Flagyl Side Effects

Generic name: metronidazole

Medically reviewed by Drugs.com. Last updated on Sep 15, 2024.

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

For healthcare professionals

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

General adverse events

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]

Gastrointestinal

• 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]

Genitourinary

• 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]

Other

• 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]

Respiratory

• Common (1% to 10%): Upper respiratory tract infection, rhinitis, sinusitis, pharyngitis
• Frequency not reported: Dyspnea, nasal congestion, hiccup^[Ref]

Musculoskeletal

• Common (1% to 10%): Myalgia
• Very rare (less than 0.01%): Arthralgia
• Frequency not reported: Muscle spasms, fleeting joint pain, stiff neck^[Ref]

Hematologic

• 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]

Dermatologic

• 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]

Hepatic

• 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]

Hypersensitivity

• 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

Psychiatric

• 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]

Ocular

• 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]

Cardiovascular

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

Metabolic

• Frequency not reported: Anorexia, decreased appetite^[Ref]

Oncologic

• 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]

Local

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

See also:

Frequently asked questions

• Can Flagyl be used to treat diarrhea?
• Does Flagyl treat UTI infections?
• What can I do about the metallic taste of Flagyl?
• Does Flagyl make your urine dark?
• Does Flagyl cause diarrhea?
• What happens if you drink alcohol with metronidazole?
• How long does it take for metronidazole to work?
• What are the most common skin conditions? (with photos)
• Is metronidazole safe for dogs? Uses, dosage, side effects?

Further information

Flagyl side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Note: Medication side effects may be underreported. If you are experiencing side effects that are not listed, submit a report to the FDA by following this guide.

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