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Flagyl ER 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 Patient Information in Detail?
Easy-to-read and understand detailed drug information and pill images for the patient or caregiver from Cerner Multum.
Flagyl ER in Detail - Patient Information: Side Effects
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.
Read the entire detailed patient monograph for Flagyl ER (Metronidazole Extended Release Tablets)
What is Patient Information Overview?
A concise overview of the drug for the patient or caregiver from First DataBank.
Flagyl ER Overview - Patient Information: Side Effects
This drug may cause urine to darken in color. This is harmless.
Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.
Tell your doctor immediately if any of these unlikely but serious side effects occur: unsteadiness, seizures, mental/mood changes (such as confusion), numbness/tingling of hands/feet, painful urination.
Tell your doctor immediately if any of these rare but very serious side effects occur: eye pain, severe/persistent headache, stiff/painful neck, sudden vision changes, sore throat, persistent fever, unusual bleeding/bruising, severe stomach pain, persistent nausea/vomiting.
Use of this medication for prolonged or repeated periods may result in oral thrush or a new vaginal yeast infection (oral or vaginal fungal infection). Contact your doctor if you notice white patches in your mouth, a change in vaginal discharge, or other new symptoms.
A very serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction may include: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.
This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.
In the US -
Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.
Read the entire patient information overview for Flagyl ER (Metronidazole Extended Release Tablets)
What is Prescribing information?
The FDA package insert formatted in easy-to-find categories for health professionals and clinicians.
Flagyl ER FDA Prescribing Information: Side Effects
In two multicenter clinical trials, a total of 270 patients received 750 mg FLAGYL ER tablets orally once daily for 7 days, and 287 were treated with a comparator agent administered intravaginally once daily for 7 days (See Clinical Studies).5,6
Most adverse events were described as being of mild or moderate severity. Among patients taking FLAGYL ER who reported headaches, 10% considered them severe, and less than 2% of reported episodes of nausea were considered severe. Metallic taste was reported by 9% of patients taking FLAGYL ER.
Adverse events reported at ≥ 2% incidence for either treatment group, irrespective of treatment causality, are summarized in the table below.
Adverse Events ( ≥ 2% Incidence Rate)-Irrespective
of Treatment Causality
|FLAGYL ER 7 days
|Headache||48 (18%)||44 (15%)|
|Vaginitis||39 (15%)||32 (12%)|
|Nausea||28 (10%)||8 (3%)|
|Taste Perversion (metallic taste)||23 (9%)||1 (0%)|
|Infection Bacterial||19 (7%)||17 (6%)|
|Influenza-like Symptoms||17 (6%)||20 (7%)|
|Pruritus Genital||14 (5%)||25 (9%)|
|Abdominal Pain||10 (4%)||13 (5%)|
|Dizziness||11 (4%)||3 (1%)|
|Diarrhea||11 (4%)||3 (1%)|
|Upper Respiratory Tract Infection||11 (4%)||10 (4%)|
|Rhinitis||12 (4%)||10 (4%)|
|Sinusitis||7 (3%)||6 (2%)|
|Urine Abnormal||7 (3%)||4 (1%)|
|Pharyngitis||8 (3%)||4 (1%)|
|Dysmenorrhea||9 (3%)||7 (2%)|
|Moniliasis||9 (3%)||8 (3%)|
|Mouth Dry||5 (2%)||2 (1%)|
|Urinary Tract Infection||6 (2%)||16 (6%)|
Vulvovaginal candidiasis is a recognized consequence of treatment with many anti-infective agents. In these multicenter clinical trials, there were no statistically significant differences in the incidence rates of yeast vaginitis for groups of patients treated with FLAGYL ER or the vaginal comparator.
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; 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.
Cardiovascular: Flattening of the T-wave may be seen in electrocardiographic tracings.
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 ER 750 mg tablets.
5. Integrated clinical and statistical report for the treatment of bacterial vaginosis with metronidazole modified release tablet- a dose duration study. G.D. Searle & Co., Protocol No. N13-95-02-015; Report No. N13-96-06-015, 19 Nov 1996.
6. Integrated clinical and statistical report for the treatment of bacterial vaginosis with metronidazole modified release tablet. G.D. Searle & Co., Protocol No. N13-95-02-017; Report No. N13-96-06-017, 11 Nov 1996.
Read the entire FDA prescribing information for Flagyl ER (Metronidazole Extended Release Tablets)
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