What is the recommended dosage for Flagyl (metronidazole)?

Flagyl (Metronidazole) Dosing Recommendations

The recommended dosage for Flagyl depends entirely on the specific infection being treated, with distinct regimens for trichomoniasis, bacterial vaginosis, amebiasis, and anaerobic bacterial infections.

Trichomoniasis

For trichomoniasis, prescribe metronidazole 500 mg orally twice daily for 7 days rather than the single 2-gram dose, as this achieves superior cure rates. 1

• The 7-day regimen (500 mg twice daily) reduces treatment failure by 45% compared to single-dose therapy (11% vs 19% failure rates) 1
• Alternative single-dose option: 2 grams orally as a one-time dose, though this has lower efficacy 2
• FDA-approved alternative: 375 mg twice daily for 7 days, though clinical equivalency data are lacking 2
• Critical caveat: Always treat sexual partners simultaneously and instruct patients to abstain from intercourse until both partners complete therapy and are asymptomatic 2

Treatment Failures

• First failure: Re-treat with 500 mg twice daily for 7 days 2
• Repeated failures: 2 grams once daily for 3-5 days 2
• Highly resistant cases require expert consultation with CDC 2

Bacterial Vaginosis (Non-Pregnant Women)

For bacterial vaginosis, prescribe metronidazole 500 mg orally twice daily for 7 days as first-line therapy. 2

• This oral regimen achieves 78-84% cure rates at 4 weeks post-treatment 2
• Alternative topical option: 0.75% metronidazole gel, one applicator (5g) intravaginally twice daily for 5 days 2
• Lower-efficacy alternative: 2 grams orally as single dose (use only when compliance is a major concern) 2
• Important: Advise patients to avoid alcohol during treatment and for 24 hours after completion 2

Pregnant Women with BV

• High-risk pregnant women (prior preterm delivery): 250 mg orally three times daily for 7 days, initiated in early second trimester 2
• Low-risk symptomatic pregnant women: 250 mg orally three times daily for 7 days 2
• Avoid clindamycin vaginal cream in pregnancy due to increased preterm delivery risk 2

Amebiasis

For intestinal amebiasis, prescribe 750 mg orally three times daily for 5-10 days; for amebic liver abscess, use 500-750 mg orally three times daily for 5-10 days. 3

• Pediatric dosing: 35-50 mg/kg/24 hours divided into three doses for 10 days 3

Anaerobic Bacterial Infections

For serious anaerobic infections, prescribe 7.5 mg/kg orally every 6 hours (approximately 500 mg for a 70-kg adult), not exceeding 4 grams per 24 hours. 3

• Typical duration: 7-10 days 3
• Bone/joint, lower respiratory tract, and endocardial infections may require longer treatment 3
• Severe hepatic disease: Reduce doses significantly and monitor plasma levels closely due to drug accumulation 3

Clostridioides difficile Infection

For C. difficile infection, metronidazole is NO LONGER recommended as first-line therapy; vancomycin or fidaxomicin should be used instead. 2

• Metronidazole 500 mg orally three times daily for 10 days may be considered ONLY in settings with limited access to vancomycin/fidaxomicin for non-severe initial episodes 2
• Avoid repeated or prolonged metronidazole courses due to cumulative neurotoxicity risk 2
• For fulminant CDI with ileus: Metronidazole 500 mg IV every 8 hours should be added to oral/rectal vancomycin 2

Special Populations

Elderly Patients

• Monitor serum levels and adjust dosing as metronidazole pharmacokinetics may be altered 3

Pregnancy

• First trimester: Contraindicated for trichomoniasis 3
• Second/third trimester: Single 2-gram dose acceptable for trichomoniasis if alternative treatments inadequate, though avoid if possible 2
• Lower doses recommended for BV to minimize fetal exposure 2

Renal Failure

• No specific dose reduction needed in anuric patients as metabolites are rapidly removed by dialysis 3