METRONIDAZOLE (Flagyl): What is used for?

What is Metronidazole?

Metronidazole is a drug used for the treatment of anaerobic bacterial infections, urogenital trichomoniasis, intestinal and hepatic amebiasis and lambliasis (giardiasis). The product ingredient is Metronidazole hydrochloride (hcl).

The brand names of Metronidazole in the United States are: Flagyl, Flagyl IV, Protostat, Metro.

Metronidazole Mechanism of Action (MOA)

Anti-infective, antibacterial and antiparasitic, possibly by interaction with DNA.

Therapeutic indications, uses, benefits and Dosage of Metronidazole

Oral dose (during the meal)

Lambliasis (Giardiasis). Patients > 10 years: 2 g/day, 3 days or 400 mg 3 times/day, 5 days or 500 mg 2 times/day, 7-10 days; 7-10 years: 1 g/day, 3 days; 3-7 years: 600-800 mg/day, 3 days; 1-3 years: 500 mg/day, 3 days. Expressed in c.p.: 15-40 mg/kg/day in 2-3 doses.

Intestinal and hepatic amebiasis. Patients > 10 years: 400-800 mg 3 times/day; 7-10 years: 200-400 mg 3 times/day; 3-7 years: 100-200 mg 4 times/day; 1-3 years: 100-200 mg 3 times/day. Duration of the tto. Duration: 5-10 days. Expressed in c.p.: 35-50 mg/kg/day in 3 doses, not exceeding 2.4 g/day.

Anaerobic infection. Adults: 500 mg/8 h. Children > 8 weeks – 12 years: 20-30 mg/kg/day as a single dose or 7.5 mg/kg/8 h, 7 days. Children < 8 weeks: 15 mg/kg/day as a single dose or 7.5 mg/kg/12 h.

Urogenital trichomoniasis. Adults and adolescents: 2 g as a single dose or 200 mg 3 times/day, 7 days or 400 mg 2 times/day, 5-7 days. Children < 10 years: 40 mg/kg as a single dose or 15-30 mg/kg/day in 2-3 doses for 7 days. Max. 2 g/day.

– Bacterial vaginosis. Adolescents: 400 mg 2 times/day for 5-7 days or 2 g as a single dose.

Intravenous Perfusion (30-60 min)

Infection by anaerobic bacteria: Bacterioids, Fusobacteria, Eubacteria, Clostridium and anaerobic Streptococci.
Adults and children > 12 years: 500 mg/8 h, max. 7 days.
Children > 8 weeks-12 years: 20-30 mg/kg/day as a single dose or 7.5 mg/kg/8 h, 7 days.
Children < 8 weeks: 15 mg/kg/day as a single dose or 7.5 mg/kg/12 h.

Prevention of postoperative infection by anaerobic bacteria, especially Bacteroids and anaerobic Streptococci.
Adults and children > 12 years: 500 mg preoperatively or 1.5 g in a dose before, during or after surgery. Colorectal surgery: 500 mg/8 h for 24 h, starting before surgery, extending 3-5 days if there is perforation of hollow viscera or gangrenous appendicitis.
Children > 8 weeks – 12 years: 20-30 mg/kg as a single dose 1-2 h before surgery.
Newborns < 40 weeks: 10 mg/kg as a single dose before surgery.

Mode of administration of Metronidazole

It should be taken orally during meals.

Contraindications of Metronidazole

Hypersensitivity to imidazoles.

Warnings and Precautions with Metronidazole

Kidney failure – reduce doses when not on dialysis and monitor serum metabolite levels.

Hepatic encephalopathy; severe acute or chronic CNS or SNP disease; risk of neurological worsening.

If necessary, lengthen the cough, assess benefit/risk, perform hematological determinations, especially leukocyte count, and monitor for risk of central or peripheral neuropathies.

It darkens the urine; antabús effect with alcohol, to avoid; to evaluate use in prolonged treatmetnt.

Hepatic failure in combination with other antibiotics; risk of thrombophlebitis (via IV) and superinfections by non-susceptible microorganisms.

Notified cases of severe hepatotoxicity and hepatic insufficiency (including a patient with a fatal outcome), in patients with Cockayne syndrome: do not administer unless there is no therapeutic alternative available (if finally administered, perform close monitoring of liver function and if any liver alteration is detected, discontinue treatment).

Liver failure

Caution in hepatic encephalopathy.

Renal insufficiency

Caution. It is recommended to reduce doses, when they are not under dialysis, and to monitor serum metabolite levels.

Interactions with Metronidazole

  • Psychotic reaction with: disulfiram (interrupt 2 weeks before treatment), alcohol.
  • Reduces hepatic metabolism and potency effect of: oral anticoagulants (adjust dose and monitor prothrombin time).
  • Increases plasma level of: lithium (monitor lithium level, creatinine and electrolytes); cyclosporine (monitor creatinine and plasma level); busulfan (severe toxicity).
  • Increased elimination and decreased plasma level by: phenytoin, phenobarbital, cimetidine.
  • Reduces lightening and increases toxicity: 5-Fluorouracil.
  • Intravenous: simultaneous but separate administration of other anti-infectives to avoid chemical incompatibility.
  • Lab: false negatives in blood of ALT, AST, LDH, triglycerides, glucose.

Pregnancy and Metronidazole

It crosses the placental barrier, no data is available to establish its security. Assess benefit/risk.


Metronidazole is excreted in human milk and therefore unnecessary administration during lactation should be avoided.

Effects on driving ability

If any of the following symptoms appear, refrain from driving or operating machinery: confusion, dizziness, hallucinations, seizures, or eye disorders.

Adverse reactions and side effects of Metronidazole

Epigastric pain, nausea, vomiting, diarrhea, oral mucositis, taste disorder, anorexia, pancreatitis (reversible), discoloration of the tongue/hairy tongue.

Angioedema, anaphylactic shock; peripheral sensory neuropathy, headaches, convulsions, vertigo, encephalopathy, acute cerebellar syndrome, aseptic meningitis.

Psychotic disorder, confusion, hallucination, depressive behavior; transient vision disorders, optic neuropathy, neuritis.
Agranulocytosis, neutropenia, thrombocytopenia; increased hepatic enzymes (AST, ALT, alkaline phosphatase), cholestatic or mixed hepatitis and hepatocellular damage, hepatic failure.

Rash, itching, hot flashes, hives, pustular rashes, Stevens-Johnson syndrome, toxic epidermal necrolysis; fever.


Source: The content of this active ingredient has been written taking into account the clinical and molecular information of all medicines authorised and marketed in the United States under the Unique Ingredient Identifier (UNII) by the Substance Registration System (SRS) of the Food and Drug Administration (FDA) and the United States Pharmacopeia (USP).

In order to know in detail the information authorized by the FDA for each drug, you should consult the corresponding medication guide authorized by the FDA.


UNII: 140QMO216E
PubChem CID: 4173
NCI Thesaurus: C651

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