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

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

• intravenous solution

Serious side effects of Flagyl

Along with its needed effects, metronidazole (the active ingredient contained in Flagyl) 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

• Agitation
• back pain
• blindness
• blurred vision
• burning, numbness, tingling, or painful sensations in the hands or feet
• changes in speech patterns
• confusion
• decreased vision
• depression
• dizziness
• drowsiness
• eye pain
• fever
• headache
• irritability
• lack of coordination
• nausea
• seeing or hearing things that are not there
• seizures
• shakiness and unsteady walk
• slurred speech
• stiff neck or back
• trouble speaking
• unsteadiness, trembling, or other problems with muscle control or coordination
• unusual tiredness or weakness
• vomiting
• weakness in the arms, hands, legs, or feet

Less common

• Black, tarry stools
• blood in the urine
• body aches or pain
• burning while urinating
• chills
• clumsiness or unsteadiness
• difficulty with breathing
• ear congestion
• fainting
• feeling of pelvic pressure
• frequent or painful urination
• irregular heartbeat
• loss of voice
• nasal congestion
• pinpoint red spots on the skin
• runny nose
• skin rash, hives, redness, itching
• sneezing
• stomach and back pain (severe)
• unusual bleeding or bruising
• vaginal irritation, discharge, or dryness not present before taking the medicine

Rare

• Bleeding gums
• bloating
• chest pain
• constipation
• cough
• dark urine
• fast heartbeat
• indigestion
• loss of appetite
• painful or difficult urination
• pains in the stomach, side, or abdomen, possibly radiating to the back
• sore throat
• sores, ulcers, or white spots on the lips or in the mouth
• swollen glands
• yellow eyes or skin

Incidence not known

• Blistering, peeling, or loosening of the skin
• bloody or cloudy urine
• burning while urinating
• continuing diarrhea
• continuing stomach pain
• diarrhea
• feeling of warmth
• increased volume of pale, dilute urine
• joint or muscle pain
• loss of bladder control
• red skin lesions, often with a purple center
• red, irritated eyes
• redness of the face, neck, arms, and occasionally, upper chest
• redness of the skin

Other side effects of Flagyl

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

• Dizziness or lightheadedness
• feeling of constant movement of self or surroundings
• heartburn
• sensation of spinning
• stomach cramps
• trouble sleeping
• weight loss

Rare

• Change in taste sensation
• congestion
• dry mouth
• pain or tenderness around the eyes and cheekbones
• tender, swollen glands in the neck
• trouble with swallowing
• unpleasant or sharp metallic taste
• voice changes

Incidence not known

• Decreased interest in sexual intercourse
• inability to have or keep an erection
• loss in sexual ability, desire, drive, or performance
• painful sexual intercourse

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

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

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: Breast cancer, colon cancer, gastrointestinal cancer, certain extraintestinal cancers^[Ref]

Local

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