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 tablet, oral tablet extended release

Get emergency medical help if you have any of these signs of an allergic reaction while taking metronidazole (the active ingredient contained in Flagyl ER) hives; difficulty breathing; swelling of your face, lips, tongue, or throat.

Call your doctor at once if you have any of these serious side effects:

• numbness or tingling in your hands or feet;

• white patches or sores inside your mouth or on your lips;

• pain or burning when you urinate;

• diarrhea that is watery or bloody;

• vision problems, pain behind your eyes;

• trouble concentrating, slurred speech, mood or behavior changes, tremors, muscle twitching, seizure (convulsions);

• fever, chills, muscle pain, confusion, headache, sore throat, neck stiffness, increased sensitivity to light, drowsiness, nausea and vomiting; or

• severe skin reaction -- fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain, followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling.

Less serious side effects of metronidazole may include:

• stomach pain, diarrhea;

• dizziness, loss of balance;

• vaginal itching or discharge;

• dry mouth or unpleasant metallic taste;

• cough, sneezing, runny or stuffy nose; or

• swollen or sore tongue.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects.

For Healthcare Professionals

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

General

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.

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 (the active ingredient contained in Flagyl ER) 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 (the active ingredient contained in Flagyl ER) 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 (the active ingredient contained in Flagyl ER) for extended periods of time.

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