Metronidazole (Flagyl): Uses, Dose, Side Effects

✓ Medically reviewed by · Last reviewed: May 2026

Pharmacy Researcher · 8 years experience

Pharmacy researcher with 8 years reviewing clinical drug information, generic formulation equivalence, and international pharmaceutical standards. Focuses on patient-facing accuracy in medication education.

Reviewed by Sophie Chen · Last updated: 23 May 2026

Key Takeaways

What it is: Metronidazole (brand name Flagyl) is a nitroimidazole antibiotic and antiprotozoal that has been in clinical use since 1960. It is on the WHO Model List of Essential Medicines.
What it treats: Anaerobic bacterial infections, bacterial vaginosis (BV), trichomoniasis, giardiasis, amoebiasis, Clostridioides difficile colitis, Helicobacter pylori (as part of triple/quadruple therapy), dental abscesses, surgical-prophylaxis cover, and certain skin and intra-abdominal infections.
How it works: Once inside an anaerobic or microaerophilic organism, metronidazole is reduced to a reactive nitro-radical that damages microbial DNA — a mechanism that aerobic human cells do not activate, which is why the drug spares normal flora that need oxygen.
Standard adult dose: 400 mg three times daily for 5–7 days for most bacterial indications, or 2 g as a single oral dose for uncomplicated trichomoniasis and BV.
Most important warning: Avoid alcohol during treatment and for at least 48 hours after the last dose — the disulfiram-like reaction can be severe.
Ordering on MedsBase: Available as Flagyl (brand, Sanofi), Metrogyl tablets (generic), Metrogyl Gel (vaginal), and Flagyl Suspension (liquid for paediatric or swallowing-difficulty patients). No prescription needed; worldwide shipping; WHO-GMP-certified manufacturers.

What is metronidazole?

Metronidazole is a small-molecule antibiotic in the nitroimidazole class. It was discovered at Rhône-Poulenc laboratories in the late 1950s as a treatment for trichomoniasis and reached the market in 1960 under the brand name Flagyl. More than six decades later it remains one of the most prescribed antibiotics in the world, with the World Health Organization listing it as an essential medicine because no comparably effective oral agent exists against several common pathogens.

Two properties set metronidazole apart from beta-lactams, macrolides, and fluoroquinolones:

It only works on anaerobes and protozoa. Aerobic bacteria like E. coli, Staphylococcus aureus, and Streptococcus pneumoniae simply do not activate the drug — making metronidazole one of the few antibiotics that preserves the aerobic gut and respiratory flora rather than wiping them out.
It crosses the blood–brain barrier and reaches abscess cavities. Tissue penetration is exceptional. That is why it is the backbone agent for brain abscesses, intra-abdominal sepsis, and deep dental infections.

The drug is well absorbed when taken by mouth (≥80% bioavailability), reaches peak plasma concentrations in 1–2 hours, and has a half-life of around 8 hours in adults with normal liver function. That pharmacokinetic profile supports the standard three-times-daily oral schedule.

How metronidazole kills bacteria and parasites

The molecule is a pro-drug — biologically inert when you swallow it. Inside an anaerobic or microaerophilic cell, the organism’s electron-transport chemistry reduces metronidazole’s nitro group to a short-lived nitro-radical anion. That radical reacts with bacterial or protozoal DNA, causing strand breaks and helix destabilisation. The result is rapid loss of cell viability, usually within hours of the drug reaching the infection site.

The reduction step requires the low redox potential found only in anaerobes and certain protozoa — that is the entire selectivity story. Human mitochondria operate at a much higher redox potential and do not activate the drug, which is why metronidazole has been so widely used so safely for so long.

What metronidazole treats (clinical indications)

Below is a clinician-style summary of where metronidazole is first- or second-line. Always work with a healthcare provider when treating yourself with an antibiotic — these summaries are for education, not prescribing.

Infection	Typical adult regimen	Where metronidazole sits
Bacterial vaginosis (BV)	400 mg PO twice daily × 7 days, or 2 g single dose, or 0.75% vaginal gel nightly × 5 nights	First-line (CDC, BASHH, WHO)
Trichomoniasis (T. vaginalis)	2 g PO single dose, or 500 mg twice daily × 7 days (CDC 2021 prefers the 7-day regimen in women)	First-line; partner must also be treated
Giardiasis	250 mg PO three times daily × 5–7 days	First-line (tinidazole and nitazoxanide are alternates)
Amoebiasis (invasive E. histolytica)	500–750 mg PO three times daily × 7–10 days, then a luminal agent (paromomycin or diloxanide)	First-line for tissue phase
Clostridioides difficile colitis (mild, first episode)	500 mg PO three times daily × 10–14 days (2017 IDSA now prefers vancomycin or fidaxomicin first-line; metronidazole reserved when those are unavailable)	Second-line
H. pylori eradication	Component of bismuth quadruple therapy (PPI + bismuth + tetracycline + metronidazole 500 mg QID × 10–14 days)	Backbone of quadruple therapy
Dental abscess / pericoronitis (with anaerobic involvement)	400 mg PO three times daily × 5 days, often added to amoxicillin	Adjunct to drainage
Intra-abdominal sepsis / peritonitis	500 mg IV every 8 hours, paired with an aerobic-coverage agent (cefotaxime, ciprofloxacin, etc.)	Standard anaerobic cover
Perioral / rosacea inflammation	0.75% or 1% topical gel/cream twice daily × 8–12 weeks	First-line topical
Crohn’s disease (perianal / fistulising)	10–20 mg/kg/day PO in divided doses, often combined with ciprofloxacin	Adjunct to biologic / immunomodulator therapy

Bacterial vaginosis — the most common reason patients search for this drug

BV is not a sexually transmitted infection in the classical sense; it is an overgrowth of anaerobes (Gardnerella vaginalis, Atopobium, Prevotella, Mobiluncus) that displaces protective Lactobacillus species. The hallmark symptoms are a thin grey-white discharge with a fishy odour that worsens after sex or during menses.

Three metronidazole-based regimens have nearly identical 7-day cure rates of 75–85% in published trials:

Oral tablets, 400 mg twice daily for 7 days — the most studied regimen; slightly higher cure rate than the single dose.
Single 2 g oral dose — convenient when adherence is a concern; relapse rates are higher.
Intravaginal 0.75% gel nightly for 5 nights — preferred when systemic side effects (nausea, taste disturbance) are intolerable. We carry this as Metrogyl Gel.

Recurrence is the major frustration with BV — up to 50% of women relapse within 12 months. For prevention strategy after a recurrence, see our guide on how to stop BV from coming back.

Trichomoniasis — partner treatment is non-negotiable

Trichomonas vaginalis is the most common non-viral sexually transmitted infection worldwide. The 2021 CDC STI Treatment Guidelines moved women from a single 2 g dose to 500 mg twice daily for 7 days, after a 2018 randomised trial (Kissinger et al., Lancet ID 2018) showed the 7-day regimen halved repeat infection. For men, the single 2 g dose remains acceptable. Sexual partners must be treated simultaneously regardless of symptoms, and patients should abstain from sex until both have completed therapy and resolved symptoms.

Dosing: a practical reference table

Doses below assume normal renal and hepatic function in adults. Reduce in severe liver disease; renal adjustment is generally not needed until creatinine clearance falls below 10 mL/min.

Research Spotlight: why “twice daily” became “three times daily” in many guidelines

The 8-hour half-life means trough concentrations dip below the minimum inhibitory concentration (MIC) for some anaerobic strains when dosed at 12-hour intervals. The 8-hour (TID) schedule maintains time-above-MIC above 70% across the dose interval, which translates into better cure rates for deep-tissue infections such as abscesses and Crohn’s-associated perianal disease. For uncomplicated BV and trich, twice-daily dosing remains effective because the urogenital tract is well perfused and the MIC is low.

Form-by-form dosing

Oral tablets (200 mg, 400 mg, 500 mg) — taken with food to reduce nausea; swallow whole with a full glass of water.
Oral suspension (200 mg/5 mL) — for paediatric use, elderly with swallowing difficulty, or patients with NG tubes. Shake well before each dose. Available as Flagyl Suspension.
Vaginal gel (0.75%) — one applicator-full (5 g) inserted at bedtime for 5 consecutive nights for BV.
Topical cream / gel (0.75%, 1%) — for rosacea and perioral dermatitis, applied twice daily after gentle cleansing.
IV infusion (500 mg in 100 mL) — hospital use only; given over 20–60 minutes every 8 hours.

Paediatric dosing

Weight-based: 7.5 mg/kg every 8 hours for most infections, with a maximum single dose of 500 mg. For trichomoniasis in adolescents, the 2 g single dose is acceptable. Always weight-dose children rather than using adult tablet strengths.

Side effects and what to do about them

Most patients tolerate metronidazole well. The side effects below are listed in approximate order of frequency.

Metallic taste in the mouth (10–20%) — usually mild, resolves after the course. Sugar-free mints or gum can mask it.
Nausea, mild abdominal cramping (5–15%) — taking each dose with food virtually eliminates this.
Dark or reddish-brown urine — harmless. Caused by a coloured metabolite excreted in the urine; not a sign of liver injury or blood.
Furred tongue, dry mouth (5%) — transient.
Headache, dizziness (3–5%) — usually mild; stop if severe or persistent.
Vaginal yeast (Candida) overgrowth — uncommon but possible because metronidazole spares aerobes, leaving the fungal niche unopposed if Lactobacillus is reduced.

Stop the medication and seek medical attention if you develop:

Neurological symptoms — tingling or numbness in hands/feet (peripheral neuropathy), unsteady gait (cerebellar toxicity), seizures, or confusion. These are rare and usually associated with prolonged or high-dose therapy, but they require immediate discontinuation.
Severe skin reaction — widespread rash, blistering, mucosal involvement (Stevens-Johnson syndrome / TEN are extremely rare but reported).
Signs of pancreatitis — severe upper-abdominal pain radiating to the back, with vomiting.
Encephalopathy — disorientation, slurred speech, ataxia. Resolves on stopping the drug but warrants urgent neurology assessment.

Drug interactions: the alcohol question and beyond

Alcohol — the disulfiram-like reaction

This is the interaction every patient asks about, and it is real. Metronidazole inhibits acetaldehyde dehydrogenase. If you drink alcohol while the drug is in your system, acetaldehyde accumulates and triggers facial flushing, tachycardia, severe nausea, vomiting, headache, and occasionally hypotension. Severity varies — some people experience mild flushing, others end up in the emergency department.

The safe rule: no alcohol during therapy and for at least 48 hours after the last dose (some sources say 72 hours). This includes mouthwash, cough syrups, and ethanol-containing herbal tinctures. Topical alcohol (hand sanitiser, perfume) is fine — only ingested alcohol triggers the reaction.

Warfarin and other anticoagulants

Metronidazole inhibits CYP2C9 and significantly increases the anticoagulant effect of warfarin. INR can rise within 3–5 days of starting therapy. If warfarin co-administration is unavoidable, the INR should be checked at day 3 and day 7, and the warfarin dose pre-emptively reduced by 25–50% by an experienced prescriber. DOACs (apixaban, rivaroxaban) are less affected but caution is still warranted.

Lithium

Metronidazole reduces renal lithium clearance and can precipitate lithium toxicity. Avoid the combination where possible or monitor lithium levels weekly.

Phenytoin, phenobarbital

These CYP-inducers can lower metronidazole levels, potentially reducing efficacy in serious infections. Higher doses may be needed.

5-fluorouracil (chemotherapy)

Metronidazole increases 5-FU exposure and toxicity. Concurrent use should be avoided.

Disulfiram (Antabuse)

The combination has been reported to cause acute psychosis and confusion. The two drugs should not be co-prescribed; allow at least two weeks between courses of one and the other.

Pregnancy, breastfeeding, and fertility

Pregnancy: Metronidazole crosses the placenta. The historical concern about first-trimester use has been largely allayed by multiple meta-analyses (Caro-Patón et al. Br J Clin Pharmacol 1997; Burtin et al. Am J Obstet Gynecol 1995) showing no increased risk of birth defects. CDC, RCOG, and WHO consider it acceptable in any trimester when the infection (especially BV in pregnancy, which carries preterm-delivery risk) warrants treatment. The 2 g single dose is usually avoided in pregnancy in favour of the 7-day regimen because the single dose produces higher peak levels in fetal serum.

Breastfeeding: Metronidazole enters breast milk in concentrations similar to maternal serum. After a 2 g single dose, some sources recommend interrupting breastfeeding for 12–24 hours and discarding expressed milk. With the standard 400 mg TID 7-day regimen, breastfeeding can continue, though watching the infant for diarrhoea or oral candidiasis is sensible.

Fertility: No human data suggest metronidazole impairs fertility in either sex.

Generic versus brand: bioequivalence and cost

Metronidazole has been off-patent since the 1970s. Flagyl (Sanofi-Aventis) is the original brand. Generic metronidazole — sold as Metrogyl, Metronide, Trichozole, and dozens of other names — contains the same active ingredient at the same strength, manufactured to bioequivalence standards. Independent pharmacokinetic studies have repeatedly confirmed that WHO-GMP-certified generic metronidazole achieves the same plasma concentration profile as brand Flagyl.

Why does this matter? The brand-versus-generic price gap on metronidazole is the largest in the antibiotic class — a 14-tablet course of Flagyl 400 mg can cost six to ten times the equivalent generic course. For an antibiotic where bioequivalence is well established and the indication usually short-course, choosing the generic is the right financial call for most patients.

Who is this guide for?

Patients prescribed metronidazole who want to understand the why and the how, not just swallow the tablet and hope.
People with recurrent BV who want a fuller picture of why this drug is first-line and what other tools exist.
Travellers or anyone considering a self-treatment “in case” stockpile for giardiasis or amoebiasis — useful background, but please do not self-prescribe without context.
Carers giving the suspension form to a child or older relative who cannot swallow tablets.

Comparing metronidazole to its closest alternatives

Tinidazole — the longer-acting cousin

Tinidazole shares metronidazole’s mechanism but has a much longer half-life (12–14 hours vs 8 hours), allowing single-dose or shorter regimens. For trichomoniasis, a single 2 g dose of tinidazole is non-inferior to metronidazole’s 7-day course, with somewhat better gastrointestinal tolerability and a less pronounced metallic taste. For BV, tinidazole 2 g daily for 2 days or 1 g daily for 5 days matches metronidazole’s cure rate. We carry tinidazole as Tinima and Tinvista. It is the right pick when adherence is a problem or when patients had a rough time with the metronidazole metallic-taste effect previously.

Clindamycin (oral or vaginal cream) — the BV alternative

Clindamycin 300 mg orally twice daily for 7 days, or 2% vaginal cream nightly for 7 nights, is the standard alternative for women who cannot tolerate metronidazole. Cure rates are comparable. Clindamycin avoids the alcohol-interaction problem but carries its own concern: a small but real risk of C. difficile-associated diarrhoea. It is generally the second-choice for that reason.

Ornidazole and secnidazole — the regional alternatives

Ornidazole (used widely in India, Europe, and parts of Asia) has an even longer half-life than tinidazole (~13 hours) and similar efficacy. Secnidazole has a half-life over 17 hours, allowing true single-dose therapy for BV (an approved indication in many countries). We stock ornidazole in combination products such as Cifran OZ (ciprofloxacin + ornidazole), used for mixed aerobic-anaerobic infections — particularly travellers’ diarrhoea with anaerobic involvement.

Doxycycline — for pelvic inflammatory disease and certain STI work-ups

Although doxycycline is not a metronidazole substitute, the two drugs are frequently paired in regimens for pelvic inflammatory disease (PID): ceftriaxone IM + doxycycline 100 mg PO BID for 14 days + metronidazole 500 mg PO BID for 14 days. We carry Doxycycline Capsules.

For a broader overview of choosing antibiotics by indication, see our evidence-backed best antibiotics guide and the ordering antibiotics online complete guide.

Storage and stability

Tablets: store at room temperature (15–25 °C), in the original blister, away from direct light and humidity. Shelf life is typically 3–5 years from manufacture.
Oral suspension: shelf-stable until first opened; once reconstituted, follow the label (usually 14 days, refrigerated).
Vaginal gel / topical cream: stored at room temperature; discard at the printed expiry.

Resistance — is metronidazole still working?

Resistance has been recognised but remains clinically manageable. Roughly 5% of Trichomonas vaginalis isolates in the US show some degree of metronidazole resistance — usually treatable with higher doses or a switch to tinidazole. H. pylori resistance to metronidazole has climbed above 30% in many regions, which is why bismuth quadruple therapy (rather than older triple therapy) has become the preferred eradication regimen. For BV, true antibiotic resistance is not the main driver of recurrence — biofilm formation is — which is why suppression strategies focus on disrupting biofilms rather than escalating antibiotic doses.

What to do if metronidazole isn’t working

If you finish a course and symptoms persist or recur within weeks:

Confirm the diagnosis. Recurrent vaginal symptoms after a single course of metronidazole may be a candida infection (yeast) opportunistically replacing the bacterial overgrowth — antibiotics won’t help, and antifungals are needed.
Check adherence and alcohol exposure. A surprising fraction of “treatment failures” turn out to be unfinished courses or doses missed during the work week.
Consider partner treatment for trichomoniasis or recurrent BV.
Switch class. Tinidazole or clindamycin for BV; high-dose tinidazole for resistant trich; vancomycin or fidaxomicin if treating C. difficile.
Ask about adjuncts. For recurrent BV, vaginal boric-acid suppositories followed by an extended metronidazole-gel maintenance schedule is the most evidence-supported recurrence-prevention strategy (Reichman et al., Sex Transm Dis 2009).

Frequently asked questions

How long does it take to start working?

Most patients with BV or trichomoniasis notice symptom improvement within 48–72 hours of the first dose. Discharge and odour typically clear by day 5 of a 7-day course. For deeper infections (dental abscess, intra-abdominal sepsis) source control — drainage or surgery — must accompany the antibiotic.

Can I take metronidazole with food?

Yes — and you should. Taking each dose with a meal dramatically reduces nausea without affecting absorption. The drug is not affected by dairy products, antacids, or supplements at any clinically relevant level.

What if I miss a dose?

Take it as soon as you remember, unless it is within 4 hours of your next scheduled dose. Do not double up. For BV and trich, missing a single dose rarely affects cure; for serious anaerobic infections, contact your prescriber.

Is metronidazole safe in pregnancy?

Modern obstetric guidelines (RCOG, ACOG, WHO) consider metronidazole acceptable when indicated in any trimester, with the 7-day oral regimen preferred over the 2 g single dose. Untreated BV in pregnancy itself carries preterm-delivery risk, so refusal of treatment is not “safer”.

Can men take metronidazole for trichomoniasis?

Yes — men are usually asymptomatic carriers and must be treated whenever a female partner tests positive, otherwise re-infection is near certain. The single 2 g dose is standard for men.

Does metronidazole treat a UTI?

No. Most UTIs are caused by aerobic gram-negative bacteria (E. coli, Klebsiella) that metronidazole does not cover. Nitrofurantoin, fosfomycin, or trimethoprim-sulfamethoxazole are first-line; see our best antibiotics guide for indication mapping.

Does metronidazole treat strep throat or a sinus infection?

No — both are caused by aerobic organisms (Streptococcus pyogenes, S. pneumoniae, H. influenzae). A beta-lactam (amoxicillin) is first-line.

Will metronidazole interact with my birth control pill?

Current evidence does not support a clinically meaningful interaction between metronidazole and combined oral contraceptives. The historical advice to use back-up contraception during any antibiotic course is not supported by data for metronidazole specifically. If in doubt, condoms during the course and for 7 days after is a low-cost safeguard.

Can metronidazole cause yeast infection?

It can. Because metronidazole leaves aerobes and yeast untouched while clearing anaerobic bacteria, the vaginal ecosystem can shift towards Candida overgrowth. Roughly 5–10% of women treated for BV develop a yeast infection within a month. An over-the-counter azole antifungal usually resolves it.

Why does my urine look dark after starting metronidazole?

A pigmented metabolite is excreted renally. It is harmless and resolves within a day of stopping the drug. It is not blood and not a sign of liver damage.

Can I drink kombucha or eat fermented foods on metronidazole?

Trace ethanol from fermented foods (sauerkraut, kombucha, kefir) is generally below the threshold for a disulfiram-like reaction. To be cautious, skip overtly alcoholic beverages and obvious ethanol sources during the course and for 48 hours after.

Can I take it with ibuprofen or paracetamol?

Yes — no clinically meaningful interaction with NSAIDs or paracetamol.

What’s the difference between Flagyl and Metrogyl?

Flagyl is the original brand from Sanofi-Aventis; Metrogyl is a generic version of metronidazole. Active ingredient, strength, and bioequivalence are the same; the price typically is not.

Ordering metronidazole on MedsBase

What you get with MedsBase

No prescription needed — order without uploading paperwork.
FDA-approved active ingredient from WHO-GMP-certified manufacturers (Cipla, J.B. Chemicals, Unique Pharmaceuticals, Sanofi).
Worldwide shipping with our Reshipment Assurance Policy — every order is covered.
Discreet billing — your card statement will show the regulated payment processor’s name, never MedsBase or any medication. See our credit card payment guide for details.

Choose the form that matches your indication:

Flagyl tablets — the original brand (Sanofi-Aventis), 200 mg and 400 mg strengths.
Metrogyl tablets — generic metronidazole, 200 mg and 400 mg. Same molecule, lower cost.
Metrogyl Gel 0.75% (vaginal) — for women who prefer intravaginal therapy or had GI side effects with the oral form.
Flagyl Suspension — 200 mg/5 mL liquid for children, elderly, or anyone with swallowing difficulty.

For broader anaerobic-cover alternatives: Tinima and Tinvista (tinidazole), and Cifran OZ (ciprofloxacin + ornidazole combo). Browse the full antibiotics catalogue for related options.

Men's Health

Erectile Dysfunction

Metronidazole (Flagyl): Mechanism, Uses, Dosing & Side Effects