A nitroimidazole antimicrobial with potent activity against anaerobic bacteria and certain protozoa. Alternative treatment for Clostridioides difficile infection when vancomycin or fidaxomicin is unavailable, bacterial vaginosis, trichomoniasis, amoebic dysentery, and anaerobic intra-abdominal/pelvic infections (typically combined with a cephalosporin or fluoroquinolone). Also used in combination regimens for H. pylori eradication.
Alternative treatment for C. difficile infection (when first-line agents unavailable)7,8
Treats bacterial vaginosis
Treats trichomoniasis
Treats amoebic dysentery and liver abscess
Treats anaerobic intra-abdominal and pelvic infections9
What to watch for
Metallic taste (very common)
Nausea and vomiting
Diarrhea
Known hypersensitivity to metronidazole or nitroimidazoles1,2
First trimester of pregnancy (for trichomoniasis, weigh risk/benefit)6
The bottom line
Evidence rating strong. Most-documented uses: alternative treatment for c. difficile infection (when first-line agents unavailable), treats bacterial vaginosis, treats trichomoniasis. 10 sources indexed (1995–2025), with 4 interaction records on file.
The science
How it works, mechanistically.
Core mechanism
A prodrug that is reduced inside anaerobic and microaerophilic organisms. The nitro group of metronidazole is reduced by ferredoxin (or flavodoxin) to form reactive cytotoxic intermediates, nitro radical anions, nitroso compounds, and hydroxylamine. These intermediates cause DNA strand breakage, helix destabilization, and inhibition of nucleic acid synthesis, leading to cell death. Selective toxicity to anaerobes results from their low-redox-potential electron transport systems required to activate the drug.
Class
Nitroimidazole Antibiotic/Antiprotozoal
Absorption
Water-soluble; take with food
Dosing
Dosing & protocol.
Common range
250-500 mg every 8 hours orally; C. diff: 500 mg TID x 10 days; bacterial vaginosis: 500 mg BID x 7 days; trichomoniasis: 2 g single dose (as prescribed by your physician)
Recommended form
Oral tablets, capsules, IV, topical gel, or vaginal gel
Oral bioavailability nearly 100%. Taking with food reduces GI upset. IMPORTANT: Avoid alcohol during treatment and for at least 3 days after, disulfiram-like reaction (nausea, vomiting, flushing, tachycardia).
Depletions
What it depletes.
Nutrients this medication can lower over time, and what to replace.
Vitamin K
Mild
Broad-spectrum antibiotic exposure can suppress gut bacteria that synthesize menaquinones, lowering vitamin K availability in susceptible patients.
Monitor PT/INROnset Usually with prolonged therapy, poor intake, or malabsorption
Safety
Full safety detail.
Side effects
Metallic taste (very common)
Nausea and vomiting
Diarrhea
Disulfiram-like reaction with alcohol
Peripheral neuropathy (with prolonged use)
Darkening of urine (harmless metabolite)
Headache
Seizures (rare, with high doses or prolonged use)
Contraindications
Known hypersensitivity to metronidazole or nitroimidazoles1,2
First trimester of pregnancy (for trichomoniasis, weigh risk/benefit)6
Metronidazole inhibits acetaldehyde dehydrogenase, and concurrent alcohol can trigger a disulfiram-like reaction including flushing, severe nausea, vomiting, headache, tachycardia, and chest pain. Although recent reviews have debated whether the reaction occurs in all patients, multiple case reports describe serious reactions and the manufacturer warning remains in place. Severe cases have caused hypotension and even death.
Recommendation: Do not drink alcohol while taking metronidazole. Wait at least 3 days (72 hours) after your last dose before consuming alcohol, including alcohol in mouthwashes, cough syrups, and food preparations.
Probiotic supplementation during metronidazole therapy reduces antibiotic-associated diarrhea and helps preserve gut microbiome diversity. This is particularly relevant because metronidazole is often used for gastrointestinal infections that already disrupt the microbiome.
Recommendation: Take probiotics throughout your metronidazole course, separated by at least 2 hours from each antibiotic dose. Continue for at least 1 week after the antibiotic ends.
Saccharomyces boulardii is a yeast probiotic with strong evidence for preventing antibiotic-associated diarrhea, and it is unaffected by metronidazole because it is fungal rather than bacterial. It is particularly useful during anaerobic-targeted therapy.
Recommendation: Take Saccharomyces boulardii throughout your metronidazole course. Timing flexibility is greater than with bacterial probiotics. Continue for at least 1 week after the antibiotic ends.
Lactobacillus rhamnosus GG reduces antibiotic-associated diarrhea during metronidazole therapy and helps restore gut microbiome balance disrupted by broad anaerobic coverage. Separation from the antibiotic dose preserves bacterial viability.
Recommendation: Take Lactobacillus rhamnosus throughout your metronidazole course, separated by at least 2 hours from each antibiotic dose. Continue for at least 1 week after the antibiotic ends.
Shahzad I, Alasmari MS, Zamir A et al.. Clinical pharmacokinetics of metronidazole: a systematic review and meta-analysis. Antimicrobial agents and chemotherapy. 2025
Taj S, Zuber M, Hanumanthaiah VB et al.. Metronidazole Induced Cutaneous Adverse Drug Reaction- A Systematic Review of Descriptive Studies. Current reviews in clinical and experimental pharmacology. 2024
Eberspacher C, Mascagni D, Pontone S et al.. Topical metronidazole after haemorrhoidectomy to reduce postoperative pain: a systematic review. Updates in surgery. 2024
Re AD, Toh JWT, Iredell J et al.. Metronidazole in the Management of Post-Open Haemorrhoidectomy Pain: Systematic Review. Annals of coloproctology. 2020
Xia W, Manning JPR, Barazanchi AWH et al.. Metronidazole following excisional haemorrhoidectomy: a systematic review and meta-analysis. ANZ journal of surgery. 2018
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