Metronidazole (Flagyl) is a nitroimidazole antibiotic and antiprotozoal used for bacterial vaginosis, Trichomonas vaginalis, C. difficile infection, H. pylori eradication, and anaerobic bacterial infections of the abdomen, skin, and CNS. It works as a prodrug activated in anaerobic organisms, generating cytotoxic free radical intermediates that cause DNA strand breaks and cell death. Common side effects include metallic taste, nausea, and dizziness. Alcohol must be strictly avoided during treatment and for 48–72 hours after the final dose — metronidazole inhibits aldehyde dehydrogenase and causes a severe disulfiram-like reaction including flushing, vomiting, and potentially serious cardiovascular effects.
Metronidazole
Uses & FDA Indications
Metronidazole is a nitroimidazole with a uniquely broad spectrum of activity against anaerobic bacteria and parasitic protozoa. Its clinical versatility has made it a foundational antibiotic across gastroenterology, infectious disease, gynecology, and dermatology.
FDA-approved indications include: bacterial vaginosis (oral and vaginal gel formulations), Trichomonas vaginalis infection (in both the infected patient and their partner), anaerobic bacterial infections of the abdomen, skin, bone and joints, CNS, lower respiratory tract, and endocardium (including Bacteroides, Clostridium, Fusobacterium, Peptostreptococcus, and related species), and amebic liver abscess and intestinal amebiasis caused by Entamoeba histolytica. The topical formulations (MetroGel, MetroCream) are FDA-approved for rosacea.
Clinically critical off-label and guideline-recommended uses include Clostridioides difficile (C. diff) infection — particularly mild-to-moderate disease, though vancomycin or fidaxomicin are now preferred by major guidelines — and Helicobacter pylori eradication as part of multi-drug regimens (typically combined with a proton pump inhibitor and clarithromycin, or in bismuth-based quadruple therapy). Metronidazole is also used in surgical prophylaxis for colorectal procedures and in the treatment of Giardia lamblia intestinal infection.
How It Works
Metronidazole is a prodrug that requires intracellular activation to exert its antimicrobial effect. After entering susceptible organisms — anaerobic bacteria and certain protozoa — the nitro group of the drug is reduced by intracellular electron transport proteins (specifically ferredoxin and related low-redox-potential electron carriers present only in anaerobic organisms). This reduction generates cytotoxic free radical intermediates that damage DNA, causing strand breaks and inhibition of DNA synthesis, ultimately leading to cell death.
The requirement for anaerobic activation is fundamental to its selectivity: aerobic bacteria and human cells cannot reduce the nitro group to the reactive form, which is why metronidazole has no activity against aerobic organisms and minimal toxicity to host cells at therapeutic concentrations. This mechanism is also why metronidazole is effective against a broad range of anaerobic pathogens regardless of species — the target is the electron transport system common to all anaerobes rather than a species-specific enzyme.
Metronidazole's activity is strictly limited to anaerobic bacteria and certain protozoa. It has no meaningful activity against aerobic bacteria, fungi, or viruses. Infections involving aerobic organisms require separate antibiotic coverage — metronidazole is frequently combined with other agents (such as fluoroquinolones or cephalosporins) to provide both aerobic and anaerobic coverage in mixed infections.
ALCOHOL INTERACTION: Metronidazole inhibits aldehyde dehydrogenase, the enzyme responsible for breaking down acetaldehyde (a toxic alcohol metabolite). Concurrent alcohol consumption causes acetaldehyde accumulation and a disulfiram-like reaction: flushing, nausea, vomiting, sweating, palpitations, and hypotension. Avoid ALL alcohol during treatment and for at least 48–72 hours after the final dose. This includes alcohol in food, mouthwash, and liquid medications.
Side Effects
Common
- Metallic or bitter taste (dysgeusia) — among the most frequently reported effects; relates to drug secretion in saliva. Temporary but often bothersome during treatment.
- Nausea, vomiting, and GI upset — particularly common with oral formulations; taking with food reduces GI irritation
- Headache
- Dizziness
- Anorexia (decreased appetite)
- Vaginal yeast infection (vulvovaginal candidiasis) — disruption of normal vaginal flora can allow Candida overgrowth, particularly when treating bacterial vaginosis
- Darkening of urine — harmless; caused by metronidazole metabolites; does not indicate liver damage
Serious
- Peripheral neuropathy — numbness, tingling, or burning in the extremities, particularly with prolonged treatment or high cumulative doses. May be irreversible if not recognized promptly. Metronidazole should generally be discontinued if peripheral neuropathy develops.
- Central nervous system toxicity — encephalopathy, cerebellar ataxia, confusion, seizures. CNS toxicity is associated with high doses, renal impairment, or accumulation in hepatic failure. MRI may show characteristic T2 hyperintensities in the cerebellum and corpus callosum.
- Disulfiram-like reaction with alcohol — see warn box above. Can be severe, with cardiovascular consequences.
- Severe allergic reactions — including anaphylaxis, urticaria, angioedema (rare)
- Clostridium difficile-associated diarrhea — paradoxically, like other antibiotics, metronidazole can predispose to or worsen C. diff in some contexts, despite being used to treat it
- Hepatotoxicity — rare but reported; metronidazole is extensively metabolized by the liver. Use with caution in severe hepatic impairment.
Drug Interactions
| Drug / Class | Interaction | Clinical Significance |
|---|---|---|
| Alcohol (ethanol) | Aldehyde dehydrogenase inhibition causes acetaldehyde accumulation — disulfiram-like reaction: flushing, nausea, vomiting, palpitations, hypotension. Applies to all alcohol sources including beverages, mouthwash, and ethanol-containing medications. | High — absolute contraindication during treatment and for 48–72 hours after last dose |
| Warfarin (Coumadin) | Metronidazole inhibits CYP2C9, the primary enzyme metabolizing the more potent S-warfarin enantiomer. This substantially increases anticoagulant effect and INR, raising bleeding risk. | High — monitor INR closely; expect warfarin dose reduction may be needed during metronidazole course |
| Disulfiram (Antabuse) | Combined use may cause acute psychosis or confusional states. Both agents can produce psychoneurological effects, and the combination is potentially dangerous. | High — avoid concurrent use; wait at least 2 weeks after stopping disulfiram before initiating metronidazole |
| Lithium | Metronidazole can increase serum lithium levels, potentially to toxic concentrations. Mechanism involves reduced renal lithium clearance. | Moderate-High — monitor lithium levels and renal function; watch for signs of lithium toxicity (tremor, confusion, nausea) |
| Phenytoin, Phenobarbital (CYP3A4/CYP2C9 inducers) | Enzyme induction increases metronidazole metabolism, potentially reducing efficacy. Metronidazole may also inhibit phenytoin clearance, raising phenytoin levels. | Moderate — monitor phenytoin levels; consider metronidazole efficacy in long-term phenobarbital or phenytoin users |
| Cyclosporine | Metronidazole inhibits cyclosporine metabolism, potentially elevating cyclosporine levels and nephrotoxicity risk. | Moderate — monitor cyclosporine concentrations and renal function |
| Busulfan | Metronidazole inhibits busulfan metabolism, raising plasma concentrations and risk of busulfan toxicity (hepatic veno-occlusive disease). | High — avoid concurrent use when possible; closely monitor if combination is unavoidable |
Warnings & Contraindications
Contraindications
- Hypersensitivity to metronidazole or other nitroimidazoles (tinidazole, secnidazole)
- Concurrent use with alcohol or disulfiram
- First trimester of pregnancy — avoid unless benefit clearly outweighs risk
Alcohol Warning
The disulfiram-like interaction between metronidazole and alcohol is one of the most clinically important drug-alcohol interactions in outpatient medicine. Patients must be explicitly counseled to avoid all forms of alcohol — including wine, beer, spirits, and products containing ethanol such as some mouthwashes, cough syrups, and topical preparations — during treatment and for at least 48 to 72 hours after completing the course. The intensity of the reaction varies between individuals but can include flushing, severe nausea and vomiting, throbbing headache, palpitations, hypotension, and in severe cases, cardiovascular compromise.
Neurological Toxicity
Metronidazole can cause peripheral neuropathy (numbness, tingling, paresthesias) with prolonged use. Patients on extended courses for conditions like Crohn's disease or refractory C. diff should be monitored for neurological symptoms. Discontinuation is typically warranted if peripheral neuropathy develops, as continuation risks irreversible nerve damage. CNS toxicity — cerebellar ataxia, encephalopathy, seizures — occurs more rarely and is associated with high doses, hepatic failure (accumulation), or renal impairment. A characteristic pattern on brain MRI (symmetric T2 hyperintensities in the dentate nuclei of the cerebellum and corpus callosum) is often called "metronidazole-induced encephalopathy" and typically reverses with drug discontinuation.
Carcinogenicity Concerns
Metronidazole has demonstrated mutagenic activity in certain bacterial assays and tumorigenic effects in animal studies at high doses. No clear evidence of carcinogenicity has been established in humans at therapeutic doses, and the drug has been used clinically for decades without an identified excess cancer risk in humans. However, this history means metronidazole is generally avoided during the first trimester of pregnancy when organogenesis is occurring, and unnecessary prolonged use should be avoided. Brief therapeutic courses carry minimal concern.
Pregnancy and Lactation
Metronidazole crosses the placenta. While it has been widely studied in pregnancy, it is generally avoided in the first trimester due to historical concerns about mutagenicity (no human teratogenicity has been established, but caution remains). Use in the second and third trimesters is considered when the benefit outweighs the risk — it is used for bacterial vaginosis in pregnancy to reduce the risk of preterm birth associated with untreated infection. Metronidazole is secreted in breast milk; brief interruption of breastfeeding (24 hours) after a single high dose is sometimes recommended, though short-course oral therapy is generally considered compatible with breastfeeding.
Check for the warfarin interaction or other interactions with metronidazole before starting treatment.
Check Drug Interactions →Frequently Asked Questions
Can you drink alcohol while taking metronidazole?
No — alcohol must be avoided during metronidazole treatment and for at least 48–72 hours after the last dose. Metronidazole inhibits aldehyde dehydrogenase, the enzyme that breaks down acetaldehyde (a toxic byproduct of alcohol metabolism). This causes acetaldehyde to accumulate, producing a disulfiram-like reaction: flushing, nausea, vomiting, headache, sweating, palpitations, and potentially severe cardiovascular effects. This interaction applies to all forms of alcohol including wine, beer, spirits, and some mouthwashes and cough syrups containing alcohol.
Why does metronidazole cause a metallic taste?
A metallic or bitter taste (dysgeusia) is one of the most commonly reported side effects of oral metronidazole, affecting a large proportion of patients. The exact mechanism is not fully understood, but it likely relates to the drug or its metabolites being secreted in saliva, where taste receptors detect the compound directly. Some patients also notice a metallic aftertaste in foods during treatment. This effect is temporary and resolves after completing the course, though it can be distressing during treatment. Staying hydrated and using sugar-free gum or mints may help.
Is metronidazole an antibiotic or an antifungal?
Metronidazole is an antibiotic and antiprotozoal — it is not an antifungal. It is highly effective against anaerobic bacteria (bacteria that thrive without oxygen) and certain parasitic protozoa, but it has no activity against fungi or aerobic bacteria. Because it does not cover common vaginal yeast (Candida), metronidazole used for bacterial vaginosis can sometimes allow yeast overgrowth, leading to a yeast infection as a secondary effect in susceptible individuals. Yeast infections that develop during or after metronidazole therapy require separate antifungal treatment.