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Flagyl

Last reviewed on RxList: 3/14/2017
Flagyl Side Effects Center

Medical Editor: John P. Cunha, DO, FACOEP

Last reviewed on RxList 03/14/2017

Flagyl, Flagyl ER, and Flagyl Injection (metronidazole) are antimicrobial drugs used to treat bacterial vaginosis, trichomonas, amebiasis, and anaerobic bacterial infections. Flagyl is available in generic form. Common side effects of Flagyl, Flagyl ER, and Flagyl Injection include:

  • nausea,
  • abdominal cramps,
  • stomach upset,
  • vomiting,
  • diarrhea,
  • constipation,
  • headache,
  • weight loss (anorexia),
  • dizziness,
  • dry mouth,
  • dark-colored urine, or
  • a metallic taste in the mouth or changes in taste.

Uncomfortable side effects that may become serious are:

  • fevers,
  • pain with urination,
  • mouth sores,
  • tingling or pricking sensations that may become permanent,
  • brain disease, and
  • seizures.

Serious but unlikely side effects of Flagyl include:

Flagyl is supplied in 250 and 500 mg strength tablets, Flagyl ER is available in 750 mg strength tablets, and Flagyl Injection in 500 mg strength in a buffered 100 ml vial. Dosage is quite variable and dependent upon the severity of disease and other considerations made by the treating physician. Most of the serious side effects may occur with any of these three preparations of Flagyl. Flagyl may cause liver enzyme levels to increase; lithium and creatinine levels should be checked to avoid lithium toxicity or renal compromise. Alcohol may increase the side effects of Flagyl. Patients on disulfiram should not take Flagyl until they have had a two week interval without taking disulfiram, especially alcoholic patients, to avoid psychotic reactions. Treatment in pregnant women (during first 3 months is not advised) or women who are breastfeeding should only be done if the benefits outweigh the potential problems. Flagyl passes into breast milk and may harm a nursing baby. Consult your doctor before breastfeeding. Except for use in amebiasis, studies in pediatric patients are not available; Flagyl ER has no pediatric studies.

Our Flagyl, Flagyl ER and Flagyl Injection Side Effects Drug Centers provide a comprehensive view of available drug information on the potential side effects when taking this medication.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Flagyl Consumer Information

Get emergency medical help if you have any of these signs of an allergic reaction: 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 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. You may report side effects to FDA at 1-800-FDA-1088.

Read the entire detailed patient monograph for Flagyl (Metronidazole)

Flagyl Professional Information

SIDE EFFECTS

The following reactions have been reported during treatment with metronidazole:

Central Nervous System

The most serious adverse reactions reported in patients treated with 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 metronidazole, 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. In addition, patients have reported headache, syncope, dizziness, vertigo, incoordination, ataxia, confusion, dysarthria, irritability, depression, weakness, and insomnia (see WARNINGS).

Gastrointestinal

The most common adverse reactions reported have been referable to the gastrointestinal tract, particularly nausea, sometimes accompanied by headache, anorexia, and occasionally vomiting; diarrhea; epigastric distress; and abdominal cramping and constipation.

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.

Dermatologic

Erythematous rash and pruritus.

Hematopoietic

Reversible neutropenia (leukopenia); rarely, reversible thrombocytopenia.

Cardiovascular

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

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.” Rare cases of pancreatitis, which generally abated on withdrawal of the drug, have been reported.

Patients with Crohn's disease 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 tablets.

Read the entire FDA prescribing information for Flagyl (Metronidazole)

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