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Flagyl Injection Side Effects Center
Medical Editor: Charles Patrick Davis, MD, PhD
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). This drug is available as a generic termed metronidazole. These preparations are designed to treat bacterial vaginosis (mainly Flagyl ER), trichomonas, amebiasis and anaerobic bacterial infections. The most common side effects for these three preparations of Flagyl are nausea; occasionally abdominal cramps, vomiting, diarrhea or constipation, headache, and anorexia occur. Most clinicians should have their laboratories check to see if the antimicrobial agent is effective against the microbes, especially bacteria, it is directed against.
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. Uncomfortable side effects that may become serious are fevers, pain with urination and mouth sores; most serious side effects are paresthesias that may become permanent, encephalopathy, and seizures. 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 levels approach serum levels in breast milk. 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.
What is Prescribing information?
The FDA package insert formatted in easy-to-find categories for health professionals and clinicians.
Flagyl Injection FDA Prescribing 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.
Dermatologic: Erythematous rash and pruritus.
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.
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)
Additional Flagyl Injection Information
Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.
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