Description
Flagyl ER (metronidazole)
Flagyl ER is an extended-release formulation of metronidazole, a nitroimidazole antibiotic active against anaerobic bacteria and certain protozoa. The ER designation refers to modified-release tablets designed to maintain more consistent plasma levels than immediate-release forms. Metronidazole is used across a range of infections: trichomoniasis, bacterial vaginosis, anaerobic bacterial infections including intra-abdominal and pelvic infections, Clostridium difficile infection, intestinal amebiasis, and as part of combination therapy for Helicobacter pylori eradication.
Dosing
Dosing depends on the indication:
Trichomoniasis: 2 g as a single dose, or 500 mg twice daily for 7 days. Partners are treated simultaneously since the infection is sexually transmitted. Bacterial and anaerobic infections: typically 500–750 mg two to three times daily for 7 to 14 days depending on severity and site. C. difficile: 500 mg three times daily for 10 to 14 days. H. pylori eradication: used in combination regimens, not as monotherapy.
Alcohol
Do not consume alcohol during metronidazole treatment and for at least 48 to 72 hours after the final dose. The combination causes a disulfiram-like reaction: flushing, nausea, vomiting, headache, and rapid heart rate. This reaction can be severe. The warning includes alcohol in any form — drinks, sauces, medications containing ethanol.
Disulfiram interaction
Metronidazole combined with disulfiram can cause acute psychosis and confusional states. This combination is contraindicated.
Contraindications and cautions
Hypersensitivity to metronidazole or other nitroimidazoles. Severe hepatic impairment — metronidazole is extensively metabolized in the liver; impaired clearance raises the risk of accumulation and neurological toxicity. Renal impairment and older patients require caution with extended or high-dose regimens.
Drug interactions
Warfarin: metronidazole inhibits warfarin metabolism and significantly raises anticoagulant effect — INR monitoring and dose reduction are needed during co-administration. Phenytoin and phenobarbital: metronidazole affects plasma levels of both. Cimetidine slows metronidazole clearance. Lithium: metronidazole can raise lithium levels; monitoring is required.
Side effects
Nausea, vomiting, diarrhea, abdominal pain, and a persistent metallic or bitter taste are the most common. Neurological effects — dizziness, headache, drowsiness — occur less frequently. Prolonged or high-dose use carries risk of peripheral neuropathy and, rarely, central nervous system effects. Hypersensitivity reactions including rash, hives, and angioedema have been reported.
