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# Metronidazole (Metrogyl)
## Overview
Metronidazole is an antibiotic and antiprotozoal agent.
## Primary Indications
* Treatment of anaerobic bacterial infections.
* Treatment of protozoal infections (e.g., trichomoniasis, giardiasis, amebiasis).
* Treatment of *Clostridioides difficile* infection (CDI) in certain situations.
* Surgical prophylaxis.
## Adult Dosing
* **Anaerobic bacterial infections:** Typically 500 mg IV or PO every 8 hours. Duration varies based on infection severity and site, often 7-14 days. Max dose generally 4 g/day.
* **Trichomoniasis:** 2 g PO single dose, or 500 mg PO BID for 7 days.
* **Giardiasis:** 250 mg PO TID for 5-7 days, or 2 g PO once daily for 3 days.
* **Amebiasis:** 500-750 mg PO TID for 5-10 days.
* ***Clostridioides difficile* infection (CDI):** 500 mg PO TID for 10-14 days. *Note: Vancomycin is generally preferred for initial episodes, especially severe ones.*
* **Surgical prophylaxis:** 1 g IV 1-2 hours before surgery, or 500 mg PO/IV every 8 hours starting preoperatively and continued postoperatively for up to 24 hours.
## Pediatric Dosing
Dosing is weight-based and depends on the indication. Consult specific pediatric guidelines or references.
* **Anaerobic bacterial infections:** 7.5 mg/kg/dose IV or PO every 8 hours. Max dose 500 mg/dose. Max 4 g/day.
* **Giardiasis:** 5 mg/kg/dose PO TID for 5-7 days. Max 250 mg/dose.
* **Amebiasis:** 10-20 mg/kg/day PO divided every 8 hours for 5-10 days. Max 500 mg/dose.
## Dose Adjustments
* **Hepatic Impairment:** Reduce dose by 50% in severe hepatic disease.
* **Renal Impairment:** No dose adjustment needed for hemodialysis. For severe renal impairment (CrCl < 10 mL/min) not on hemodialysis, consider reducing dose by 50% if prolonged therapy is anticipated. However, many sources state no dose adjustment is typically needed.
## Contraindications
* Hypersensitivity to metronidazole or other nitroimidazole derivatives.
* Concomitant use with disulfiram or alcohol (within 3 days of last dose).
## Adverse Effects
* **Common:** Nausea, metallic taste, headache, anorexia, diarrhea, abdominal pain.
* **Serious:** Peripheral neuropathy (especially with prolonged use), central nervous system effects (dizziness, ataxia, seizures), Stevens-Johnson syndrome, neutropenia, disulfiram-like reaction with alcohol.
## Key Drug Interactions
* **Alcohol:** Disulfiram-like reaction (flushing, nausea, vomiting, tachycardia). Avoid alcohol during and for at least 3 days after treatment.
* **Warfarin:** Metronidazole can potentiate the anticoagulant effect. Monitor INR closely and adjust warfarin dose as needed.
* **Lithium:** Metronidazole can increase lithium levels, potentially leading to toxicity. Monitor lithium levels.
* **Phenytoin/Phenobarbital:** May decrease metronidazole effectiveness.
* **Disulfiram:** Avoid concurrent use.
## Monitoring
* Monitor for signs and symptoms of infection resolution.
* Monitor for adverse effects, particularly neuropathy and CNS symptoms, especially with prolonged therapy.
* Monitor INR if patient is on warfarin.
* Monitor lithium levels if patient is on lithium.
## Clinical Pearls
* Administer IV formulation undiluted or diluted in compatible IV fluids.
* Oral formulations can be taken with or without food.
* A metallic taste is a common side effect.
* Urine may turn reddish-brown; this is harmless.
* Advise patients to avoid alcohol during and for at least 3 days after therapy.
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*This information is intended for healthcare professionals and should not replace clinical judgment or current prescribing information. Always verify with the most recent drug monographs and institutional protocols.*
