What type of antibiotic (abx) is Flagyl (metronidazole)?

Flagyl (Metronidazole) Classification

Flagyl (metronidazole) is a nitroimidazole antibiotic with selective activity against anaerobic bacteria and certain protozoa. 1, 2

Antimicrobial Spectrum

Anaerobic Bacterial Coverage

• Metronidazole is bactericidal against most obligate anaerobic bacteria, including Gram-negative anaerobes (Bacteroides species including B. fragilis group, Fusobacterium species) and Gram-positive anaerobes (Clostridium species, Peptostreptococcus species, Peptococcus niger) 1, 2
• It is considered the most active agent available against Bacteroides fragilis, the most resistant anaerobic pathogen, achieving bactericidal activity at low concentrations (MIC ≤1 mcg/mL for most strains) 1, 3
• Kill-curve studies demonstrate a 2-5 log decrease in colony forming units within one hour of exposure 3

Protozoal Coverage

• Metronidazole possesses direct trichomonacidal and amebacidal activity against Trichomonas vaginalis, Entamoeba histolytica, and Giardia lamblia 1, 2
• The CDC recommends metronidazole as the only oral medication available in the United States for treating trichomoniasis, with cure rates of 90-95% 4

Important Limitations

• Metronidazole has NO clinically relevant activity against facultative anaerobes or obligate aerobes 1, 3
• Resistant anaerobes include occasional anaerobic cocci, some non-sporulating Gram-positive bacilli, and Propionibacterium species 3
• Must be combined with other agents (typically aminoglycosides or cephalosporins) when treating mixed aerobic-anaerobic infections 3, 5

Clinical Applications in Guidelines

Anaerobic Infections

• Recommended for necrotizing fasciitis (ceftriaxone + metronidazole, with or without vancomycin) 6
• Essential for anaerobic coverage in animal bites (combined with beta-lactams) 6
• Key component for surgical site infections involving intestinal or genitourinary tract (combined with ceftriaxone or fluoroquinolones) 6
• Used in intra-abdominal infections requiring anaerobic coverage, typically combined with agents covering aerobic Gram-negatives 6

Protozoal Infections

• First-line treatment for Giardia lamblia (alternative to tinidazole) 6
• Treatment for Clostridium difficile infection as a second-line agent when vancomycin or fidaxomicin cannot be obtained 6
• Preferred treatment for trichomoniasis with metronidazole 500 mg twice daily for 7 days or 2g single dose 4, 7

Mechanism and Pharmacology

• Metronidazole is a 5-nitroimidazole compound that requires reduction by anaerobic organisms to become active 2, 8
• Peak plasma concentrations occur 1-2 hours after oral administration, with excellent tissue penetration including cerebrospinal fluid, saliva, and abscesses 1, 5
• Approximately 20% of circulating metronidazole is bound to plasma proteins, with an elimination half-life of 8 hours 1

Critical Pitfall to Avoid

Never use metronidazole vaginal gel for trichomoniasis—it achieves <50% efficacy and is only FDA-approved for bacterial vaginosis, not systemic protozoal infections 4, 7