What is the recommended dosing for Flagyl (metronidazole)?

Flagyl (Metronidazole) Dosing

The recommended dosing for Flagyl varies significantly by indication: for trichomoniasis use 2 g orally as a single dose or 500 mg twice daily for 7 days; for bacterial vaginosis use 500 mg orally twice daily for 7 days; for amebiasis use 500-750 mg orally three times daily for 5-10 days; and for anaerobic bacterial infections use 500 mg (7.5 mg/kg) orally every 6 hours for 7-10 days, though metronidazole is now second-line for C. difficile infection. 1, 2

Trichomoniasis Dosing

Single-dose regimen (preferred for compliance):

• Metronidazole 2 g orally as a single dose achieves 90-95% cure rates 1
• This regimen ensures compliance, especially when directly observed 1
• Results in higher serum levels that can reach fetal circulation, so avoid in pregnancy 1, 2

Alternative multi-day regimen:

• Metronidazole 500 mg orally twice daily for 7 days 1
• Or 250 mg orally three times daily for 7 days 1, 2
• May achieve slightly higher cure rates and minimize reinfection by covering the treatment period for sexual contacts 1

For pregnant patients:

• Use 250 mg orally three times daily for 7 days (lower dose to minimize fetal exposure) 1
• Avoid single-dose 2 g regimen due to higher fetal drug levels 1, 2
• Do not treat during first trimester 2

Bacterial Vaginosis Dosing

• Metronidazole 500 mg orally twice daily for 7 days 1
• Alternative: 250 mg orally three times daily for 7 days 1
• Single-dose 2 g regimen is less effective for BV than for trichomoniasis 1
• Topical metronidazole gel is not recommended for trichomoniasis but can be used for BV 1

Amebiasis Dosing

Acute intestinal amebiasis (acute amebic dysentery):

• Adults: 750 mg orally three times daily for 5-10 days 2

Amebic liver abscess:

• Adults: 500-750 mg orally three times daily for 5-10 days 2

Pediatric patients:

• 35-50 mg/kg/24 hours divided into three doses for 10 days 2

Anaerobic Bacterial Infections Dosing

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

Once-daily alternative for serious intraabdominal/pelvic infections:

• Metronidazole 1 g IV every 24 hours appears as efficacious as multiple daily dosing for B. fragilis infections 3
• Pharmacokinetic studies demonstrate activity for 12-24 hours after 1 g dose with post-antibiotic effect extending beyond 3 hours 4

C. difficile Infection Dosing (Now Second-Line)

Critical update: Metronidazole is no longer first-line for C. difficile infection 1, 5, 6

First-line therapy:

• Vancomycin 125 mg orally four times daily for 10 days OR fidaxomicin 200 mg twice daily for 10 days 1, 5, 6

Metronidazole use only when vancomycin/fidaxomicin unavailable:

• For non-severe CDI only: 500 mg orally three times daily for 10 days 1, 5, 6
• Cure rates inferior to vancomycin (84% vs 97% overall; 76% vs 97% in severe disease) 6
• Never use for severe CDI 1, 6

Fulminant CDI:

• Vancomycin 500 mg orally four times daily PLUS IV metronidazole 500 mg every 8 hours 1, 5, 6

Giardiasis Dosing

• Metronidazole 250 mg orally three times daily for 5-7 days 6
• Note: Tinidazole is now preferred first-line agent 6
• Expect gastrointestinal side effects including metallic taste 5, 6

Special Population Adjustments

Elderly patients:

• Monitor serum levels as pharmacokinetics may be altered 2, 7
• Reduced renal excretion of parent drug and hydroxy metabolite 7
• Consider dosage adjustment based on monitoring 2

Severe hepatic disease:

• Reduce doses below usual recommendations 2
• Metronidazole accumulates due to slow metabolism 2, 7
• Close monitoring of plasma levels and toxicity recommended 2

Renal failure:

• No specific dose reduction needed for parent drug 2, 7
• Metabolites accumulate but no documented toxicity 7
• Hemodialysis removes substantial amounts; peritoneal dialysis has limited effect 7

Critical Safety Warnings

Neurotoxicity risk:

• Avoid repeated or prolonged courses due to cumulative and potentially irreversible neurotoxicity 1, 6
• For recurrent CDI, never use metronidazole—switch to vancomycin tapered/pulsed regimens 5, 6

Treatment failure management:

• If initial regimen fails, re-treat with 500 mg twice daily for 7 days 1
• For repeated failures, use 2 g once daily for 3-5 days 1
• Consider susceptibility testing for culture-documented resistant infections 1

Drug interactions:

• Avoid alcohol (disulfiram-like reaction) 8
• Monitor warfarin and phenytoin levels 8

Monitoring:

• Total and differential leukocyte counts before and after re-treatment 2
• Allow 4-6 weeks between repeat courses 2
• Reconfirm infection presence before re-treatment 2