Flagyl

Last updated on RxList: 3/22/2021
Flagyl Side Effects Center

Medical Editor: John P. Cunha, DO, FACOEP

What Is Flagyl

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.

What Are Side Effects of Flagyl

Common side effects of Flagyl, Flagyl ER, and Flagyl Injection include:

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:

Dosage for Flagyl

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.

What Drugs, Substances, or Supplements Interact with Flagyl

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.

Flagyl and Pregnancy

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.

Additional Information

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.

QUESTION

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Flagyl Consumer Information

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Get emergency medical help if you have signs of an allergic reaction (hives, itching, warmth or tingling; fever, joint pain; dry mouth, dry vagina; stuffy nose, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning eyes, skin pain, red or purple skin rash with blistering and peeling).

Call your doctor at once if you have:

  • new or worsening symptoms of infection;
  • painful or difficult urination;
  • confusion;
  • a light-headed feeling (like you might pass out); or
  • blisters or ulcers in your mouth, red or swollen gums, trouble swallowing.

Stop taking the medicine and call your doctor right away if you have neurologic side effects (more likely to occur while taking metronidazole long term):

  • numbness, tingling, or burning pain in your hands or feet;
  • vision problems, pain behind your eyes, seeing flashes of light;
  • muscle weakness, problems with speech or coordination;
  • trouble speaking or understanding what is said to you;
  • a seizure; or
  • fever, neck stiffness, and increased sensitivity to light.

Metronidazole can cause life-threatening liver problems in people with Cockayne syndrome. If you have this condition, stop taking metronidazole and contact your doctor if you have signs of liver failure--nausea, stomach pain (upper right side), dark urine, clay-colored stools, or jaundice (yellowing of the skin or eyes).

Side effects may be more likely in older adults.

Common side effects may include:

  • depression, trouble sleeping, feeling irritable;
  • headache, dizziness, weakness;
  • nausea, vomiting, loss of appetite, stomach pain;
  • diarrhea, constipation;
  • unpleasant metallic taste;
  • rash, itching;
  • vaginal itching or discharge, pain during sex;
  • mouth sores; or
  • swollen, red, or "hairy" 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)

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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: QT prolongation has been reported, particularly when metronidazole was administered with drugs with the potential for prolonging the QT interval. 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.

DRUG INTERACTIONS

Disulfiram

Psychotic reactions have been reported in alcoholic patients who are using metronidazole and disulfiram concurrently. Metronidazole should not be given to patients who have taken disulfiram within the last two weeks (see CONTRAINDICATIONS).

Alcoholic Beverages

Abdominal cramps, nausea, vomiting, headaches, and flushing may occur if alcoholic beverages or products containing propylene glycol are consumed during or following metronidazole therapy (see CONTRAINDICATIONS).

Warfarin And Other Oral Anticoagulants

Metronidazole has been reported to potentiate the anticoagulant effect of warfarin and other oral coumarin anticoagulants, resulting in a prolongation of prothrombin time.

When FLAGYL is prescribed for patients on this type of anticoagulant therapy, prothrombin time and INR should be carefully monitored.

Lithium

In patients stabilized on relatively high doses of lithium, short-term metronidazole therapy has been associated with elevation of serum lithium and, in a few cases, signs of lithium toxicity. Serum lithium and serum creatinine levels should be obtained several days after beginning metronidazole to detect any increase that may precede clinical symptoms of lithium intoxication.

Busulfan

Metronidazole has been reported to increase plasma concentrations of busulfan, which can result in an increased risk for serious busulfan toxicity. Metronidazole should not be administered concomitantly with busulfan unless the benefit outweighs the risk. If no therapeutic alternatives to metronidazole are available, and concomitant administration with busulfan is medically needed, frequent monitoring of busulfan plasma concentration should be performed and the busulfan dose should be adjusted accordingly.

Drugs That Inhibit CYP450 Enzymes

The simultaneous administration of drugs that decrease microsomal liver enzyme activity, such as cimetidine, may prolong the half-life and decrease plasma clearance of metronidazole.

Drugs That Induce CYP450 Enzymes

The simultaneous administration of drugs that induce microsomal liver enzymes, such as phenytoin or phenobarbital, may accelerate the elimination of metronidazole, resulting in reduced plasma levels; impaired clearance of phenytoin has also been reported.

Drugs That Prolong The QT Interval

QT prolongation has been reported, particularly when metronidazole was administered with drugs with the potential for prolonging the QT interval.

Read the entire FDA prescribing information for Flagyl (Metronidazole)

© Flagyl Patient Information is supplied by Cerner Multum, Inc. and Flagyl Consumer information is supplied by First Databank, Inc., used under license and subject to their respective copyrights.

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