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Flagyl IV Side Effects

Generic name: metronidazole

Medically reviewed by Last updated on Aug 12, 2023.

Note: This document contains side effect information about metronidazole. Some dosage forms listed on this page may not apply to the brand name Flagyl IV.

Applies to metronidazole: oral capsule, oral powder for suspension, oral tablet. Other dosage forms:


Oral route (Tablet; Tablet, Extended Release)

Metronidazole has been shown to be carcinogenic in mice and rats. Unnecessary use of the drug should be avoided. Its use should be reserved only for conditions for which it is approved.

Oral route (Suspension)

Potential For CarcinogenicityMetronidazole has been shown to be carcinogenic in mice and rats. Avoid unnecessary use of metronidazole. Reserve metronidazole for use in the following indications: trichomoniasis, amebiasis and anaerobic bacterial infections.

Serious side effects of Flagyl IV

Along with its needed effects, metronidazole (the active ingredient contained in Flagyl IV) may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking metronidazole:

More common

Less common


Incidence not known

Other side effects of Flagyl IV

Some side effects of metronidazole may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

More common


Incidence not known

For Healthcare Professionals

Applies to metronidazole: compounding powder, intravenous powder for injection, intravenous solution, oral capsule, oral suspension, oral tablet, oral tablet extended release.


The most serious side effects reported were convulsive seizures, encephalopathy, aseptic meningitis, optic neuropathy, and peripheral neuropathy (characterized by numbness/paresthesia of an extremity).[Ref]

Nervous system

Very common (10% or more): Headache (up to 18%)

Common (1% to 10%): Dysgeusia/taste perversion (e.g., metallic taste, sharp unpleasant metallic taste, taste of alcoholic beverages modified), dizziness

Rare (0.01% to 0.1%): Encephalopathy (e.g., confusion, fever, headache, hallucinations, paralysis, light sensitivity, disturbances in sight and movement, stiff neck), aseptic meningitis, seizure, somnolence, peripheral neuropathy, ataxia

Very rare (less than 0.01%): Subacute cerebellar syndrome (e.g., ataxia, dysarthria, gait impaired, nystagmus, tremor), drowsiness, convulsions, incoordination

Frequency not reported: Dysarthria, hypoesthesia, paresthesia, peripheral sensory neuropathy, transient epileptiform seizures, hearing impaired/hearing loss (including sensorineural), tinnitus, taste disorders/unpleasant taste in mouth, nystagmus, convulsive seizure, numbness, syncope, vertigo, paralysis[Ref]


Very common (10% or more): Nausea (up to 10%)

Common (1% to 10%): Abdominal pain, diarrhea, dry mouth, stomatitis, glossitis

Rare (0.01% to 0.1%): Pancreatitis, upper abdominal pain/epigastralgia, vomiting

Frequency not reported: Constipation, tongue discoloration, oral mucositis, furry/furred tongue, gastrointestinal disturbances (e.g., epigastric pain/distress), proctitis, sudden overgrowth of Candida in mouth, abdominal cramping, dyspepsia, pseudomembranous colitis, mucositis[Ref]


Very common (10% or more): Vaginitis (up to 15%)

Common (1% to 10%): Genital pruritus, abnormal urine, dysmenorrhea, moniliasis, urinary tract infection

Rare (0.01% to 0.1%): Chromaturia

Very rare (less than 0.01%): Darkened urine

Frequency not reported: Vaginal dryness, vulva dryness, dysuria, vaginal candidiasis, dyspareunia, polyuria, incontinence, cystitis, pyuria, proliferation of Candida in vagina[Ref]


Common (1% to 10%): Bacterial infection, influenza-like symptoms

Uncommon (0.1% to 1%): Asthenia

Rare (0.01% to 0.1%): Mucosal inflammation, pyrexia/fever

Very rare (less than 0.01%): Flushing

Frequency not reported: Malaise, face edema, peripheral edema, chest pain, chills, overgrowth of Candida, disulfiram-like reaction, weakness, sensation of pelvic pressure[Ref]


Common (1% to 10%): Upper respiratory tract infection, rhinitis, sinusitis, pharyngitis

Frequency not reported: Dyspnea, nasal congestion, hiccup[Ref]


Common (1% to 10%): Myalgia

Very rare (less than 0.01%): Arthralgia

Frequency not reported: Muscle spasms, fleeting joint pain, stiff neck[Ref]


Uncommon (0.1% to 1%): Leukopenia

Rare (0.01% to 0.1%): Neutropenia, thrombocytopenia, pancytopenia, agranulocytosis

Frequency not reported: Eosinophilia, bone marrow aplasia, bone marrow depression, blood dyscrasia[Ref]


Rare (0.01% to 0.1%): Stevens-Johnson syndrome, toxic epidermal necrolysis, angioedema, erythema multiforme

Very rare (less than 0.01%): Skin rash, pustular eruptions, acute generalized exanthematous pustulosis, pruritus

Frequency not reported: Fixed drug eruption, swelling face, urticaria, hyperhidrosis, erythema, drug reaction with eosinophilia and systemic symptoms (DRESS), erythematosus rash, mild erythematous eruptions, pustulosis[Ref]


Rare (0.01% to 0.1%): Cholestatic hepatitis

Very rare (less than 0.01%): Increased liver enzymes (AST, ALT, alkaline phosphatase), cholestatic/mixed hepatitis, hepatocellular liver injury, jaundice, liver failure requiring liver transplant, abnormal liver function tests

Frequency not reported: Increased liver enzyme, hepatotoxicity/liver failure in patients with Cockayne syndrome, drug-induced hepatitis[Ref]


Rare (0.01% to 0.1%): Anaphylaxis, anaphylactic shock, Jarisch-Herxheimer reaction

Frequency not reported: Serum sickness-like reaction, immediate/delayed hypersensitivity reactions, anaphylactic reaction, hypersensitivity


Rare (0.01% to 0.1%): Hallucinations

Very rare (less than 0.01%): Confusion/confusional state, psychotic disorders

Frequency not reported: Depression, insomnia, depressed mood, decreased libido, irritability, psychosis, disorientation, psychotic reaction[Ref]


Rare (0.01% to 0.1%): Optic neuropathy, diplopia, myopia

Very rare (less than 0.01%): Vision disorders (e.g., diplopia, myopia)

Frequency not reported: Optic neuropathy/neuritis, saccadic eye movement, blurred vision, decreased visual acuity, changed color vision, light sensitivity[Ref]


Frequency not reported: Flattening of T-wave on ECG, tachycardia, palpitations, QT interval prolonged on ECG[Ref]


Frequency not reported: Anorexia, decreased appetite[Ref]


Frequency not reported: Breast cancer, colon cancer, gastrointestinal cancer, certain extraintestinal cancers[Ref]

An increased incidence of gastrointestinal and certain extraintestinal cancers have been reported in patients with Crohn's disease. Breast and colon cancer have been reported in medical literature in Crohn's disease patient treated with high doses of this drug for a prolonged duration; causality was not established.[Ref]


Frequency not reported: Injection site reaction, thrombophlebitis[Ref]

Frequently asked questions


1. (2002) "Product Information. Flagyl (metronidazole)." Searle

2. Cerner Multum, Inc. "UK Summary of Product Characteristics."

3. Cerner Multum, Inc. "Australian Product Information."

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.