What is the recommended dose of metronidazole?

Recommended Dosage of Metronidazole

The recommended dose of metronidazole varies by indication, with 500 mg orally three times daily for 10 days being the standard regimen for most non-severe Clostridium difficile infections, while other infections have different dosing protocols. 1

Dosing by Indication

Clostridium difficile Infection (CDI)

• Initial non-severe episode:

  • Metronidazole 500 mg orally 3 times daily for 10 days 1
  • Note: Vancomycin (125 mg orally 4 times daily) or fidaxomicin (200 mg twice daily) are now preferred over metronidazole for initial CDI episodes 1

• Fulminant CDI (with hypotension, shock, ileus, or megacolon):

  • Metronidazole 500 mg intravenously every 8 hours in combination with oral vancomycin 500 mg 4 times daily 1
  • If ileus present, add rectal vancomycin 500 mg every 6 hours as retention enema 1

Trichomoniasis

• Standard regimen: Metronidazole 500 mg orally twice daily for 7 days 2
• Alternative regimen: Single 2 g oral dose 3
  • Note: Recent evidence suggests 7-day regimen is more effective (11% failure rate vs 19% with single dose) 4

Anaerobic Infections (IV administration)

• Loading dose: 15 mg/kg infused over one hour (approximately 1 g for 70-kg adult) 5
• Maintenance dose: 7.5 mg/kg infused over one hour every six hours (approximately 500 mg for 70-kg adult) 5

Bacterial Vaginosis

• Oral regimen: Metronidazole 500 mg twice daily for 7 days 2, 6
• Topical regimen: 0.75% metronidazole gel intravaginally (5g containing 37.5 mg metronidazole) 7

Special Populations and Considerations

Hepatic Impairment

• Patients with severe hepatic disease metabolize metronidazole more slowly
• Lower doses should be administered cautiously with close monitoring of plasma levels 5

Renal Impairment

• No specific dose reduction needed in anuric patients as metabolites can be removed by dialysis 5
• However, renal failure affects excretion of metabolites significantly 8

Elderly Patients

• Pharmacokinetics may be altered
• Monitoring of serum levels may be necessary to adjust dosage accordingly 5
• Reduced renal excretion of both parent drug and hydroxy metabolite has been observed 8

Important Warnings and Precautions

• Avoid prolonged or repeated courses due to risk of cumulative and potentially irreversible neurotoxicity 1
• Metronidazole has limited plasma protein binding but achieves favorable tissue distribution, including into the CNS 8
• Bioavailability approaches 100% when given orally 8
• Patients receiving metronidazole via nasogastric tube may have reduced serum levels due to removal in aspirate 5
• Metronidazole is extensively metabolized by the liver to form two primary oxidative metabolites 8

Comparative Efficacy

• Recent evidence suggests vancomycin and fidaxomicin are superior to metronidazole for initial CDI episodes 1
• For trichomoniasis, the 7-day regimen (500 mg twice daily) shows higher cure rates than the single 2g dose 4
• For bacterial vaginosis, 7-day oral regimen (500 mg twice daily) remains highly efficacious 6

Remember that metronidazole should be used with caution and for appropriate indications only, with careful consideration of potential side effects and drug interactions, particularly with alcohol (disulfiram-like reaction).