Flagyl 400mg

Flagyl 400mg is a branded metronidazole tablet manufactured by Sanofi. Metronidazole is a nitroimidazole antimicrobial active against anaerobic bacteria and several protozoan parasites. The 400 mg strength is the standard adult dose used in most short-course oral metronidazole regimens worldwide, including bacterial vaginosis, dental infections, intra-abdominal anaerobic infections, and Helicobacter pylori eradication protocols.

Metronidazole was first introduced in 1959 for trichomoniasis and has since become one of the most clinically essential antimicrobial molecules ever developed. It is on the WHO Model List of Essential Medicines. The drug works by entering microbial cells and being reduced into reactive nitro radicals that damage microbial DNA. The selectivity comes from the fact that this reduction happens efficiently only in low-oxygen environments, which is where anaerobes and certain protozoa live (PMID 25232137).

What 400 mg is used for

The 400 mg adult dose is used in several standard regimens:

• Bacterial vaginosis: 400 mg twice daily for 5 to 7 days, with cure rates of 70 to 80 percent.
• Dental abscess and odontogenic infections: 400 mg three times daily for 5 days alongside drainage and amoxicillin.
• H. pylori eradication: 400 mg twice daily as part of triple or quadruple therapy with a proton pump inhibitor and amoxicillin or clarithromycin.
• Anaerobic bacterial infections of the abdomen, pelvis, or skin: 400 mg three times daily.
• Giardiasis: 400mg three times daily for 5 days. Tinidazole single-dose regimens are now generally preferred but metronidazole remains effective. See our intestinal worm treatment guide for diagnosis and follow-up.
• Amoebic dysentery: 800 mg three times daily for 5 to 10 days.

How Flagyl 400 mg differs from other metronidazole brands

Metronidazole is the same active molecule across all brands, including Metrogyl 400 mg, Metrogyl 200 mg, and generic Flagyl alternatives. Flagyl is the original Sanofi brand. The choice between brands does not change the antimicrobial activity; it changes the price and the manufacturer. For a lower-dose option used in pediatric or step-down adult therapy, see Flagyl 200 mg.

Side effects

The most common adverse effects are metallic taste (20 to 30 percent of users), nausea (10 to 20 percent), and headache (5 to 10 percent). Dry mouth and a furry tongue sensation are common. Urine may turn a dark reddish-brown color due to harmless metabolite excretion. Vomiting, diarrhea, and abdominal cramps occur less commonly. Peripheral neuropathy is rare but is the main reason metronidazole courses are kept short and not repeated without clinician input. Prolonged use beyond 10 to 14 days raises the neuropathy risk.

The disulfiram-like reaction

Metronidazole inhibits aldehyde dehydrogenase. Combining the drug with alcohol can produce nausea, vomiting, flushing, headache, and tachycardia within minutes of drinking. The reaction is unpleasant but rarely dangerous, and the warning to avoid alcohol during treatment and for 48 hours after is standard. Some recent literature questions whether the reaction is as common or severe as originally believed, but the safe and conservative recommendation remains complete avoidance during a course.

Contraindications and interactions

Avoid in known hypersensitivity. Interacts with warfarin (potentiates anticoagulation, requires INR monitoring), lithium (raises lithium levels), 5-fluorouracil, and disulfiram (do not combine). Use with caution in significant hepatic impairment, where dose reduction may be needed. Generally avoided in the first trimester of pregnancy.

How to take

Take with food to minimize GI side effects. Complete the prescribed course even if symptoms resolve early, particularly for bacterial vaginosis and H. pylori regimens where incomplete courses promote resistance. Avoid alcohol during treatment and for 48 hours after the final dose.

For related antiparasitic options, see Niclosamide 500 mg for tapeworm infections or Biltree 600 mg (praziquantel) for schistosomiasis.

References

1. Lofmark S et al. Clin Infect Dis 2010. PMID 20067414.
2. Loke YK et al. BMC Med 2014. PMID 25232137.
3. Workowski KA et al. MMWR Recomm Rep 2021. CDC STI Treatment Guidelines.

Related Guides

• Flagyl Metronidazole: Complete Guide to Uses, Dosage and Side Effects
• Intestinal Worm Treatment: Complete Guide
• Niclosamide: Antiparasitic Guide (Complementary Option)