion. Patients should be cautioned about operating machinery or driving until they are certain the drug does not affect them adversely.
| Alcohol (Ethanol) | Disulfiram-like reaction: severe nausea, vomiting, flushing, tachycardia, headache. | Major |
| Warfarin and other Coumarin anticoagulants | Ornidazole may potentiate anticoagulant effect by inhibiting metabolism, increasing INR and risk of bleeding. | Major |
| Phenobarbital, Phenytoin, Rifampicin | Inducers of CYP450 enzymes may increase metabolism of ornidazole, reducing its plasma concentration and efficacy. | Moderate |
| Cimetidine, Ketoconazole | Inhibitors of CYP3A4 may decrease metabolism of ornidazole, increasing its plasma concentration and risk of toxicity. | Moderate |
| Lithium | May increase lithium levels and risk of toxicity; mechanism unclear. | Moderate |
| Cyclosporine, Tacrolimus | Ornidazole may increase levels of these calcineurin inhibitors, increasing risk of nephrotoxicity and neurotoxicity. | Moderate |