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# Metronidazole (Metrogyl)
## Overview
Metronidazole is a nitroimidazole antibiotic effective against anaerobic bacteria and certain protozoa.
## Primary Indications
* **Bacterial Vaginosis:** Treatment of bacterial vaginosis.
* **Trichomoniasis:** Treatment of symptomatic trichomoniasis.
* **Amebiasis:** Treatment of intestinal amebiasis (amebic dysentery) and amebic liver abscess.
* **Giardiasis:** Treatment of giardiasis.
* **Anaerobic Infections:** Treatment of serious anaerobic bacterial infections, often in combination therapy (e.g., intra-abdominal infections, gynecologic infections, skin and soft tissue infections, bone and joint infections, CNS infections, lower respiratory tract infections, endocarditis, and bacteremia).
* **Prophylaxis:** Preoperative prophylaxis in patients undergoing certain colorectal surgeries.
* **Pseudomembranous Colitis:** Treatment of *Clostridioides difficile*-associated diarrhea (CDAD).
## Adult Dosing
* **Bacterial Vaginosis:** 500 mg orally twice daily for 7 days, OR 2 g orally as a single dose, OR 0.75% vaginal gel 5 g intravaginally once daily for 5 days.
* **Trichomoniasis:** 2 g orally as a single dose, OR 250 mg orally twice daily for 7 days, OR 375 mg orally twice daily for 7 days.
* **Amebiasis:**
* Intestinal: 500-675 mg orally three times daily for 5-10 days.
* Liver Abscess: 500-750 mg orally three times daily for 5-10 days, followed by a luminal agent if abscess is drained.
* **Giardiasis:** 250 mg orally three times daily for 5-7 days, OR 2 g orally once daily for 3 days.
* **Anaerobic Infections:** 500 mg intravenously or orally every 8 hours. Typical duration is 7-14 days, depending on severity and infection site. Higher doses (e.g., 750 mg every 8 hours) may be used for severe infections.
* **Prophylaxis (Colorectal Surgery):** 1 g intravenously 1-2 hours before surgery, followed by 500 mg orally or intravenously every 8 hours postoperatively for up to 24 hours.
* **Pseudomembranous Colitis:** 500 mg orally three times daily for 10-14 days. This is generally reserved for patients who cannot tolerate vancomycin due to cost or availability.
## Pediatric Dosing
Dosing is typically weight-based and depends on the indication. Specific protocols may vary.
* **Intestinal Amebiasis:** 35-50 mg/kg/day orally, divided into three doses, for 5-7 days. Maximum daily dose: 750 mg.
* **Giardiasis:** 15-20 mg/kg/day orally, divided into three doses, for 5-7 days. Maximum daily dose: 750 mg.
* **Anaerobic Infections:** 30 mg/kg/day intravenously or orally, divided into three doses. Maximum daily dose: 4 g. Treatment duration is typically 7-14 days.
## Dose Adjustments
* **Hepatic Impairment:** No specific dose adjustment is universally recommended for mild to moderate hepatic impairment. However, caution is advised. For severe hepatic impairment, reduce the dose by 30-50% and monitor for toxicity.
* **Renal Impairment:** No dose adjustment is typically required for renal impairment. Metronidazole and its metabolites are largely removed by hemodialysis.
## Contraindications
* Hypersensitivity to metronidazole or other nitroimidazole derivatives.
* Concurrent use of disulfiram or alcohol within 3 days (due to disulfiram-like reaction).
## Adverse Effects
* **Common:** Nausea, vomiting, diarrhea, abdominal pain, metallic taste, headache, dizziness, dark urine.
* **Serious:** Peripheral neuropathy (especially with prolonged use), seizures, encephalopathy, aseptic meningitis, Stevens-Johnson syndrome, toxic epidermal necrolysis, pancreatitis, leukopenia, thrombocytopenia, anorexia, candidiasis of the vagina.
## Key Drug Interactions
* **Alcohol:** Disulfiram-like reaction (flushing, nausea, vomiting, headache, tachycardia). Avoid alcohol during and for at least 3 days after metronidazole therapy.
* **Disulfiram:** Increased risk of psychotic reactions. Avoid concurrent use.
* **Warfarin:** Potentiates anticoagulant effect. Monitor INR closely and adjust warfarin dose as needed.
* **Lithium:** Increased lithium levels, potentially leading to toxicity. Monitor lithium levels.
* **Phenobarbital:** May decrease metronidazole levels.
* **5-Fluorouracil (5-FU) and capecitabine:** Metronidazole may increase toxicity of these agents.
## Monitoring
* Clinical signs and symptoms of infection.
* Complete blood count (CBC) may be considered with prolonged therapy or high doses.
* Renal and hepatic function tests, especially in patients with pre-existing impairment or on prolonged therapy.
* Neurologic status for signs of peripheral neuropathy or CNS effects.
## Clinical Pearls
* Metronidazole has a metallic taste that can lead to medication non-adherence.
* Urine may turn reddish-brown due to metabolites.
* Treatment of *C. difficile*-associated diarrhea (CDAD) with metronidazole should be considered only if vancomycin is not an option. Guidelines now favor vancomycin or fidaxomicin as first-line agents.
* Oral metronidazole can be given with or without food. Intravenous administration is typically preferred for severe infections.
* Concurrent therapy with a luminal agent is recommended for amebiasis after the metronidazole course to eradicate cysts in the intestinal lumen.
**Disclaimer:** This information is intended for healthcare professionals and does not replace a thorough review of the most current prescribing information, including all warnings, contraindications, and adverse effects, from the manufacturer or other authoritative sources before prescribing or administering this medication. Local guidelines and institutional protocols should also be consulted.
