Flagyl Side Effects

Generic Name: metronidazole

Please note - some side effects for Flagyl may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

Side Effects of Flagyl - for the Consumer

Flagyl

All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using Flagyl:

Appetite loss; constipation; diarrhea; dizziness; headache; metallic taste; nausea; stomach upset; vomiting.

Seek medical attention right away if any of these SEVERE side effects occur when using Flagyl:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); bloody stools; decreased coordination; increased or decreased urination; numbness, tingling, or burning of the arms, hands, legs, or feet; red, swollen, blistered, or peeling skin; seizures; severe diarrhea; severe or persistent dizziness or headache; sore throat, chills, or fever; speech problems; stiff neck; stomach pain or cramps; vaginal itching, odor, or discharge; vision loss or other vision changes; white patches in the mouth.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.

Flagyl Capsules

All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using Flagyl Capsules:

Appetite loss; constipation; diarrhea; dizziness; headache; metallic taste; nausea; stomach upset; vomiting.

Seek medical attention right away if any of these SEVERE side effects occur when using Flagyl Capsules:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); bloody stools; decreased coordination; increased or decreased urination; numbness, tingling, or burning of the arms, hands, legs, or feet; red, swollen, blistered, or peeling skin; seizures; severe diarrhea; severe or persistent dizziness or headache; sore throat, chills, or fever; speech problems; stiff neck; stomach pain or cramps; vaginal itching, odor, or discharge; vision loss or other vision changes; white patches in the mouth.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.

Flagyl ER Extended-Release Tablets

All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome when using Flagyl ER Extended-Release Tablets:

Appetite loss; constipation; diarrhea; dizziness; headache; metallic taste; nausea; stomach upset; vomiting.

Seek medical attention right away if any of these SEVERE side effects occur when using Flagyl ER Extended-Release Tablets:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); bloody stools; decreased coordination; increased or decreased urination; numbness, tingling, or burning of the arms, hands, legs, or feet; red, swollen, blistered, or peeling skin; seizures; severe diarrhea; severe or persistent dizziness or headache; sore throat, chills, or fever; speech problems; stiff neck; stomach pain or cramps; vaginal itching, odor, or discharge; vision loss or other vision changes; white patches in the mouth.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. To report side effects to the appropriate agency, please read the Guide to Reporting Problems to FDA.

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Flagyl Side Effects - for the Professional

Flagyl

The most serious adverse reactions reported in patients treated with Flagyl (metronidazole) have been convulsive seizures, encephalopathy, aseptic meningitis, optic and peripheral neuropathy, the latter characterized mainly by numbness or paresthesia of an extremity. Since persistent peripheral neuropathy has been reported in some patients receiving prolonged administration of Flagyl, patients should be specifically warned about these reactions and should be told to stop the drug and report immediately to their physicians if any neurologic symptoms occur.

The most common adverse reactions reported have been referable to the gastrointestinal tract, particularly nausea reported by about 12% of patients, sometimes accompanied by headache, anorexia, and occasionally vomiting; diarrhea; epigastric distress; and abdominal cramping. Constipation has also been reported.

The following reactions have also been reported during treatment with Flagyl (metronidazole):

Mouth: A sharp, unpleasant metallic taste is not unusual. Furry tongue, glossitis, and stomatitis have occurred; these may be associated with a sudden overgrowth of Candida which may occur during therapy.

Hematopoietic: Reversible neutropenia (leukopenia); rarely, reversible thrombocytopenia.

Cardiovascular: Flattening of the T-wave may be seen in electrocardiographic tracings.

Central Nervous System: Encephalopathy, aseptic meningitis, convulsive seizures, optic neuropathy, peripheral neuropathy, dizziness, vertigo, incoordination, ataxia, confusion, dysarthria, irritability, depression, weakness, and insomnia.

Hypersensitivity: Urticaria, erythematous rash, Stevens-Johnson Syndrome, toxic epidermal necrolysis, flushing, nasal congestion, dryness of the mouth (or vagina or vulva), and fever.

Renal: Dysuria, cystitis, polyuria, incontinence, and a sense of pelvic pressure. Instances of darkened urine have been reported by approximately one patient in 100,000. Although the pigment which is probably responsible for this phenomenon has not been positively identified, it is almost certainly a metabolite of metronidazole and seems to have no clinical significance.

Other: Proliferation of Candida in the vagina, dyspareunia, decrease of libido, proctitis, and fleeting joint pains sometimes resembling "serum sickness." If patients receiving Flagyl drink alcoholic beverages, they may experience abdominal distress, nausea, vomiting, flushing, or headache. A modification of the taste of alcoholic beverages has also been reported. Rare cases of pancreatitis, which generally abated on withdrawal of the drug, have been reported.

Crohn's disease patients are known to have an increased incidence of gastrointestinal and certain extraintestinal cancers. There have been some reports in the medical literature of breast and colon cancer in Crohn's disease patients who have been treated with metronidazole at high doses for extended periods of time. A cause and effect relationship has not been established. Crohn's disease is not an approved indication for Flagyl.

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Side Effects by Body System - for Healthcare Professionals

General

The most serious side effects reported with metronidazole 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.

Nervous system

Nervous system side effects have included encephalopathy, aseptic meningitis, optic neuropathy, peripheral neuropathy, convulsive seizures, headache, dizziness, vertigo, syncope, incoordination, ataxia, confusion, dysarthria, weakness, and insomnia. Disorientation, agitation, hallucinations, and reversible abnormalities in the cerebellar dentate nuclei have been reported.

A rare but serious adverse effect of metronidazole is peripheral neuropathy which is thought to be related to dose and duration of therapy. Most cases occur after doses of 1000 to 2400 mg per day for at least 30 days, or a total dose of 50 grams. Neuropathy is generally located in the lower extremities and presents 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 are rare, but do 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.

Gastrointestinal

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.

Gastrointestinal side effects have been common and have included nausea (up to 12%), anorexia, dry mouth, a strange/unpleasant metallic taste, diarrhea, vomiting, abdominal discomfort, epigastric distress, abdominal cramping, constipation, proctitis, coating of the tongue (furry tongue), glossitis, and stomatitis. A modification of the taste of alcoholic beverages has been reported. Rarely, pseudomembranous colitis and pancreatitis have been reported.

Hypersensitivity

Hypersensitivity side effects have included urticaria, erythematous rash, Stevens-Johnson syndrome, toxic epidermal necrolysis, flushing, nasal congestion, dryness of the mouth (or vagina or vulva), and fever. A serum sickness-like reaction has been reported. Fixed drug eruptions have been reported and cross-sensitivity to other nitroimidazole derivatives has been demonstrated.

Metronidazole has rarely been associated with a serum sickness-like reaction which presents as arthralgias, malaise, fever, chills, pruritus, and rash.

Other

Other side effects have included fever, instances of darkened urine, and overgrowth of Candida. Metronidazole may produce a disulfiram-like reaction (abdominal distress, nausea, vomiting, flushing, or headache) when taken concurrently with ethanol.

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.

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.

Hematologic

Hematologic side effects have included reversible neutropenia (leukopenia) and reversible thrombocytopenia (rare).

Genitourinary

Genitourinary side effects have included dysuria, cystitis, polyuria, incontinence, vaginal candidiasis, vaginal itching, darkened urine, dyspareunia, and a sense of pelvic pressure.

Cardiovascular

Cardiovascular side effects have included flattening of the T-wave in electrocardiographic tracings.

Dermatologic

Dermatologic side effects have included erythematous rash and pruritus.

Local

Local side effects have included thrombophlebitis following intravenous infusion.

Thrombophlebitis can be minimized or avoided by avoiding prolonged use of indwelling intravenous catheters.

Psychiatric

Psychiatric side effects have included irritability, depression, and decreased libido.

Musculoskeletal

Musculoskeletal side effects have included fleeting joint pains sometimes resembling "serum sickness".

Oncologic

Oncologic side effects have included breast and colon cancer in Crohn's disease patients treated with high doses of metronidazole for extended periods of time.

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