Drug Monograph
Metronidazole
Brand names: Flagyl, Flagyl ER, Flagyl IV RTU
Pharmacokinetic Profile
Half-Life
~8 h (healthy adults)
Metabolism
Hepatic (side-chain oxidation, glucuronide conjugation)
Protein Binding
<20%
Bioavailability
Near-complete (~100% oral)
Volume of Distribution
~0.55 L/kg
Clinical Information
Drug Class
Nitroimidazole (antibacterial & antiprotozoal)
Available Doses
250 mg, 375 mg, 500 mg tabs; 750 mg ER tab; IV 500 mg/100 mL; oral suspension; topical gel/cream/lotion
Route
Oral, Intravenous, Topical, Vaginal
Renal Adjustment
Not required for parent drug; ESRD: monitor for metabolite accumulation; supplement post-hemodialysis
Hepatic Adjustment
Child-Pugh C: reduce dose by 50%; A/B: monitor, no dose change
Pregnancy
Caution — crosses placenta; contraindicated in 1st trimester for trichomoniasis; no confirmed teratogenicity in large studies
Lactation
Present in breast milk at levels similar to plasma; pump and discard during therapy + 48 h after last dose, or weigh risks vs benefits
Schedule / Legal Status
Prescription only (Rx)
Generic Available
Yes
Black Box Warning
Yes — carcinogenic in rodents; avoid unnecessary use
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Trichomoniasis (T. vaginalis — symptomatic, asymptomatic, sexual partner treatment) | Adults | Monotherapy | FDA Approved |
| Amebiasis (acute intestinal amebic dysentery and amebic liver abscess) | Adults & children | Monotherapy (often with luminal agent) | FDA Approved |
| Anaerobic bacterial infections — intra-abdominal, gynecologic, skin/soft tissue, bone/joint, CNS, lower respiratory tract, endocarditis, bacteremia (Bacteroides, Clostridium, Fusobacterium, Peptostreptococcus spp.) | Adults | Mono or combination therapy | FDA Approved |
| Bacterial vaginosis (ER formulation) | Non-pregnant females | Monotherapy | FDA Approved |
| Surgical prophylaxis — elective colorectal surgery | Adults (IV formulation) | Perioperative prophylaxis | FDA Approved |
Metronidazole is the foundational nitroimidazole antibiotic, active against obligate anaerobes and key protozoal pathogens. Its near-complete oral bioavailability, penetration into CNS and abscess cavities, and bactericidal concentration-dependent killing make it a first-line agent for mixed anaerobic infections and protozoal disease. For anaerobic infections, it is frequently combined with agents active against aerobes (e.g., fluoroquinolones or cephalosporins) since metronidazole has no clinically relevant aerobic activity.
Notable Off-Label Uses
Clostridioides difficile infection (non-severe, initial episode) — 500 mg PO TID for 10 days. IDSA 2021 update: metronidazole only if vancomycin and fidaxomicin are unavailable; ACG 2021: acceptable in low-risk, younger outpatients. No longer preferred first-line. Evidence quality: High (inferior to vancomycin).
C. difficile fulminant disease — IV 500 mg q8h added to high-dose oral vancomycin. IDSA 2021 recommended. Evidence quality: Moderate.
Helicobacter pylori eradication — component of multiple regimens (e.g., bismuth quadruple therapy: PPI + bismuth + metronidazole 500 mg TID/QID + tetracycline). ACG 2017 guideline. Evidence quality: High.
Giardiasis — 250 mg PO TID for 5–7 days (adults). CDC-recommended alternative to tinidazole. Evidence quality: High.
Bacterial vaginosis (standard tablets) — 500 mg PO BID for 7 days. CDC 2021 STI treatment guidelines. Evidence quality: High.
Perianal Crohn’s disease / pouchitis — 250–500 mg TID; duration variable (weeks to months). Evidence quality: Moderate.
Tetanus (adjunctive) — 500 mg PO/IV q6–8h for 7–10 days. WHO recommendation. Evidence quality: Moderate.
Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Trichomoniasis — single-dose regimen | 2 g PO as a single dose | — | 2 g (single dose) | Treat sexual partners simultaneously; higher cure rates reported with 7-day regimen FDA PI; CDC STI guidelines 2021 |
| Trichomoniasis — 7-day regimen | 500 mg PO BID | 500 mg PO BID | 1 g/day | 7 days; CDC 2021 now recommends 500 mg BID as preferred for women (replacing prior 2 g single-dose preference) FDA PI lists 250 mg TID × 7 d as alternative; CDC 2021 uses 500 mg BID × 7 d |
| Acute intestinal amebiasis (amebic dysentery) | 750 mg PO TID | 750 mg PO TID | 2.25 g/day | 5–10 days; follow with luminal agent (paromomycin or iodoquinol) FDA PI |
| Amebic liver abscess | 500–750 mg PO TID | 500–750 mg PO TID | 2.25 g/day | 5–10 days; aspiration/drainage may be needed for large or refractory abscesses FDA PI |
| Serious anaerobic infections (IV to PO) | Loading: 15 mg/kg IV, then 7.5 mg/kg IV q6h | 500 mg PO q6–8h after clinical improvement | 4 g/day | Duration 7–10 days; bone/joint, endocarditis, and lung infections may require longer FDA PI; ~500 mg q6h for 70 kg adult |
| Bacterial vaginosis | 500 mg PO BID | 500 mg PO BID | 1 g/day | 7 days; alternative: Flagyl ER 750 mg daily × 7 days CDC 2021 STI guidelines |
| C. difficile — non-severe, initial episode (only if vancomycin/fidaxomicin unavailable) | 500 mg PO TID | 500 mg PO TID | 1.5 g/day | 10 days; no longer preferred first-line per IDSA 2021 IDSA/SHEA 2021 focused update |
| C. difficile — fulminant (adjunct to oral vancomycin) | 500 mg IV q8h | 500 mg IV q8h | 1.5 g/day | Given with oral vancomycin 500 mg q6h ± rectal vancomycin IDSA/SHEA 2021 |
| H. pylori eradication — bismuth quadruple therapy | 250 mg PO QID | 250 mg PO QID | 1 g/day | 10–14 days; given with PPI BID + bismuth subsalicylate 524 mg QID + tetracycline 500 mg QID ACG H. pylori guideline 2017; some international regimens use 500 mg TID |
| Giardiasis | 250 mg PO TID | 250 mg PO TID | 750 mg/day | 5–7 days Alternative to tinidazole; CDC recommendation |
| Colorectal surgical prophylaxis (IV) | 15 mg/kg IV infused over 30–60 min, completed ~1 h before incision | 7.5 mg/kg IV q6h | 4 g/day | Discontinue within 12 h postoperatively FDA PI for IV metronidazole |
Pediatric Dosing
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Amebiasis (intestinal & hepatic) | 35–50 mg/kg/day PO divided TID | Same | 2.25 g/day | 10 days; follow with luminal agent FDA PI; only FDA-approved pediatric indication for oral metronidazole |
| Giardiasis (off-label) | 15 mg/kg/day PO divided TID | Same | 750 mg/day | 5–7 days AAP Red Book recommendation |
| Anaerobic infections (off-label) | 30 mg/kg/day IV/PO divided q6h | Same | 4 g/day | 7–10 days Neonates: significantly prolonged half-life (22–109 h); use lower doses, extended intervals |
Dose Adjustments
| Population | Adjustment | Maximum Dose | Dialysis | Notes |
|---|---|---|---|---|
| Mild-moderate hepatic impairment (Child-Pugh A/B) | No dose change | Standard | — | Monitor for adverse events (AUC increased ~53–54%) |
| Severe hepatic impairment (Child-Pugh C) | Reduce dose by 50% | 50% of standard max | — | AUC increased 114% vs healthy controls (FDA PI) |
| Renal impairment (mild-moderate) | No dose change | Standard | — | Parent drug PK not significantly altered |
| ESRD / Hemodialysis | Standard dose; supplement post-HD | Standard | HD removes 40–65% of dose; give supplemental dose post-dialysis | CAPD removes ~10%; no dose adjustment needed for CAPD |
| Elderly (>70 years) | Standard dose, monitor closely | Standard | — | Active metabolite AUC 40–80% higher; monitor for neurotoxicity |
Clinical Pearl: IV-to-Oral Switch
Metronidazole achieves near-identical plasma levels via oral and IV routes, making it one of the easiest antibiotics to transition from IV to oral. For intra-abdominal infections started on IV metronidazole, early switch to oral dosing (typically 500 mg every 8 hours) is appropriate once the patient can tolerate oral intake, with no loss of efficacy.
Pharmacology
Mechanism of Action
Metronidazole is a prodrug that requires reductive activation within anaerobic or microaerophilic organisms. After passive diffusion into the cell, low-redox-potential electron transport proteins (such as ferredoxin) donate electrons to the nitro group, generating reactive nitroso and free radical intermediates. This reduction creates a concentration gradient favouring further drug uptake. The cytotoxic intermediates cause oxidative damage to DNA through strand breakage and helix destabilization, ultimately leading to cell death. This mechanism is selective for organisms with sufficiently low intracellular redox potentials, which is why metronidazole has no clinically relevant activity against facultative anaerobes or obligate aerobes. The drug also demonstrates concentration-dependent bactericidal activity against anaerobes and protozoa including Entamoeba histolytica and Trichomonas vaginalis.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Near-complete; Tmax 1–2 h; dose-proportional plasma levels (250 mg → 6 mcg/mL, 500 mg → 12 mcg/mL); food delays Tmax but does not reduce overall absorption | Oral and IV routes are essentially bioequivalent, enabling straightforward IV-to-oral step-down without efficacy loss |
| Distribution | Vd ~0.55 L/kg; protein binding <20%; penetrates CSF, saliva, breast milk, pus, and abscess cavities at concentrations similar to plasma | Excellent CNS and deep tissue penetration; appropriate for brain abscess, meningitis, and hepatic abscess without dose adjustment for site |
| Metabolism | Hepatic: side-chain oxidation (hydroxyl metabolite retains activity) and glucuronide conjugation; unchanged drug accounts for ~20% of urinary excretion | Severe hepatic impairment (Child-Pugh C) increases AUC by 114%, necessitating 50% dose reduction; the hydroxyl metabolite has antimicrobial activity |
| Elimination | 60–80% renal (as metabolites and parent drug); 6–15% fecal; t½ ~8 h; renal clearance 10 mL/min/1.73 m² | Hemodialysis removes 40–65% of drug — supplement after dialysis sessions; CAPD removes only ~10%, no supplement needed; neonatal t½ ranges 22–109 h depending on gestational age |
Side Effects
≥10%Very Common
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Nausea (with or without vomiting, anorexia) | 10–12% | Most frequently reported adverse effect; dose-related; taking with food may reduce severity |
| Metallic or sharp unpleasant taste (dysgeusia) | ~12% | Very characteristic of metronidazole; self-limiting and resolves after completion of therapy |
1–10%Common
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Headache | 5–10% | Usually mild; more common at higher doses |
| Diarrhea | 3–8% | Distinguish from C. difficile-associated diarrhea if persistent |
| Abdominal cramping / epigastric distress | 3–5% | Usually mild and self-limiting |
| Dizziness | 1–5% | Counsel patients regarding driving and machinery operation |
| Dry mouth / stomatitis / furry tongue | 1–5% | May be associated with oral Candida overgrowth |
| Darkened urine | Common (under-reported clinically) | Harmless metabolite discoloration; FDA PI cites ~1 per 100,000 reports, but clinical experience suggests this occurs frequently when patients observe it; counsel patient that this is expected and not blood in urine |
| Vaginal candidiasis | 1–5% | Disruption of normal flora; may require treatment with antifungal |
SeriousSerious (Regardless of Frequency)
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Peripheral neuropathy (sensory) | Uncommon (dose- and duration-dependent) | Weeks to months of prolonged use | Discontinue immediately; may be irreversible if continued; some cases persistent after cessation |
| CNS encephalopathy (cerebellar toxicity — ataxia, dysarthria, dizziness) | Rare | Days to weeks; more common with prolonged or high-dose use | Discontinue; generally reversible within days to weeks; MRI lesions (typically cerebellar/brainstem) also reversible |
| Seizures | Rare | Variable | Discontinue; evaluate for alternative causes; supportive management |
| Aseptic meningitis | Rare | Hours after dose administration | Discontinue; symptoms generally resolve promptly after drug withdrawal |
| Optic neuropathy | Very rare | Weeks to months | Discontinue immediately; ophthalmology referral; may be irreversible |
| Stevens-Johnson syndrome / TEN / DRESS / AGEP | Very rare | Days to weeks | Discontinue immediately; urgent dermatology referral; supportive care |
| QT prolongation | Rare (particularly with concurrent QT-prolonging drugs) | Variable | ECG monitoring in patients with risk factors; avoid concurrent QT-prolonging agents |
| Reversible neutropenia / leukopenia | Uncommon | During therapy | Monitor CBC; no persistent hematologic abnormalities reported in clinical studies |
| Hepatotoxicity (fatal in Cockayne syndrome) | Very rare (near-universal in Cockayne syndrome) | As early as 2 days after initiation | Metronidazole is absolutely contraindicated in Cockayne syndrome; in other patients, discontinue if hepatotoxicity develops |
| Disulfiram-like reaction (with alcohol) | Common when alcohol ingested | Minutes to hours after alcohol consumption | Avoid all alcohol and propylene glycol-containing products during therapy and for ≥3 days after; supportive treatment if reaction occurs |
| Pancreatitis | Very rare | Variable | Discontinue; generally resolves on drug withdrawal |
DiscontinuationDiscontinuation Rates
Standard-Dose Courses (General Population)
~2–5%
Top reasons: GI intolerance (nausea, vomiting), metallic taste, neurologic symptoms
Prolonged / High-Dose Use (e.g., Crohn’s disease)
10–20%
Top reasons: Peripheral neuropathy, GI intolerance; neuropathy risk increases significantly beyond 4 weeks of continuous use
| Reason for Discontinuation | Incidence | Context |
|---|---|---|
| GI intolerance | 1–5% | Nausea and metallic taste most common drivers |
| Neurologic symptoms | 1–3% | Higher with prolonged courses (>14 days); peripheral neuropathy may be irreversible |
| Allergic/hypersensitivity reaction | <1% | Rash, urticaria; severe cutaneous reactions very rare |
Peripheral Neuropathy: The Duration-Dependent Risk
Sensory peripheral neuropathy (numbness, paresthesia) is the most clinically important long-term risk of metronidazole. The risk is strongly linked to cumulative dose and duration. At standard courses (7–14 days), the risk is low. However, with prolonged use beyond 4 weeks (as sometimes employed in Crohn’s disease or chronic infections), neuropathy incidence rises substantially. Critically, some cases of peripheral neuropathy are irreversible even after drug discontinuation. Patients on prolonged courses should be warned to report any numbness or tingling immediately, and the drug should be stopped at the first sign of neuropathic symptoms.
Drug Interactions
Metronidazole has a moderate interaction profile. Its most clinically significant interactions involve alcohol (disulfiram-like reaction), warfarin (anticoagulant potentiation), lithium (toxicity risk), and disulfiram (psychotic reactions). Unlike many antibiotics, metronidazole does not exert its interactions primarily through CYP450 inhibition, although it is metabolized hepatically and its clearance can be affected by CYP enzyme inducers and inhibitors.
MajorDisulfiram
MechanismUnclear; possible additive neurotoxicity and altered dopamine metabolism
EffectPsychotic reactions reported in alcoholic patients receiving both drugs concurrently
ManagementContraindicated: do not administer metronidazole to patients who have used disulfiram within the past 2 weeks
FDA PIMajorAlcohol & Propylene Glycol-Containing Products
MechanismNot fully established; traditionally attributed to inhibition of aldehyde dehydrogenase, though this has been questioned in recent reviews
EffectAbdominal cramps, nausea, vomiting, headaches, and flushing
ManagementAvoid all alcohol and propylene glycol-containing products during therapy and for at least 3 days after last dose
FDA PIMajorBusulfan
MechanismMetronidazole increases busulfan plasma concentrations (mechanism not fully characterized)
EffectIncreased risk of serious busulfan toxicity (hepatic veno-occlusive disease, myelosuppression)
ManagementAvoid concomitant use unless no alternative; if used, monitor busulfan plasma levels and adjust dose accordingly
FDA PIModerateWarfarin & Oral Anticoagulants
MechanismMetronidazole potentiates the anticoagulant effect; likely via inhibition of warfarin metabolism
EffectProlonged prothrombin time and increased INR; risk of bleeding
ManagementMonitor PT/INR closely during and after metronidazole therapy; consider empiric warfarin dose reduction
FDA PIModerateLithium
MechanismMetronidazole may reduce renal lithium clearance
EffectElevated serum lithium levels; signs of lithium toxicity (tremor, confusion, renal impairment)
ManagementObtain serum lithium and creatinine levels several days after starting metronidazole; monitor for toxicity symptoms
FDA PIModeratePhenytoin / Phenobarbital (CYP inducers)
MechanismCYP450 inducers accelerate metronidazole metabolism; metronidazole may also impair phenytoin clearance
EffectReduced metronidazole plasma levels (potential treatment failure); elevated phenytoin levels (toxicity risk)
ManagementMonitor phenytoin levels; consider higher metronidazole doses or alternative antibiotic if enzyme inducer is essential
FDA PIModerateQT-Prolonging Agents
MechanismAdditive QT prolongation
EffectIncreased risk of ventricular arrhythmia, torsade de pointes
ManagementAvoid concurrent use with class IA/III antiarrhythmics; monitor ECG in patients with multiple risk factors for QT prolongation
FDA PIMinorCimetidine (CYP inhibitor)
MechanismDecreases hepatic microsomal enzyme activity, reducing metronidazole clearance
EffectProlonged metronidazole half-life; potential for increased adverse effects
ManagementMonitor for metronidazole-related side effects; consider alternative H2-blocker if prolonged courses needed
FDA PIMonitoring
- CBC (WBC with differential)Before and after therapy
RoutineMild, reversible leukopenia has been observed. Total and differential leukocyte counts recommended before and after therapy per FDA labeling. More frequent monitoring for courses exceeding 10 days. - Neurologic AssessmentEach visit; heightened vigilance if >14 days
RoutineAssess for peripheral neuropathy symptoms (numbness, tingling, paresthesia of extremities), cerebellar signs (ataxia, dysarthria), and visual changes (optic neuropathy). Discontinue immediately if neurologic symptoms emerge. - Hepatic FunctionBaseline for patients with liver disease; as clinically indicated
Trigger-basedScreen for Cockayne syndrome before initiating (absolute contraindication). In patients with known hepatic impairment, monitor for accumulation and adverse effects. Reduce dose 50% in Child-Pugh C. - INR / Prothrombin TimeDuring and after metronidazole in patients on warfarin
Trigger-basedMetronidazole potentiates warfarin; closely monitor INR. May need warfarin dose reduction. - Serum LithiumSeveral days after starting metronidazole in lithium-treated patients
Trigger-basedMetronidazole can elevate lithium levels and precipitate toxicity. Also check serum creatinine. - Candidal OvergrowthClinical assessment during therapy
RoutineOral or vaginal candidiasis may become more prominent during therapy; treat with appropriate antifungal if symptomatic.
Contraindications & Cautions
Absolute Contraindications
- Hypersensitivity to metronidazole or other nitroimidazole derivatives
- Cockayne syndrome — severe, irreversible hepatotoxicity/acute liver failure with fatal outcomes reported within days of initiation
- Concurrent disulfiram use (or use within the past 2 weeks) — risk of psychotic reactions
- First trimester pregnancy for trichomoniasis treatment — per FDA labeling
Relative Contraindications (Specialist Input Recommended)
- Active CNS disease — patients with seizure disorders or active neurologic conditions may be at higher risk for metronidazole neurotoxicity
- Severe hepatic impairment (Child-Pugh C) — can be used with 50% dose reduction and close monitoring, but alternative agents preferred if available
- History of blood dyscrasias — use with caution; monitor CBC
- Concurrent use with alcohol or propylene glycol-containing products — disulfiram-like reaction risk
Use with Caution
- Prolonged courses (>14 days) — increasing risk of peripheral neuropathy and CNS toxicity; limit duration where possible
- Elderly patients (>70 years) — higher active metabolite levels; monitor for neurotoxicity
- Neonates — markedly prolonged half-life (22–109 h depending on gestational age); adjust dosing intervals
- Patients on warfarin, lithium, or phenytoin — clinically significant pharmacokinetic interactions
- Patients with pre-existing neuropathy — baseline deficit makes detection of new drug-induced changes difficult
FDA Boxed Warning
Carcinogenicity in Rodents
Metronidazole has been shown to be carcinogenic in mice and rats, with tumors detected in the liver, lungs, mammary glands, and lymphatic tissues. These findings have not been confirmed in hamsters or in human epidemiologic studies (with over 50 years of clinical use). Nonetheless, the FDA requires that unnecessary use be avoided and that metronidazole be reserved for its approved indications. This warning is particularly relevant when considering prolonged, high-dose, or off-label chronic use (e.g., Crohn’s disease maintenance).
Patient Counselling
Purpose of Therapy
Metronidazole is an antibiotic and antiparasitic medication that works by disrupting the DNA of anaerobic bacteria and certain parasites. It is used to treat a wide range of infections including bacterial vaginosis, gastrointestinal infections, pelvic infections, and others. It does not work against viruses such as colds or influenza. Completing the full prescribed course is essential even if symptoms improve early.
How to Take
Take metronidazole with food to reduce stomach upset. Swallow tablets whole with a full glass of water. Extended-release tablets should not be crushed, split, or chewed. If using the liquid suspension, measure each dose carefully using the provided device rather than a household spoon.
Alcohol Avoidance
Tell patientDo not drink any alcoholic beverages or use products containing alcohol or propylene glycol while taking metronidazole AND for at least 3 full days after your last dose. Mixing metronidazole with alcohol can cause severe nausea, vomiting, cramps, headache, and facial flushing. Check medication labels, mouthwashes, and cooking extracts for hidden alcohol content.
Call prescriberIf you accidentally consume alcohol and develop severe nausea, vomiting, chest pain, or rapid heartbeat, seek medical attention.
Metallic Taste & GI Side Effects
Tell patientA metallic or bitter taste in the mouth is very common with this medication and does not indicate a problem. It resolves after you finish the course. Taking the drug with food may help reduce nausea. Sugar-free gum or mints may mask the taste.
Call prescriberContact your doctor if nausea or vomiting is severe enough that you cannot keep the medication down, or if you develop bloody or watery diarrhea.
Numbness or Tingling (Nerve Damage Warning)
Tell patientMetronidazole can occasionally damage peripheral nerves, particularly with longer courses. This presents as numbness, tingling, or a “pins and needles” sensation in the hands or feet. The risk increases with treatment lasting more than 2 weeks.
Call prescriberStop the medication and contact your doctor immediately if you notice any new numbness, tingling, or weakness in your hands or feet, or if you develop difficulty with coordination, walking, or vision changes.
Dark Urine
Tell patientYour urine may turn a dark reddish-brown colour while taking metronidazole. This is a harmless drug metabolite and is not blood. It does not require any action and will resolve after you stop the medication.
Call prescriberIf you experience pain or difficulty urinating, or notice blood clots in the urine, contact your prescriber as this may indicate a different issue.
Skin Rash or Allergic Reaction
Tell patientAlthough uncommon, metronidazole can cause serious skin reactions. Any new rash during treatment should be evaluated.
Call prescriberStop the medication and seek immediate medical attention if you develop blistering skin, mouth sores, skin peeling, high fever, or swelling of the face/tongue.
Yeast Infection
Tell patientMetronidazole can disrupt normal body flora and may lead to vaginal yeast infection (itching, white discharge) or oral thrush (white patches in the mouth). These are treatable side effects.
Call prescriberLet your doctor know if you develop symptoms of yeast infection so appropriate antifungal treatment can be arranged.
Sources
Regulatory (PI / SmPC)
- FLAGYL (metronidazole) tablets. Full Prescribing Information. Pfizer Labs. Revised July 2024. FDA LabelPrimary regulatory reference for oral metronidazole: all approved indications, dosing, contraindications, adverse reactions, and drug interactions including the 2024 labeling revision.
- FLAGYL ER (metronidazole) extended-release tablets 750 mg. Full Prescribing Information. Pfizer Labs. Revised July 2024. FDA LabelRegulatory label for the extended-release formulation approved for bacterial vaginosis; includes identical PK, safety, and interaction data.
- FLAGYL 375 (metronidazole) capsules. Full Prescribing Information. Pfizer Labs. Revised July 2024. FDA LabelCapsule formulation label; includes additional PK data under fed/fasted conditions (Tmax shift with food).
Key Clinical Trials & Meta-Analyses
- Zar FA, Bakkanagari SR, Moorthi KM, et al. A comparison of vancomycin and metronidazole for the treatment of Clostridium difficile-associated diarrhea, stratified by disease severity. Clin Infect Dis. 2007;45(3):302–307. DOILandmark RCT demonstrating vancomycin superiority over metronidazole for severe CDI, forming the basis for guideline changes away from metronidazole as first-line CDI treatment.
- Nelson RL, Suda KJ, Evans CT. Antibiotic treatment for Clostridium difficile-associated diarrhoea in adults. Cochrane Database Syst Rev. 2017;3(3):CD004610. DOICochrane meta-analysis of 22 trials confirming vancomycin superiority over metronidazole for CDI, supporting the shift to vancomycin/fidaxomicin as preferred first-line agents.
Guidelines
- Johnson S, Lavergne V, Skinner AM, et al. Clinical Practice Guideline by the IDSA and SHEA: 2021 Focused Update Guidelines on Management of Clostridioides difficile Infection in Adults. Clin Infect Dis. 2021;73(5):e1029–e1044. DOICurrent IDSA/SHEA guideline relegating metronidazole to last resort for non-severe CDI; recommends IV metronidazole as adjunct in fulminant CDI.
- Kelly CR, Fischer M, Allegretti JR, et al. ACG Clinical Guidelines: Prevention, Diagnosis, and Treatment of Clostridioides difficile Infections. Am J Gastroenterol. 2021;116(6):1124–1147. DOIACG 2021 guideline allowing metronidazole as an option in low-risk, younger outpatients with initial non-severe CDI as a cost-saving alternative.
- Workowski KA, Bachmann LH, Chan PA, et al. Sexually Transmitted Infections Treatment Guidelines, 2021. MMWR Recomm Rep. 2021;70(4):1–187. DOICDC 2021 STI treatment guidelines recommending 7-day metronidazole regimen (500 mg BID) as now preferred for trichomoniasis in women over the single-dose regimen.
- Chey WD, Leontiadis GI, Howden CW, et al. ACG Clinical Guideline: Treatment of Helicobacter pylori Infection. Am J Gastroenterol. 2017;112(2):212–239. DOIACG guideline for H. pylori including metronidazole as a component of bismuth quadruple therapy; provides dosing and duration recommendations.
Mechanistic / Basic Science
- Edwards DI. Mechanisms of selective toxicity of metronidazole and other nitroimidazole drugs. Br J Vener Dis. 1980;56(5):285–290. DOIClassic paper detailing the reductive activation mechanism, ferredoxin-mediated electron transfer, and DNA damage pathway that underpins metronidazole’s selective anaerobic toxicity.
- Lófmark S, Edlund C, Nord CE. Metronidazole is still the drug of choice for treatment of anaerobic infections. Clin Infect Dis. 2010;50(Suppl 1):S16–S23. DOIComprehensive review of metronidazole’s spectrum, resistance patterns, and clinical role in anaerobic infections with pharmacokinetic context.
Pharmacokinetics / Special Populations
- Lamp KC, Freeman CD, Klutman NE, et al. Pharmacokinetics of metronidazole in special populations. Pharmacotherapy. 1999;19(4):473–489. DOIDetailed PK review covering geriatric, neonatal, hepatically impaired, and renally impaired populations including dialysis pharmacokinetics.
- Weir CB, Le JK. Metronidazole. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023. StatPearlsContinuously updated clinical pharmacology overview including off-label indications, dosing recommendations, and adverse effect profiles.