Metronidazole (Flagyl): Uses, Warnings, and Key Facts

What is metronidazole?

Metronidazole is a drug used for the treatment of anaerobic bacterial infections and certain parasitic infections. The source also identifies the ingredient as metronidazole hydrochloride (HCl).

In the United States, brand names listed in the source include Flagyl, Flagyl IV, Protostat, and Metro.

What is metronidazole used for?

The original content lists these main uses:

• Urogenital trichomoniasis
• Intestinal and hepatic amebiasis
• Lambliasis (giardiasis)
• Anaerobic bacterial infections
• Prevention of postoperative infection by anaerobic bacteria
• Bacterial vaginosis

Exact use depends on the formulation, age group, and clinical situation. The official leaflet is the best source for the approved indication and dosing details.

How metronidazole works

Metronidazole is described as an anti-infective, antibacterial, and antiparasitic medicine. The source notes that it may act through interaction with DNA.

Dosage information from the source

The original content includes oral and intravenous dosing examples. Because dosing varies by age, indication, and formulation, the details below are presented only as a summary of the source content.

Oral administration

The source states that oral metronidazole should be taken during meals.

• Giardiasis: multiple age-based regimens are listed, including short-course and divided-dose options.
• Intestinal and hepatic amebiasis: age-based divided-dose regimens are listed for several pediatric groups and adults.
• Anaerobic infection: adult and pediatric regimens are listed, with different schedules by age.
• Urogenital trichomoniasis: single-dose and multi-day regimens are listed for adults, adolescents, and children.
• Bacterial vaginosis: adolescent regimens are listed.

Intravenous infusion

The source also lists intravenous use for anaerobic infections and for prevention of postoperative infection by anaerobic bacteria. It notes infusion over 30–60 minutes and provides age-based regimens.

Contraindications

The source lists hypersensitivity to imidazoles as a contraindication.

Warnings and precautions

The original content highlights several precautions that should be reviewed before use:

• Kidney failure: dose reduction may be needed when not on dialysis, with monitoring of serum metabolite levels.
• Hepatic encephalopathy: caution is advised.
• Severe central or peripheral nervous system disease: risk of neurological worsening is noted.
• Prolonged treatment: the source mentions blood count monitoring, especially leukocytes, and watching for neuropathies.
• Urine discoloration: metronidazole may darken the urine.
• Alcohol: the source warns of an antabuse-like effect and says to avoid alcohol.
• IV use: thrombophlebitis and superinfection by non-susceptible microorganisms are mentioned.
• Cockayne syndrome: the source reports severe hepatotoxicity and hepatic failure cases and advises close liver monitoring if used only when no alternative exists.

Liver and kidney considerations

Liver failure

The source advises caution in hepatic encephalopathy.

Renal insufficiency

The source recommends caution, dose reduction when not on dialysis, and monitoring of serum metabolite levels.

Drug interactions

The original content lists several interactions that may matter in practice:

• Disulfiram and alcohol: psychotic reactions are mentioned.
• Oral anticoagulants: metronidazole may reduce hepatic metabolism and increase effect; monitoring is noted in the source.
• Lithium: plasma levels may increase.
• Cyclosporine: levels may increase.
• Busulfan: severe toxicity is mentioned.
• Phenytoin, phenobarbital, cimetidine: the source notes changes in elimination and plasma levels.
• 5-fluorouracil: the source notes increased toxicity.
• IV anti-infectives: separate administration is advised to avoid chemical incompatibility.
• Laboratory tests: false negatives are mentioned for some blood tests, including ALT, AST, LDH, triglycerides, and glucose.

Pregnancy and breastfeeding

The source says metronidazole crosses the placental barrier and that available data are not enough to establish safety, so benefit and risk should be assessed.

It also states that metronidazole is excreted in human milk and that unnecessary use during lactation should be avoided.

Effects on driving and machinery

If confusion, dizziness, hallucinations, seizures, or eye disorders occur, the source advises avoiding driving or operating machinery.

Possible adverse reactions

The original content lists the following side effects and adverse reactions:

• Epigastric pain, nausea, vomiting, diarrhea, oral mucositis, taste disorder, anorexia
• Reversible pancreatitis
• Discoloration of the tongue or hairy tongue
• Angioedema and anaphylactic shock
• Peripheral sensory neuropathy, headache, convulsions, vertigo, encephalopathy, acute cerebellar syndrome, aseptic meningitis
• Psychotic disorder, confusion, hallucinations, depressive behavior
• Transient vision disorders, optic neuropathy, neuritis
• Agranulocytosis, neutropenia, thrombocytopenia
• Increased liver enzymes, cholestatic or mixed hepatitis, hepatocellular damage, hepatic failure
• Rash, itching, hot flashes, hives, pustular rashes, Stevens-Johnson syndrome, toxic epidermal necrolysis, fever

Safety reminder

Before using metronidazole, check the official medication guide for the exact indication, dose, duration, and warnings for the specific product you have. If you have liver disease, kidney disease, neurological symptoms, or take other medicines, a pharmacist or clinician should review the full interaction and safety profile.

Frequently asked questions

Source note

This article is based on the original content and on official reference resources linked below. For exact product-specific information, consult the FDA medication guide and the package insert for the medicine in question.