Flagyl Injection Side Effects Center

Last updated on RxList: 6/2/2021
Flagyl Injection Side Effects Center

Medical Editor: John P. Cunha, DO, FACOEP

What Is Flagyl Injection?

Flagyl, Flagyl ER and Flagyl Injection (metronidazole) are all almost identical antimicrobial drugs that differ mainly in design for use; oral (Flagyl,) oral extended release (Flagyl ER), or intravenous (Flagyl injection) are used to treat bacterial vaginosis (mainly Flagyl ER), trichomonas, amebiasis and anaerobic bacterial infections. Flagyl is available as a generic termed metronidazole.

What Are Side Effects of Flagyl Injection?

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

Uncomfortable side effects of Flagyl, Flagyl ER and Flagyl Injection that may become serious are:

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

Dosage for Flagyl Injection

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.

What Drugs, Substances, or Supplements Interact with Flagyl Injection?

Flagyl may interact with anticoagulants, phenytoin, phenobarbital, cimetidine, alcohol, and disulfiram. Tell your doctor all medications and supplements you use.

Flagyl Injection During Pregnancy or Breastfeeding

Tell your doctor if you are pregnant or plan to become pregnant before using Flagyl; it is unknown if it will harm a fetus. Flagyl passes into breast milk. Breastfeeding while using Flagyl is not recommended.

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.

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Flagyl Injection Consumer Information

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);
  • vaginal itching or discharge; 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.

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Flagyl Injection Professional Information

SIDE EFFECTS

The most serious adverse reactions reported in patients treated with metronidazole injection 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 oral administration of metronidazole, patients should be observed carefully if neurologic symptoms occur and a prompt evaluation made of the benefit/risk ratio of the continuation of therapy.

The following reactions have also been reported during treatment with Metronidazole Injection, USP RTU.

Gastrointestinal: Nausea, vomiting, abdominal discomfort, diarrhea and an unpleasant metallic taste.

Hematopoietic: Reversible neutropenia (leukopenia).

Dermatologic: Erythematous rash and pruritus.

Central Nervous System: Encephalopathy, aseptic meningitis, optic neuropathy, headache, dizziness, syncope, ataxia, confusion and dysarthria.

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

Local Reactions: Thrombophlebitis after intravenous infusion. This reaction can be minimized or avoided by avoiding prolonged use of indwelling intravenous catheters.

Other: Fever. Instances of a darkened urine have also been reported, and this manifestation has been the subject of a special investigation. 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.

The following adverse reactions have been reported during treatment with oral metronidazole:

Gastrointestinal: Nausea, sometimes accompanied by headache, anorexia and occasionally vomiting; diarrhea, epigastric distress, 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 effective 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, flushing, nasal congestion, dryness of the mouth (or vagina or vulva) and fever.

Renal: Dysuria, cystitis, polyuria, incontinence, a sense of pelvic pressure and darkened urine.

Other: Proliferation of Candida in the vagina, dyspareunia, decrease of libido, proctitis and fleeting joint pains sometimes resembling “serum sickness.” If patients receiving metronidazole 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 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 Metronidazole Injection, USP RTU.

Read the entire FDA prescribing information for Flagyl Injection (Metronidazole Injection)

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

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