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# Metronidazole (Metrogyl)
## Overview
Metronidazole is an antimicrobial agent effective against anaerobic bacteria and certain protozoa.
## Primary Indications
* Bacterial vaginosis
* Trichomoniasis
* Amebiasis
* Giardiasis
* Anaerobic bacterial infections (e.g., intra-abdominal infections, gynecologic infections, skin and soft tissue infections, bone and joint infections, central nervous system infections, lower respiratory tract infections, endocarditis, bacteremia)
* *Clostridioides difficile* infection (CDI) - *Note: Oral vancomycin is generally preferred for initial episodes of CDI, especially severe cases.*
* Surgical prophylaxis (colorectal surgery)
* *Helicobacter pylori* eradication (part of combination therapy)
## Adult Dosing
Dosing varies significantly by indication.
* **Bacterial Vaginosis:** 500 mg orally twice daily for 7 days OR 2 grams orally as a single dose.
* **Trichomoniasis:** 2 grams orally as a single dose OR 250 mg orally three times daily for 7 days.
* **Amebiasis (intestinal):** 500-750 mg orally three times daily for 5-10 days.
* **Amebiasis (hepatic abscess):** 500-750 mg orally or IV three times daily for 5-10 days (may require aspiration/drainage).
* **Giardiasis:** 250 mg orally three times daily for 5-7 days OR 2 grams orally once daily for 3 days.
* **Anaerobic Bacterial Infections (non-CNS):** 500 mg IV or orally every 8 hours. Dosing duration depends on severity and site of infection, typically 7-14 days.
* **CNS Anaerobic Infections:** 15 mg/kg IV loading dose, followed by 7.5 mg/kg IV every 6 hours (or 500 mg IV every 8 hours), or sometimes up to 15 mg/kg IV every 8 hours. Duration typically 10-14 days or longer.
* **CDI:** 500 mg orally three times daily for 10-14 days. *Consider oral vancomycin for severe or initial episodes.*
* **Surgical Prophylaxis (colorectal):** 1 gram IV 1 hour before surgery, or 500 mg orally every 8 hours starting 12-24 hours before surgery and continued postoperatively for 24 hours.
* ***H. pylori* Eradication:** Typically 500 mg orally twice daily in combination with other agents.
## Pediatric Dosing
Dosing varies by indication and is weight-based.
* **Amebiasis:** 35-50 mg/kg/day orally divided into 3 doses for 5-10 days (maximum 750 mg/dose).
* **Giardiasis:** 15 mg/kg/day orally divided into 3 doses for 5-7 days (maximum 250 mg/dose).
* **Anaerobic Bacterial Infections:** 7.5 mg/kg IV or orally every 8 hours (or 15 mg/kg every 12 hours) for 7-14 days. Maximum dose typically 4 grams/day.
## Dose Adjustments
* **Renal Impairment:** No dose adjustment typically needed for moderate to severe renal impairment. Hemodialysis effectively removes metronidazole.
* **Hepatic Impairment:** Reduce dose by 33-50% in severe hepatic impairment.
## Contraindications
* Hypersensitivity to metronidazole or other nitroimidazole derivatives.
* Use within 14 days of disulfiram (due to potential for disulfiram-like reaction).
* Use with alcohol (due to potential for disulfiram-like reaction).
## Adverse Effects
* **Common:** Nausea, vomiting, diarrhea, abdominal pain, metallic taste, headache, dizziness.
* **Serious:** Peripheral neuropathy (especially with prolonged use), seizures, Stevens-Johnson syndrome, encephalopathy, aseptic meningitis, pancreatitis.
* **Other:** Darkened urine, opportunistic infections (e.g., candidiasis).
## Key Drug Interactions
* **Alcohol:** Disulfiram-like reaction (flushing, headache, nausea, vomiting, abdominal cramps). Advise patients to avoid alcohol during and for at least 3 days after therapy.
* **Warfarin:** Increased anticoagulant effect. Monitor INR closely and adjust warfarin dose as needed.
* **Lithium:** Increased lithium levels, potentially leading to toxicity. Monitor lithium levels.
* **Phenytoin, Phenobarbital:** May decrease metronidazole levels.
* **Disulfiram:** Contraindicated use due to potential for severe psychiatric symptoms.
* **Cyclosporine:** Increased cyclosporine levels. Monitor cyclosporine levels.
* **5-Fluorouracil (5-FU):** Metronidazole may increase the toxicity of 5-FU.
## Monitoring
* Monitor for signs and symptoms of peripheral neuropathy (numbness, tingling) with prolonged therapy.
* Monitor for CNS side effects (dizziness, seizures, encephalopathy).
* Monitor for signs of opportunistic infections.
* If used for *H. pylori*, monitor for eradication success per local guidelines.
* If used with warfarin, monitor INR.
* If used with lithium, monitor lithium levels.
## Clinical Pearls
* A metallic taste is a common and expected side effect.
* Advise patients to avoid alcohol during treatment and for at least 3 days after completion.
* For anaerobic infections, IV administration can be switched to oral once the patient is tolerating oral intake and clinically improving.
* Metronidazole can cause a dark urine, which is harmless.
* If a dose is missed, take it as soon as remembered unless it is close to the time for the next dose. Do not double the dose.
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*Disclaimer: This information is intended for healthcare professionals. Always consult the official prescribing information and institutional protocols for complete details before making any treatment decisions.*
