Flagyl (Metronidazole): Uses, Dosage & Safety Information

Flagyl is the Pfizer brand of metronidazole, a nitroimidazole antibiotic that works against anaerobic bacteria — organisms that grow where there is no oxygen — and against several protozoa. It is not an antifungal, and it does nothing against ordinary aerobic bacteria.

The brand has narrowed. The 250 mg and 500 mg Flagyl tablets, approved on 18 July 1963, are discontinued, as is Flagyl ER 750 mg. What survives under the name is a 375 mg capsule and an intravenous ready-to-use infusion.² A prescription for metronidazole tablets today is filled with a generic.

Two things dominate what patients need to know about it, and both are on the label rather than in folklore. The first is the boxed warning above. The second is alcohol: oral metronidazole is associated with a disulfiram-like reaction — abdominal cramps, nausea, vomiting, headache and flushing — and the label directs stopping alcohol and products containing propylene glycol during treatment and for at least three days afterwards.¹

The register entries for the discontinued tablets carry a Federal Register determination that they were not withdrawn for reasons of safety or effectiveness. The brand shrank commercially; the medicine did not fall out of use.

No alcohol during the course, and none for at least three days afterwards. This includes products containing propylene glycol, which is present in some cough syrups, mouthwashes and prepared foods. The reaction is not an interaction to manage carefully; the label directs discontinuing alcohol outright.

For trichomoniasis in women the capsule regimen is 375 mg twice daily for seven consecutive days. A seven-day course is preferred because it protects the patient long enough for sexual contacts to be treated.¹

Where repeat courses are needed, the label recommends four to six weeks between them and reconfirmation of the organism by laboratory testing before starting again.

The course is finished even after symptoms settle, and dosing is at even intervals. Metronidazole is an antibiotic: stopping early leaves the least susceptible organisms behind.

Substances that must not be combined with it are listed once, in the interactions table below.

Most people who take a course notice nothing worse than a metallic taste and an upset stomach. The effects that change management are neurological, and they are uncommon.

Source: ³

The neurological effects are worth naming in advance precisely because they are reversible. Someone who recognises new unsteadiness or tingling as a possible drug effect stops and is assessed; someone who assumes it is unrelated continues the course.

Flagyl is licensed for three groups of infection: trichomoniasis, both symptomatic and asymptomatic; acute intestinal amoebiasis and amoebic liver abscess; and serious infections caused by susceptible anaerobic bacteria.¹

Trichomoniasis is treated only when the organism has been confirmed by laboratory testing rather than assumed from symptoms. It is a sexually transmitted infection, so asymptomatic partners are treated at the same time where the organism has been found — otherwise the treated person is reinfected.

In amoebic liver abscess the label is explicit that the drug does not remove the need for aspiration or drainage of pus. An antibiotic sterilises; it does not empty a cavity.

The anaerobic indications are the ones most people meet in hospital: intra-abdominal infection including peritonitis and abscess, pelvic inflammatory disease and gynaecological infection, lung abscess, bone and joint infection, endocarditis and central nervous system infection. In mixed infections, a second antibiotic covering the aerobic organisms is needed, and surgery is done where surgery is what the problem calls for.

Metronidazole is also the mainstay for bacterial vaginosis and related bacterial vaginitis and bacterial vulvovaginitis, and for pseudomembranous colitis caused by Clostridioides difficile. It has no activity against yeast, so thrush needs fluconazole or nystatin instead — a distinction worth holding, because vaginal discharge can come from either and the two are treated with opposite drugs.

Dental and gum infections such as periodontitis and dental abscess are the other common setting, again as part of combination therapy rather than alone.

Flagyl is prescription-only in the United States. Under the brand, only the 375 mg capsule and the ready-to-use infusion remain marketed; the 250 mg and 500 mg tablets and the extended-release 750 mg tablet are discontinued.²

The discontinued entries carry a Federal Register determination that they were not withdrawn for reasons of safety or effectiveness, so the boxed warning is not the reason the tablets left the market. Generic metronidazole in those strengths remains widely dispensed.⁴

The two rules that travel with the molecule regardless of which label is on the box are set out once each: alcohol is in Drug interactions with Flagyl, and the boxed warning and neurological cautions are in Critical warnings for Flagyl use.

Carcinogenicity in animals

The boxed warning states that metronidazole has been shown to be carcinogenic in mice and rats, that unnecessary use should be avoided, and that use should be reserved for the listed indications. It is a restriction on casual prescribing, not a finding in humans, and it is the reason a course is given for a confirmed infection rather than a suspected one.

Alcohol and disulfiram

Alcohol and propylene glycol are stopped during treatment and for at least three days after it. Metronidazole is not given to anyone who has taken disulfiram in the previous two weeks, because psychotic reactions have occurred with the combination.

Cockayne syndrome

Flagyl is contraindicated in patients with Cockayne syndrome. Severe irreversible hepatotoxicity and acute liver failure with fatal outcomes have been reported after starting metronidazole in these patients.

Nervous system effects

Encephalopathy and peripheral neuropathy, including optic neuropathy, have been reported. Encephalopathy has occurred with cerebellar toxicity — unsteadiness, dizziness and slurred speech — and with lesions visible on MRI. Central nervous system symptoms are generally reversible within days to weeks after the drug is stopped, which makes prompt recognition the thing that matters.¹

Pregnancy and trichomoniasis

In trichomoniasis, Flagyl is contraindicated during the first trimester of pregnancy.

Hypersensitivity

A prior reaction to metronidazole or another nitroimidazole is a contraindication.

Source: ¹

Doses for the anaerobic indications are set by the treating team and depend on the site and severity, which is why they are not reduced to a single line here.

The first two rows are the reason this section exists at all. Both involve alcohol or a drug that behaves like it, and both are stated as contraindications rather than cautions.¹

Source:

This table is the only place on this page where interacting substances are listed. The alcohol row is the one people most often discover the hard way, because it is easy to assume a single drink at the end of a course is outside the rule. The label sets the boundary three days after the last dose.

Evidence scope

What has been reported to the FDA

As of the FAERS data cut of 30 July 2026, the FDA Adverse Event Reporting System held 64,630 reports naming metronidazole, 54,322 of them coded serious — a serious-coded share of 84%.³

Who was this medicine studied in?

No FDA Drug Trials Snapshot exists for Flagyl. Snapshots are published only for new molecular entities approved from January 2015 onward; the original tablets were approved on 18 July 1963 and the surviving capsule on 3 May 1995.

The label reflects that age. It reports adverse effects as narrative categories rather than as rates against a comparator, and its most quantified statements concern regimens and intervals — seven days of treatment, four to six weeks between repeat courses, three days of alcohol avoidance after the last dose — rather than proportions of patients affected.

Who this is and is not appropriate for

Typically considered for: a person with laboratory-confirmed trichomoniasis, with intestinal amoebiasis or amoebic liver abscess, or with a serious anaerobic bacterial infection, who can abstain from alcohol for the course and three days beyond it.

Not appropriate for: anyone with Cockayne syndrome, where severe irreversible liver injury has followed the first doses; anyone who has taken disulfiram in the previous two weeks; anyone with a prior reaction to a nitroimidazole; and, for trichomoniasis, anyone in the first trimester of pregnancy.

Needs a conversation first: anyone taking warfarin, lithium, busulfan, phenytoin or phenobarbital; anyone with existing neuropathy, since the drug can add to it; and anyone whose infection has not been confirmed, because the boxed warning turns unnecessary use into a specific instruction rather than a general preference.

Where to go next

• Report a side effect: FDA MedWatch, or MHRA Yellow Card in the UK
• Full prescribing information: DailyMed
• Pregnancy-specific enquiries: MotherToBaby, or Bumps in the UK
• Discharge that turns out to be fungal rather than bacterial: our fluconazole page

1. FDA Prescribing Information — FLAGYL 375 (metronidazole) capsules, Pfizer. Boxed Warning; Indications and Usage; Contraindications, including Interaction with Alcohol, Psychotic Reaction with Disulfiram and Cockayne Syndrome; Warnings, Central and Peripheral Nervous System Effects; Dosage and Administration.
   Label revised 2026-05 · Accessed 2026-08-30
   Used for: carcinogenicity warning, licensed indications, alcohol and disulfiram contraindications, Cockayne syndrome, neurological effects, trichomoniasis regimen and repeat-course interval
   DailyMed SPL a2883ca1-5a9a-4259-9d80-46ab67274384
2. Drugs@FDA via openFDA — applications recorded under the brand name FLAGYL: NDA 012623 tablets 250 mg and 500 mg approved 1963-07-18, Discontinued with a Federal Register determination that they were not withdrawn for safety or effectiveness reasons; NDA 020334 capsule 375 mg approved 1995-05-03, Prescription; NDA 020868 Flagyl ER 750 mg approved 1997-11-26, Discontinued; NDA 018353 Flagyl I.V., Discontinued; NDA 018657 Flagyl I.V. RTU, Baxter Healthcare, Prescription.
   Accessed 2026-08-30
   Used for: approval dates, which forms remain marketed under the brand, the basis on which the others were discontinued
   Drugs@FDA, products.brand_name FLAGYL
3. FDA Adverse Event Reporting System via openFDA, query on metronidazole.
   Data cut 2026-07-30 · Accessed 2026-08-30
   Used for: report counts, serious-coded share, most frequently coded reaction terms
   openFDA FAERS
4. FDA National Drug Code Directory via openFDA, query on metronidazole.
   Accessed 2026-08-30
   Used for: which presentations are currently listed and their marketing categories, including generic tablets in the strengths the brand no longer supplies
   openFDA NDC Directory

This page is medical information, not medical advice. It describes a prescription antibiotic and does not replace assessment by a clinician who can confirm the infection, check your other medicines and decide whether this drug is the right one.