What are the indications and dosages for metronidazole (Flagyl)?

Metronidazole (Flagyl) Indications and Dosages

Metronidazole is indicated for trichomoniasis, bacterial vaginosis, amebiasis, giardiasis, and anaerobic bacterial infections, with specific dosing regimens for each condition. 1, 2, 3

Trichomoniasis

Recommended Regimens

• First-line treatment: Metronidazole 500 mg orally twice daily for 7 days (preferred regimen with 90-95% cure rate) 1, 2
• Alternative regimen: Metronidazole 2 g orally in a single dose 4, 2

Special Considerations

• Sex partners: All sexual partners should be treated simultaneously to prevent reinfection 2
• Follow-up: Routine follow-up unnecessary if symptoms resolve 2
• Treatment failure:
  • First failure: Re-treat with metronidazole 500 mg twice daily for 7 days 2
  • Repeated failure: Metronidazole 2 g once daily for 3-5 days 4, 2

Bacterial Vaginosis (BV)

Recommended Regimens

• Metronidazole 500 mg orally twice daily for 7 days 4
• Metronidazole gel 0.75%, one full applicator (5 g) intravaginally, once daily for 5 days 4
• Clindamycin cream 2%, one full applicator (5 g) intravaginally at bedtime for 7 days 4

Alternative Regimens

• Metronidazole 2 g orally in a single dose (lower efficacy) 4
• Clindamycin 300 mg orally twice daily for 7 days 4
• Clindamycin ovules 100 g intravaginally once at bedtime for 3 days 4

Amebiasis

Dosage

• Acute intestinal amebiasis (dysentery): 750 mg orally three times daily for 5-10 days 3
• Amebic liver abscess: 500 mg or 750 mg orally three times daily for 5-10 days 3
• Pediatric patients: 35-50 mg/kg/24 hours, divided into three doses, orally for 10 days 3

Anaerobic Bacterial Infections

Dosage

• Adults: 7.5 mg/kg every six hours (approximately 500 mg for a 70 kg adult) 3
• Maximum daily dose: 4 g in a 24-hour period 3
• Duration: Typically 7-10 days; longer treatment may be required for bone/joint infections, lower respiratory tract infections, and endocarditis 3, 5

Special Populations

Pregnancy

• First trimester: Metronidazole is contraindicated 4, 1
• Second and third trimesters:
  • For trichomoniasis: 2 g orally in a single dose 4, 1
  • For BV: Oral metronidazole can be used; vaginal metronidazole gel or clindamycin cream may be preferable 4

HIV Infection

• Same treatment regimens as HIV-negative individuals 1, 2

Elderly Patients

• Pharmacokinetics may be altered; monitoring of serum levels may be necessary to adjust dosage 3

Hepatic Disease

• Doses should be administered cautiously in severe hepatic disease due to slower metabolism 3
• Close monitoring of plasma metronidazole levels and toxicity is recommended 3

Important Clinical Considerations

• Patients should avoid alcohol during treatment with metronidazole and for 24 hours afterward due to disulfiram-like reaction 4, 6
• Metronidazole gel is NOT recommended for trichomoniasis treatment despite its approval for bacterial vaginosis (efficacy <50% compared to oral therapy) 4, 2
• Metronidazole has excellent tissue penetration, including into the central nervous system 7, 5
• For patients with metronidazole allergy, options are limited as effective alternatives are not readily available; desensitization may be considered 1, 2

Common Adverse Effects

• Gastrointestinal symptoms (nausea, metallic taste) 6
• Reversible neutropenia with prolonged use 6
• Peripheral neuropathy with extended treatment 6

Metronidazole remains a cost-effective agent with excellent efficacy against anaerobic organisms and specific protozoal infections, often considered the "gold standard" for anaerobic coverage 5.