What is the dosing of oral Flagyl (metronidazole)?

Oral Metronidazole (Flagyl) Dosing

The dosing of oral metronidazole varies significantly by indication, ranging from 250 mg three times daily to 2 grams as a single dose, with specific regimens determined by the infection being treated.

Dosing by Clinical Indication

Trichomoniasis

• Single-dose regimen: 2 grams orally as a single dose (preferred for compliance, achieves 90-95% cure rates) 1
• Alternative: 500 mg orally twice daily for 7 days (may minimize reinfection and provide higher cure rates in some comparative studies) 1, 2
• For pregnant patients beyond the first trimester, use the 7-day regimen rather than single-dose to avoid higher fetal serum levels 2

Bacterial Vaginosis

• 500 mg orally twice daily for 7 days (recommended by CDC and ACOG) 1
• Alternative: 250 mg orally three times daily for 7 days 1

Giardiasis

• 250 mg orally three times daily for 5-7 days 1
• Pediatric dosing: 15 mg/kg/day divided into three doses for 5 days 1
• Important caveat: Tinidazole is now preferred as first-line therapy; metronidazole is an alternative 1

Amebiasis

• Acute intestinal amebiasis: 750 mg orally three times daily for 5-10 days 2
• Amebic liver abscess: 500-750 mg orally three times daily for 5-10 days 2
• Pediatric: 35-50 mg/kg/24 hours divided into three doses for 10 days 2

C. difficile Infection (CDI)

• Critical update: Metronidazole is NO LONGER first-line therapy for CDI 1, 3
• For non-severe CDI: 500 mg orally three times daily for 10 days ONLY when vancomycin (125 mg four times daily) or fidaxomicin is unavailable 1, 3
• Vancomycin and fidaxomicin have superior cure rates (97% vs 84% overall; 97% vs 76% in severe disease) 1
• For fulminant CDI: Intravenous metronidazole 500 mg every 8 hours PLUS oral vancomycin 500 mg four times daily 4, 3
• Never use metronidazole for recurrent CDI beyond the first episode due to cumulative neurotoxicity risk 1, 3

Anaerobic Bacterial Infections

• 7.5 mg/kg orally every 6 hours (approximately 500 mg for a 70 kg adult) 2
• Maximum: 4 grams per 24 hours 2
• Usual duration: 7-10 days; bone/joint, lower respiratory tract, and endocardium infections may require longer treatment 2

Critical Safety Considerations

Neurotoxicity Warning

• Avoid repeated or prolonged courses due to risk of cumulative and potentially irreversible neurotoxicity 1
• This is particularly important in CDI where multiple treatment courses may be tempting but should be avoided 1, 3

Special Populations

• Elderly patients: Monitor serum levels as pharmacokinetics may be altered; adjust dosing accordingly 2
• Severe hepatic disease: Use doses below usual recommendations with close monitoring of plasma levels and toxicity 2
• Anuric patients: No specific dose reduction needed as metabolites are rapidly removed by dialysis 2
• Pregnancy: Contraindicated in first trimester; use 7-day regimens (not single-dose) if treatment is essential after first trimester 2

Drug Interactions

• Avoid alcohol during treatment and for 24 hours after completion (disulfiram-like reaction) 1
• Important interactions with disulfiram and warfarin 5

Common Pitfalls to Avoid

• Do not use metronidazole as first-line for CDI - vancomycin or fidaxomicin are superior 1, 3
• Do not use metronidazole for second or subsequent CDI recurrences - neurotoxicity risk outweighs benefits 1, 3
• Do not extend treatment courses unnecessarily - cumulative neurotoxicity is a real concern 1
• Do not use single-dose regimens in pregnancy - higher serum levels reach fetal circulation 2
• Do not exceed 4 grams per 24 hours for anaerobic infections 2