What is Flagyl (metronidazole)?

What is Flagyl (Metronidazole)?

Flagyl is the brand name for metronidazole, a synthetic antiprotozoal and antibacterial medication that specifically targets anaerobic bacteria and certain protozoa through DNA strand breakage via toxic metabolites. 1, 2

Mechanism and Spectrum of Activity

Metronidazole is a 5-nitroimidazole compound that demonstrates selective bactericidal activity against anaerobic microorganisms at low concentrations. 2, 3 The drug works by producing toxic metabolites that cause DNA strand breakage in susceptible organisms. 4

The medication is most active against:

• Bacteroides fragilis (the most resistant anaerobic bacteria) 2
• Clostridium perfringens 2
• Trichomonas vaginalis 5
• Entamoeba histolytica (amebiasis) 2
• Giardia lamblia 2, 6

The drug achieves a 2 to 5 log decrease in colony forming units within one hour for organisms like B. fragilis and C. perfringens. 2

Primary Clinical Uses

Metronidazole treats three main categories of infections:

Protozoal Infections

• Trichomonal vaginitis with 90-95% cure rates using either 2g single dose or 500mg twice daily for 7 days 5, 7
• Extraintestinal amebiasis 2
• Giardiasis 2, 6

Bacterial Infections

• Bacterial vaginosis using 500mg orally twice daily for 7 days 8
• Intra-abdominal sepsis 2
• Brain abscesses 2
• Pelvic infections 2
• Preoperative bowel preparation for colorectal surgery 6, 3

Helicobacter pylori

• Used in combination regimens with proton pump inhibitors 4

Pharmacokinetics

Metronidazole demonstrates excellent absorption and tissue penetration:

• Oral bioavailability exceeds 90% for tablets 4
• Volume of distribution: 0.51 to 1.1 L/kg 4
• Protein binding less than 20% 4
• Reaches 60-100% of plasma concentrations in most tissues, including the central nervous system 4
• Extensively metabolized by the liver to 5 metabolites, with the hydroxy metabolite retaining 30-65% biological activity 4

Critical Safety Warnings

Patients must completely avoid all alcoholic beverages during treatment and for at least 24 hours (preferably one day) after the last dose, as the combination causes severe disulfiram-like reactions including abdominal cramps, nausea, vomiting, headaches, and flushing. 8, 1

Additional important precautions include:

• Should not be given to patients who have taken disulfiram within the last two weeks due to risk of psychotic reactions 1
• Use cautiously in patients with severe hepatic disease, as doses below usual recommendations should be administered 1
• Monitor for leukopenia with total and differential leukocyte counts recommended before and after therapy 1
• Potentiates anticoagulant effects of warfarin, requiring prothrombin time monitoring 1
• Can cause peripheral and autonomic neuropathy, particularly with chronic therapy or large acute doses 9
• Has shown carcinogenic activity in chronic animal studies 1

Common Adverse Effects

Mild gastrointestinal upset and unpleasant metallic taste in the mouth are common side effects. 8 Metronidazole can cause reversible autonomic neuropathy, manifesting as burning pain in extremities and temperature dysregulation. 9

Special Populations

Pregnancy: Metronidazole is safe during pregnancy, though dosing may require adjustment. 8 For bacterial vaginosis in pregnant women, the CDC recommends 250mg orally three times daily for 7 days to minimize fetal exposure. 8

HIV-infected patients: Use the same treatment regimens as for HIV-negative patients. 7, 8

Hepatic impairment: Doses below usual recommendations should be administered cautiously due to decreased clearance. 1, 4

Renal dysfunction: Pharmacokinetics are unaffected by acute or chronic renal failure, and dosage alterations are unnecessary despite reduced elimination of metabolites. 4