Flagyl ER Side Effects
Generic name: metronidazole
Note: This document contains side effect information about metronidazole. Some of the dosage forms listed on this page may not apply to the brand name Flagyl ER.
Some side effects of Flagyl ER may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.
For the Consumer
Applies to metronidazole: oral capsule, oral suspension, oral tablet, oral tablet extended release
Other dosage forms:
Along with its needed effects, metronidazole (the active ingredient contained in Flagyl ER) 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
- back pain
- blurred vision
- burning, numbness, tingling, or painful sensations in the hands or feet
- changes in speech patterns
- decreased vision
- eye pain
- lack of coordination
- mood or mental changes
- shakiness and unsteady walk
- slurred speech
- stiff neck or back
- trouble with speaking
- unsteadiness, trembling, or other problems with muscle control or coordination
- unusual tiredness or weakness
- weakness in the arms, hands, legs, or feet
- Black, tarry stools
- blood in the urine or stools
- body aches or pain
- clumsiness or unsteadiness
- difficulty with breathing
- ear congestion
- feeling of pelvic pressure
- frequent or painful urination
- loss of voice
- nasal congestion
- pinpoint red spots on the skin
- runny nose
- skin rash, hives, redness, or itching
- stomach and back pain (severe)
- unusual bleeding or bruising
- vaginal irritation, discharge, or dryness not present before taking the medicine
- Bleeding gums
- chest pain
- dark-colored urine
- fast heartbeat
- loss of appetite
- painful or difficult urination
- pains in the stomach, side, or abdomen, possibly radiating to the back
- shortness of breath
- sore throat
- sores, ulcers, or white spots on the lips or in the mouth
- swollen glands
- yellow eyes or skin
- Blistering, peeling, or loosening of the skin
- bloody or cloudy urine
- burning while urinating
- continuing diarrhea
- continuing stomach pain
- feeling of warmth
- increased volume of pale, dilute urine
- joint or muscle pain
- loss of bladder control
- red irritated eyes
- red skin lesions, often with a purple center
- redness of the face, neck, arms, and occasionally, upper chest
- redness of the skin
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
- Abdominal or stomach cramps
- dizziness or lightheadedness
- feeling of constant movement of self or surroundings
- sensation of spinning
- trouble sleeping
- unable to sleep
- weight loss
- Change in taste sensation
- dry mouth
- pain or tenderness around the eyes and cheekbones
- stuffy nose
- tender, swollen glands in the neck
- trouble with swallowing
- unpleasant or sharp metallic taste
- voice changes
- 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 tablet, oral tablet extended release
The most serious side effects reported with metronidazole (the active ingredient contained in Flagyl ER) have included convulsive seizures, encephalopathy, aseptic meningitis, optic and peripheral neuropathy (primarily characterized by numbness or paresthesia of an extremity). Persistent peripheral neuropathy has been reported with prolonged oral administration of metronidazole.
Headache (18%) and dizziness (4%) were reported in patients using the extended-release tablet formulation. Headaches were considered severe in 10% of these patients.
A rare but serious side effect of metronidazole (the active ingredient contained in Flagyl ER) was peripheral neuropathy which was thought to be related to dose and duration of therapy. Most cases occurred after doses of 1000 to 2400 mg per day for at least 30 days, or a total dose of 50 grams. Neuropathy was generally located in the lower extremities and presented with numbness, tingling, and paresthesias. Patients should be advised to discontinue metronidazole if they experience these effects. Neuropathy may be persistent despite discontinuation of metronidazole. Seizures associated with metronidazole were rare, but did occur.
Several cases of MRI abnormalities of the dentate nuclei associated with symptoms of ataxia and dysarthria have been reported. The symptoms and dentate nuclei lesions resolved several weeks after discontinuation of metronidazole. These symptoms and metronidazole-induced lesions in the brain stem and cerebellar nuclei have also been described in rat studies.
Very common (10% or more): Headache
Common (1% to 10%): Dizziness
Frequency not reported: Encephalopathy, aseptic meningitis, peripheral neuropathy (characterized mainly by numbness or paresthesia of an extremity), convulsive seizures, vertigo, syncope, incoordination, ataxia, confusion, dysarthria, weakness, reversible abnormalities in the cerebellar dentate nuclei
Very common (10% or more): Nausea
Common (1% to 10%): Taste perversion (metallic taste), abdominal pain, diarrhea, dry mouth
Rare (less than 0.1%): Pseudomembranous colitis, pancreatitis
Frequency not reported: Sharp unpleasant metallic taste, vomiting, abdominal discomfort, epigastric distress, abdominal cramping, constipation, proctitis, coating of the tongue (furry tongue), glossitis, stomatitis, taste of alcoholic beverages modified
Nausea (10%), taste perversion (metallic taste; 9%), abdominal pain (4%), diarrhea (4%), and dry mouth (2%) in patients using the extended-release tablet formulation. Nausea was considered severe in less than 2% of these patients.
Furry tongue, glossitis, and stomatitis may be associated with sudden overgrowth of Candida.
Although metronidazole has been used to treat pseudomembranous colitis, it has rarely also been implicated as a causative agent. These rare cases have been successfully treated with vancomycin.
Very common (10% or more): Vaginitis
Common (1% to 10%): Genital pruritus, abnormal urine, dysmenorrhea, urinary tract infection
Rare (less than 0.1%): Darkened urine
Frequency not reported: Dysuria, cystitis, polyuria, incontinence, vulvovaginal candidiasis, vaginal itching, dyspareunia, sense of pelvic pressure
Vaginitis (15%), genital pruritus (5%), abnormal urine (3%), dysmenorrhea (3%), and urinary tract infection (2%) were reported in patients using the extended-release tablet formulation.
Instances of darkened urine have been investigated. The pigment responsible has not been positively identified; however, it is most likely a metabolite of metronidazole and appears to have no clinical significance.
Common (1% to 10%): Bacterial infection, influenza-like symptoms, moniliasis
Frequency not reported: Fever, overgrowth of Candida, disulfiram-like reaction (abdominal distress, nausea, vomiting, flushing, hypotension, or headache) with ethanol
Bacterial infection (7%), influenza-like symptoms (6%), and moniliasis (3%) were reported in patients using the extended-release tablet formulation.
A disulfiram-like reaction has been reported in some patients drinking ethanol while taking metronidazole. Patients have experienced abdominal distress, vomiting, flushing, nausea, headache, and hypotension. Patients should be advised to refrain from all forms of ethanol while taking metronidazole and for at least 72 hours after the last dose.
Common (1% to 10%): Upper respiratory tract infection, rhinitis, sinusitis, pharyngitis
Upper respiratory tract infection (4%), rhinitis (4%), sinusitis (3%), and pharyngitis (3%) were reported in patients using the extended-release tablet formulation.
Metronidazole (the active ingredient contained in Flagyl ER) has rarely been associated with a serum sickness-like reaction which presented as arthralgias, malaise, fever, chills, pruritus, and rash.
Rare (less than 0.1%): Serum sickness-like reaction
Frequency not reported: Hypersensitivity (including urticaria, erythematous rash, Stevens-Johnson syndrome, toxic epidermal necrolysis, flushing, nasal congestion, dryness of the mouth [or vagina or vulva], fever), fixed drug eruptions, cross-sensitivity to other nitroimidazole derivatives
Rare (less than 0.1%): Reversible thrombocytopenia
Frequency not reported: Reversible neutropenia (leukopenia)
Frequency not reported: Flattening of the T-wave in ECG tracings
Frequency not reported: Erythematous rash, pruritus
Frequency not reported: Thrombophlebitis (after IV infusion)
Thrombophlebitis can be minimized or avoided by avoiding prolonged use of indwelling IV catheters.
Frequency not reported: Irritability, depression, decreased libido, insomnia, disorientation, agitation, hallucinations
Frequency not reported: Optic neuropathy
Frequency not reported: Anorexia
Frequency not reported: Fleeting joint pains (sometimes resembling "serum sickness")
Breast and colon cancer have been reported in Crohn's disease patients treated with high doses of metronidazole (the active ingredient contained in Flagyl ER) for extended periods of time.
Frequency not reported: Breast cancer, colon cancer
More Flagyl ER resources
- Flagyl ER Prescribing Information (FDA)
- Flagyl ER extended-release tablets MedFacts Consumer Leaflet (Wolters Kluwer)
- Flagyl ER Advanced Consumer (Micromedex) - Includes Dosage Information
- Flagyl Prescribing Information (FDA)
- Flagyl Monograph (AHFS DI)
- Flagyl Consumer Overview
- Flagyl MedFacts Consumer Leaflet (Wolters Kluwer)
- Metronidazole Professional Patient Advice (Wolters Kluwer)
- Metronidazole Prescribing Information (FDA)
- metronidazole Intravenous Advanced Consumer (Micromedex) - Includes Dosage Information
- metronidazole MedFacts Consumer Leaflet (Wolters Kluwer)
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