What is the recommended metronidazole (Flagyl) dosage for treating Trichomonas vaginalis infection in adults and children?

Metronidazole Dosing for Trichomoniasis

The preferred treatment for trichomoniasis is metronidazole 500 mg orally twice daily for 7 days, which achieves superior cure rates compared to single-dose therapy. 1

Recommended Treatment Regimens

First-Line Therapy

• Metronidazole 500 mg orally twice daily for 7 days is the preferred regimen, with cure rates of approximately 90-95%. 1
• This multi-day regimen demonstrates significantly better efficacy than single-dose treatment, with an 8% absolute reduction in treatment failure (11% vs 19% failure rates). 2

Alternative Single-Dose Regimen

• Metronidazole 2 g orally as a single dose remains an acceptable alternative when adherence to multi-day therapy is a concern. 3, 4
• While convenient and historically recommended, this regimen has higher failure rates compared to the 7-day course. 2
• An additional FDA-approved option is Flagyl 375 mg orally twice daily for 7 days, though clinical equivalency data are limited. 3

The choice between regimens should prioritize the 7-day course for optimal cure rates, particularly given that treatment failure leads to persistent infection, ongoing transmission risk, and the need for more intensive retreatment. 2

Critical Management Principles

Partner Treatment (Non-Negotiable)

• All sexual partners must be treated simultaneously, regardless of symptoms or test results. 1, 4
• Patients must abstain from sexual intercourse until both partners complete treatment and are asymptomatic. 3, 1
• Reinfection from untreated partners is the most common cause of apparent treatment failure. 4

Treatment Failure Protocol

If initial therapy fails:

1. First failure: Re-treat with metronidazole 500 mg twice daily for 7 days. 3, 1
2. Second failure: Metronidazole 2 g orally once daily for 3-5 days. 3
3. Persistent failure: Consult infectious disease specialist and obtain susceptibility testing after excluding reinfection. 3, 1

Special Populations

Pregnancy

• Metronidazole 2 g orally as a single dose is recommended after the first trimester. 3, 1
• Treatment is important given associations with preterm delivery, premature rupture of membranes, and low birth weight. 1, 4
• Metronidazole is contraindicated in the first trimester per older guidelines 3, though more recent evidence shows no consistent teratogenic effects. 3

HIV-Infected Patients

• Use the same treatment regimens as HIV-negative patients. 3, 1
• Treatment is particularly important as untreated trichomoniasis increases HIV transmission risk. 4

Metronidazole Allergy

• Desensitization is the recommended approach, as no effective alternatives exist. 3, 4
• Topical therapies achieve cure rates <50% and should not be used. 3

Common Pitfalls to Avoid

• Never use metronidazole vaginal gel for trichomoniasis—it achieves <50% efficacy and is only approved for bacterial vaginosis. 3, 4
• Do not skip partner treatment even if the partner is asymptomatic, as the organism is difficult to isolate in men. 4
• Always exclude reinfection from untreated partners before assuming true treatment failure. 4
• Routine follow-up is unnecessary for patients who become asymptomatic after treatment. 3, 1