Formulary
Metronidazole: Uses, Dosing, Side Effects and Indian Brand Names
Metronidazole is a nitroimidazole selectively activated by anaerobic organisms, first-line for anaerobic infections, amoebiasis, giardiasis, and trichomoniasis, with a disulfiram-like alcohol reaction.
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Metronidazole: Uses, Dosing, Side Effects and Indian Brand Names
Metronidazole is a nitroimidazole selectively activated by anaerobic organisms, first-line for anaerobic infections, amoebiasis, giardiasis, and trichomoniasis, with a disulfiram-like alcohol reaction.
NEET PG High-Yield: Selectively activated by anaerobes (ferredoxin-mediated reduction). Disulfiram-like reaction with alcohol -- favourite MCQ. Drug of choice for amoebiasis (+ luminal agent), giardiasis, trichomoniasis. Metallic taste. Dark urine (harmless). Ames test positive (mutagenic in bacteria). Peripheral neuropathy with prolonged use. Crosses BBB (effective in brain abscess). Warfarin interaction (CYP2C9). Must follow tissue amoebicide with luminal amoebicide (diloxanide/paromomycin).
Clinical overview
Metronidazole is a nitroimidazole antibiotic with bactericidal activity against virtually all clinically significant anaerobic bacteria (Bacteroides fragilis, Clostridium species, Fusobacterium, Prevotella, Peptostreptococcus) and several protozoa (Entamoeba histolytica, Giardia lamblia, Trichomonas vaginalis). Its selective toxicity to anaerobes, through a redox-dependent activation mechanism, makes it one of the most elegant examples of selective drug targeting in pharmacology. Metronidazole has excellent tissue penetration including the CNS (crossing the blood-brain barrier readily), making it effective for brain abscesses, which are frequently caused by anaerobes. It is first-line for amoebic liver abscess (followed by a luminal agent like diloxanide furoate to eliminate cysts in the gut), giardiasis, and trichomonal vaginitis. In surgical practice, metronidazole is a standard component of prophylaxis and treatment for intra-abdominal and pelvic infections where anaerobic coverage is needed. The drug has a disulfiram-like reaction with alcohol (inhibiting aldehyde dehydrogenase, causing acetaldehyde accumulation with nausea, vomiting, flushing, and headache), and patients must abstain from alcohol during treatment and for 48 hours after completion. Metronidazole has mutagenic potential in bacterial (Ames test positive) but not in mammalian systems; carcinogenicity has been shown in rodents at very high doses. Despite this, extensive human use has not demonstrated increased cancer risk at therapeutic doses. Peripheral neuropathy and seizures are the most serious adverse effects, typically with prolonged use.
Pharmacological class
Metronidazole belongs to the Nitroimidazole Antibiotics / Antiprotozoal Agents class. Prodrug that enters anaerobic and microaerophilic organisms by passive diffusion. Inside the cell, the nitro group is reduced by ferredoxin (or ferredoxin-like electron transport proteins) that exist only in anaerobic metabolism. The reduced intermediates are cytotoxic -- they damage DNA by forming adducts, causing strand breakage and inhibiting DNA repair. The drug is selectively toxic to anaerobes because aerobic organisms lack the low-redox-potential electron transport system needed to activate it.
Indian brand names and formulations
Available as: Flagyl (Abbott/Sanofi), Metrogyl (J B Chemicals), Aristogyl (Aristo), Metris (Alkem).
Tablets: 200 mg, 400 mg. IV infusion: 500 mg/100 mL. Suspension: 200 mg/5 mL. Topical gel: 1% (for rosacea, wounds). Vaginal gel: 0.75%. Suppository: 500 mg.
Regulatory status
Metronidazole is classified under Schedule H in India under the Drugs and Cosmetics Act, 1940. Prescription-only. Listed on NLEM. One of the most widely used antibiotics in India for both hospital and outpatient settings.
Indications
- Anaerobic infections (intra-abdominal, pelvic, CNS abscess, aspiration pneumonia, dental infections)
- Amoebic liver abscess and intestinal amoebiasis (Entamoeba histolytica)
- Giardiasis (Giardia lamblia)
- Trichomonal vaginitis (Trichomonas vaginalis)
- Clostridioides difficile infection (mild cases; oral vancomycin preferred for moderate-severe)
- H. pylori eradication (with amoxicillin/clarithromycin + PPI)
- Surgical prophylaxis (colorectal, gynaecological surgery -- anaerobic coverage)
- Bacterial vaginosis
Dosing
Anaerobic infections: 400-500 mg three times daily (oral) or 500 mg every 8 hours (IV) for 7-14 days. Amoebic abscess: 750 mg three times daily for 5-10 days, followed by diloxanide furoate for luminal cysts. Giardiasis: 400 mg three times daily for 5 days. Trichomoniasis: 2 g single dose or 400 mg twice daily for 7 days. C. difficile: 400-500 mg three times daily for 10 days. Surgical prophylaxis: 500 mg IV at induction.
Contraindications
- First trimester of pregnancy (theoretical teratogenicity from mutagenic potential -- though human data is reassuring)
- Active CNS disease (lowers seizure threshold)
- Known hypersensitivity to nitroimidazoles
- Concurrent disulfiram use (psychotic reactions reported)
Adverse effects
- Metallic taste (very common, distinctive -- patients often report it spontaneously)
- Nausea, vomiting, and anorexia
- Disulfiram-like reaction with alcohol (nausea, vomiting, flushing, tachycardia -- avoid alcohol during treatment and for 48 hours after)
- Peripheral neuropathy (sensory, with prolonged use >4 weeks; usually reversible on discontinuation)
- Seizures (rare, with high doses or prolonged courses; metronidazole crosses BBB)
- Dark/red-brown urine (harmless metabolite -- warn patients)
- Dry mouth and furry tongue
Drug interactions
- Alcohol: disulfiram-like reaction (inhibition of aldehyde dehydrogenase); avoid during and 48 hours after treatment
- Warfarin: metronidazole inhibits CYP2C9, potentiating warfarin effect and increasing INR; reduce warfarin dose and monitor INR
- Lithium: metronidazole reduces lithium renal clearance, risk of toxicity
- Disulfiram: concurrent use may cause acute psychosis; contraindicated
- Phenytoin and phenobarbital: may reduce metronidazole levels via enzyme induction
Pregnancy and lactation
Traditionally avoided in the first trimester (Ames test positive for mutagenicity in bacteria). However, multiple large studies and meta-analyses have NOT shown increased teratogenic risk in humans. Category B. Used in pregnancy for trichomoniasis and serious anaerobic infections when benefit outweighs theoretical risk. Safe in second and third trimesters.
Exam-style clinical scenario
A 35-year-old man presents with fever, right upper quadrant pain, and hepatomegaly. Ultrasound shows a single large liver abscess. Amoebic serology is positive. Treatment? Metronidazole 750 mg three times daily for 10 days for the invasive trophozoites, FOLLOWED by diloxanide furoate 500 mg three times daily for 10 days as a luminal agent to eliminate cysts and prevent recurrence.
Cost in India
Rs 10-25 for a strip of 10 tablets (400 mg). IV: Rs 20-40 per 100 mL infusion (500 mg). Topical gel: Rs 30-60.
Clinical governance
Author: MedNext Editorial Team. Clinical reviewer: Awaiting clinical review. Jurisdiction: India (Drugs and Cosmetics Act, 1940). Sources: CIMS India, Indian Pharmacopoeia. Publication state: Awaiting clinical review. Correction: Report errors at support@mednext.academy.
Frequently Asked Questions
Why does metronidazole only work against anaerobes?
Metronidazole's nitro group must be reduced to produce cytotoxic intermediates that damage DNA. This reduction requires a very low-redox-potential electron donor -- ferredoxin or flavodoxin -- which is found only in anaerobic or microaerophilic organisms. In aerobes, oxygen competes as the electron acceptor and re-oxidises the drug before it can cause DNA damage (futile cycling). This makes metronidazole selectively toxic to anaerobes.
Why must a luminal agent follow metronidazole in amoebiasis?
Metronidazole is a tissue amoebicide that effectively kills invasive trophozoites in the liver and intestinal wall but does NOT reach therapeutic concentrations in the intestinal lumen where cysts reside. If cysts are not eradicated, the patient remains a carrier and can relapse or transmit infection. Diloxanide furoate or paromomycin (luminal amoebicides) must follow to eliminate cysts from the gut.
What is the disulfiram-like reaction and how does it work?
Metronidazole inhibits aldehyde dehydrogenase, the enzyme that metabolises acetaldehyde (produced from ethanol by alcohol dehydrogenase) to acetate. Acetaldehyde accumulates, causing nausea, vomiting, flushing, tachycardia, and hypotension -- the same mechanism as the drug disulfiram (Antabuse). Patients must avoid ALL alcohol (including alcohol-containing mouthwashes and medicines) during treatment and for 48 hours after the last dose.
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