Mechanism of action
Metronidazole is a prodrug that requires activation by reduction of its nitro group. In anaerobic organisms and certain protozoa, low-redox potential electron transport proteins (such as ferredoxin) reduce the nitro group of metronidazole to form reactive nitro radicals and other toxic intermediates. These free radicals cause DNA strand breaks, inhibit DNA repair, and damage other macromolecules, leading to cell death.
The selective toxicity of metronidazole arises because only organisms with sufficiently low intracellular redox potential can reduce the drug. Aerobic bacteria and human cells lack the extremely low-redox electron transport proteins necessary for activation, making metronidazole selectively toxic to:
- Anaerobic bacteria: including Bacteroides fragilis, Clostridium species (including C. difficile), Peptostreptococcus, Fusobacterium, and Prevotella
- Protozoa: Trichomonas vaginalis, Giardia lamblia, Entamoeba histolytica
- Microaerophilic bacteria: Helicobacter pylori (in combination therapy)
Metronidazole has excellent tissue penetration, including across the blood-brain barrier, making it effective for CNS anaerobic infections and brain abscesses.
- Metronidazole requires anaerobic reduction to form toxic free radicals that damage DNA; selective for anaerobes and protozoa.
- Covers Bacteroides fragilis, C. difficile, Trichomonas, Giardia, and Entamoeba—know these associations cold.
- Disulfiram-like reaction with alcohol: avoid during treatment and for 48 hours after; causes flushing, nausea, tachycardia.
- Crosses blood-brain barrier; first-line for brain abscesses involving anaerobes.
- Metallic taste is a classic and very common side effect.
- Peripheral neuropathy may be irreversible; risk increases with prolonged use (weeks to months).
- Increases warfarin effect by inhibiting metabolism; monitor INR closely.
- Treat both sexual partners simultaneously for trichomoniasis to prevent reinfection.
Indications
Metronidazole is indicated for infections caused by anaerobic bacteria and susceptible protozoa:
Anaerobic bacterial infections
- Intra-abdominal infections (often combined with other antibiotics for polymicrobial coverage)
- Pelvic inflammatory disease and tubo-ovarian abscess
- Brain abscess and CNS anaerobic infections
- Aspiration pneumonia and lung abscess
- Bacterial vaginosis (topical or oral)
- Clostridium difficile infection (first-line for mild-moderate cases)
- Dental and oral infections
- Necrotizing fasciitis and other soft tissue infections involving anaerobes
- Postoperative intra-abdominal sepsis
Protozoal infections
- Trichomoniasis (treat sexual partners simultaneously)
- Giardiasis
- Amebiasis (intestinal and hepatic)
Other indications
- Helicobacter pylori eradication (as part of triple or quadruple therapy)
- Rosacea (topical formulation)
- Crohn disease (off-label, for perianal disease)
- Prophylaxis for colorectal surgery
Metronidazole is often combined with other antibiotics (such as ceftriaxone, fluoroquinolones, or aminoglycosides) for mixed aerobic-anaerobic infections.
Pharmacokinetics
Absorption: Metronidazole is well absorbed orally with bioavailability exceeding 90%. Food does not significantly affect absorption. Intravenous formulations are used for severe infections or when oral administration is not feasible.
Distribution: Metronidazole distributes widely into tissues and body fluids, including:
- Cerebrospinal fluid (achieves therapeutic concentrations even without meningeal inflammation)
- Brain tissue and abscesses
- Saliva
- Bile
- Vaginal secretions
- Breast milk
The volume of distribution approximates total body water. Protein binding is low (less than 20%).
Metabolism: Primarily hepatic via oxidation and glucuronidation. The major metabolites include the hydroxy metabolite and the acid metabolite, both of which retain some antimicrobial activity. Metabolism may be reduced in severe hepatic impairment.
Excretion: Approximately 60-80% is excreted in urine (as parent drug and metabolites), with 6-15% excreted in feces. The elimination half-life is approximately 8 hours in adults with normal hepatic function. Dosage adjustment is required in severe hepatic impairment but generally not needed in renal impairment, though metabolites may accumulate.
Adverse effects
Common adverse effects
- Gastrointestinal: metallic taste (very common and characteristic), nausea, anorexia, vomiting, dyspepsia, abdominal cramping
- CNS: headache, dizziness
- Local: darkening of urine (due to metabolites, not clinically significant)
Serious adverse effects
- Peripheral neuropathy: dose-related, may be irreversible; presents with paresthesias, numbness, and weakness. Risk increases with prolonged therapy (weeks to months). Discontinue if signs develop.
- CNS toxicity: encephalopathy, seizures, cerebellar dysfunction, aseptic meningitis (rare but serious)
- Disulfiram-like reaction: severe nausea, vomiting, flushing, tachycardia, and hypotension when combined with alcohol due to inhibition of aldehyde dehydrogenase. Patients must avoid alcohol during treatment and for 48 hours after the last dose.
- Hematologic: leukopenia, neutropenia (rare); thrombocytopenia
- Hepatotoxicity: rare but reported
Other considerations
- Carcinogenicity: Animal studies showed increased tumor rates with chronic high-dose exposure, but human data do not confirm carcinogenicity at therapeutic doses. Use the shortest effective duration.
- Pregnancy: Crosses the placenta. Generally avoided in first trimester when possible, though often used for trichomoniasis and bacterial vaginosis in second and third trimesters when benefits outweigh risks.
- Candidal overgrowth: may cause secondary vaginal candidiasis
Contraindications and warnings
Absolute contraindications
- Hypersensitivity to metronidazole or other nitroimidazole derivatives
- Concurrent or recent (within 2 weeks) disulfiram use (may cause confusion and psychosis)
- First trimester of pregnancy (relative; avoid when alternatives exist)
- Alcohol consumption during therapy and for 48 hours after discontinuation
Warnings and precautions
- Neurologic disease: Use with caution in patients with pre-existing CNS disorders or peripheral neuropathy. Discontinue immediately if neuropathy develops.
- Hepatic impairment: Reduce dose in severe hepatic dysfunction (Child-Pugh C); metabolites accumulate and increase toxicity risk.
- Blood dyscrasias: Monitor complete blood counts with prolonged therapy.
- Cockayne syndrome: Severe, irreversible hepatotoxicity reported; contraindicated.
- Pregnancy and lactation: Avoid in first trimester when possible. Excreted in breast milk; some sources recommend withholding breastfeeding during therapy and for 12-24 hours after the last dose, though others consider it compatible with breastfeeding.
- Prolonged use: Use the shortest effective duration due to potential carcinogenicity in animal models and risk of neuropathy.
Black box warning
Metronidazole does not carry a black box warning, but prescribers should be aware of the potential for irreversible peripheral neuropathy and CNS toxicity with prolonged use.
Drug interactions
Major interactions
- Alcohol: Disulfiram-like reaction with nausea, vomiting, flushing, tachycardia, and hypotension. Avoid during therapy and for 48 hours after the last dose.
- Disulfiram: Concurrent use may cause acute psychosis and confusion. Avoid metronidazole within 2 weeks of disulfiram.
- Warfarin: Metronidazole inhibits warfarin metabolism, significantly increasing INR and bleeding risk. Monitor INR closely and adjust warfarin dose.
- Lithium: Metronidazole may increase lithium levels, risking toxicity. Monitor lithium concentrations.
- Phenytoin and phenobarbital: These drugs induce metronidazole metabolism, reducing its efficacy. Conversely, metronidazole may increase phenytoin levels.
- Busulfan: Metronidazole may increase busulfan plasma concentrations and toxicity.
- 5-Fluorouracil: Metronidazole may decrease clearance of 5-FU, increasing toxicity.
Other interactions
- CYP450: Metronidazole is a weak inhibitor of CYP2C9 and CYP3A4.
- Cimetidine: May increase metronidazole levels by reducing hepatic metabolism.
- Mebendazole: Rare reports of Stevens-Johnson syndrome with concurrent use.
Always review the patient's medication list for potential interactions, particularly with narrow therapeutic index drugs.
Clinical pearls
- Metallic taste is almost pathognomonic for metronidazole and is a common reason for non-adherence.
- Metronidazole is one of the few antibiotics with excellent CNS penetration, making it first-line for brain abscesses involving anaerobes.
- Always warn patients about the disulfiram-like reaction with alcohol—this includes alcohol-containing medications, mouthwashes, and even some desserts.
- For C. difficile infection, metronidazole has largely been replaced by oral vancomycin and fidaxomicin for first-line therapy based on updated guidelines, but it remains an option for mild cases when cost is a barrier.
- Peripheral neuropathy is dose- and duration-dependent. It may be irreversible, so use the shortest effective course.
- Treat both partners simultaneously for trichomoniasis to prevent reinfection (ping-pong effect).
- Metronidazole turns urine dark or reddish-brown due to metabolites; reassure patients this is harmless.
- For intra-abdominal infections, metronidazole provides anaerobic coverage but must be combined with agents covering gram-negative aerobes (e.g., ceftriaxone, ciprofloxacin).
- Bacteroides fragilis is the classic anaerobe covered by metronidazole in polymicrobial intra-abdominal and pelvic infections.
- In bacterial vaginosis, intravaginal metronidazole gel has fewer systemic side effects than oral therapy.
- Metronidazole resistance in H. pylori is increasing globally, which may affect eradication rates in triple therapy regimens.
GET GAP on Metronidazole: Giardia, Entamoeba, Trichomonas, Gardnerella (bacterial vaginosis), Anaerobes, + no alcohol Please (disulfiram-like reaction)
Practice questions
1. A 32-year-old woman presents with malodorous vaginal discharge and is diagnosed with bacterial vaginosis. She is prescribed oral metronidazole. Three days later, she attends a wedding and drinks two glasses of wine. Within 30 minutes, she develops severe nausea, vomiting, facial flushing, and palpitations. Her heart rate is 118/min and blood pressure is 98/62 mm Hg. Which of the following best explains her symptoms?
2. A 58-year-old man with a history of alcoholism presents with fever, headache, and focal neurologic deficits. CT scan reveals a ring-enhancing lesion in the right frontal lobe consistent with a brain abscess. Aspiration culture grows Bacteroides fragilis. Which property of metronidazole makes it an appropriate treatment choice?
3. A 45-year-old woman being treated with metronidazole for a pelvic abscess develops numbness and tingling in her feet and hands after 6 weeks of therapy. Physical examination reveals decreased sensation to light touch in a stocking-glove distribution and absent ankle reflexes. Which adverse effect of metronidazole is most likely responsible?
Frequently asked questions
How does metronidazole work?
Metronidazole is a prodrug that is reduced by low-redox potential proteins in anaerobic bacteria and protozoa to form toxic nitro radicals. These radicals cause DNA strand breaks and inhibit DNA repair, leading to cell death. The drug selectively targets organisms with sufficiently low intracellular redox potential, sparing aerobic cells and human tissues. This mechanism makes it highly effective against anaerobic bacteria like Bacteroides and protozoa such as Trichomonas and Giardia.
What are the side effects of metronidazole?
Common side effects include a metallic taste, nausea, and darkening of urine. Serious adverse effects include peripheral neuropathy (which may be irreversible), CNS toxicity (seizures, encephalopathy), and a disulfiram-like reaction if taken with alcohol (causing severe nausea, vomiting, flushing, and tachycardia). Patients must avoid alcohol during treatment and for 48 hours afterward. Long-term use carries a small risk of leukopenia and neuropathy.
Can I drink alcohol while taking metronidazole?
No, you must avoid all alcohol during metronidazole treatment and for at least 48 hours after your last dose. Metronidazole inhibits aldehyde dehydrogenase, causing a disulfiram-like reaction when combined with alcohol. This reaction includes severe nausea, vomiting, flushing, headache, tachycardia, and potentially dangerous hypotension. This applies to alcoholic beverages as well as alcohol-containing medications and mouthwashes.
What infections does metronidazole treat?
Metronidazole treats infections caused by anaerobic bacteria and certain protozoa. It is used for Clostridium difficile colitis, bacterial vaginosis, trichomoniasis, giardiasis, amebiasis, intra-abdominal infections involving Bacteroides, brain abscesses, pelvic inflammatory disease, aspiration pneumonia, and dental infections. It is also part of combination therapy for Helicobacter pylori eradication and is used topically for rosacea.
Metronidazole vs vancomycin for C. difficile—what is the difference?
Both treat Clostridium difficile infection, but oral vancomycin is now preferred as first-line therapy for most cases based on updated guidelines showing superior cure rates and lower recurrence. Metronidazole is systemically absorbed when given orally, whereas oral vancomycin stays in the gut lumen, providing higher local concentrations. Metronidazole remains an alternative for mild cases when cost is a major concern, but vancomycin (or fidaxomicin) is standard for moderate-to-severe or recurrent disease.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: A01AB17WHO ATC/DDD index: D06BX01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.