Nitroimidazole Antibiotic / Antiprotozoal Pregnancy: There are no adequate and well controlled studies of metronidazole in pregnant women. Metronidazole crosses the placental barrier and its effects on human fetal organogenesis are not known. Most published studies did not show an increased risk of congenital anomalies or other adverse fetal outcomes, though one study reported an increased risk of cleft lip with or without cleft palate that was not confirmed. Contraindicated for trichomoniasis in the first trimester; in pregnant patients for whom alternative treatment has been inadequate, the one-day course should not be used as it produces higher serum levels reaching the fetal circulation.

Metronidazole

Brand names: Flagyl, Metrolyl (IV)

Metronidazole is a nitroimidazole antimicrobial used to treat anaerobic bacterial infections and certain protozoal infections such as amoebiasis, giardiasis and trichomoniasis, and is a mainstay of Clostridioides difficile and intra-abdominal infection regimens.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Anaerobic bacterial infections: 7.5 mg/kg orally every six hours (approximately 500 mg for a 70-kg adult)

Route: Oral (tablets). In the treatment of most serious anaerobic infections, intravenous metronidazole is usually administered initially

Frequency: Every six hours

Max: 4 g should not be exceeded during a 24-hour period

General medicine context. US labelling (no eMC SPC was retrieved for this record — UK posology must be confirmed). Usual duration of therapy for anaerobic infection is 7 to 10 days; infections of the bone and joint, lower respiratory tract, and endocardium may require longer treatment. Other indications in the same label: amebiasis — acute intestinal amebiasis (acute amebic dysentery) 750 mg orally three times daily for 5 to 10 days; amebic liver abscess 500 mg or 750 mg orally three times daily for 5 to 10 days. Trichomoniasis (female) — 2 g as a single dose or as two divided 1 g doses on the same day, or 250 mg three times daily for seven consecutive days; treatment in the male should be individualised as for the female. Severe hepatic impairment (Child-Pugh C): reduce the dose by 50%. Elderly: decreased liver function can result in increased metronidazole concentrations that may necessitate dosage adjustment, and monitoring for metronidazole-associated adverse events is recommended.

Paediatric dose

Route: Oral

Frequency: Divided into three doses daily, for 10 days

US labelling gives a paediatric dose for amebiasis only, and as a range rather than a single per-kg value: 35 to 50 mg/kg/24 hours, divided into three doses, orally for 10 days. Safety and effectiveness in paediatric patients have not been established except for the treatment of amebiasis. Verify all paediatric dosing against a children's formulary before prescribing.

Dose adjustments

Renal

Haemodialysis removes significant amounts of metronidazole and its metabolites from the systemic circulation; clearance depends on the dialysis membrane, session duration and other factors. If administration cannot be separated from the haemodialysis session, consider supplementing the metronidazole dose after the session depending on the patient's clinical situation.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Prior history of hypersensitivity to metronidazole or other nitroimidazole derivatives
Trichomoniasis during the first trimester of pregnancy
Patients who have taken disulfiram within the last two weeks (psychotic reactions reported)
Alcohol or products containing propylene glycol during, and for at least three days after, therapy (disulfiram-like reaction)
Cockayne syndrome — severe irreversible hepatotoxicity/acute liver failure with fatal outcomes reported

Side effects

Nausea, sometimes accompanied by headache and anorexia; occasionally vomiting, diarrhoea, epigastric distress, abdominal cramping and constipation (most common)
A sharp, unpleasant metallic taste; furry tongue, glossitis and stomatitis (may be associated with Candida overgrowth)
Convulsive seizures, encephalopathy and aseptic meningitis (most serious reported reactions)
Optic and peripheral neuropathy — mainly numbness or paraesthesia of an extremity; persistent peripheral neuropathy reported with prolonged administration, stop the drug and report neurological symptoms immediately
Headache, syncope, dizziness, vertigo, incoordination, ataxia, tinnitus, hearing impairment or loss, confusion, dysarthria, irritability, depression, weakness and insomnia

Interactions

Disulfiram — psychotic reactions reported; do not give metronidazole to patients who have taken disulfiram within the last two weeks
Alcoholic beverages or products containing propylene glycol — abdominal cramps, nausea, vomiting, headaches and flushing may occur during or following therapy
Warfarin and other oral coumarin anticoagulants — anticoagulant effect potentiated with prolongation of prothrombin time; monitor prothrombin time and INR carefully
Lithium — short-term metronidazole therapy in patients on relatively high lithium doses has been associated with elevated serum lithium and, in a few cases, signs of lithium toxicity; check serum lithium and creatinine several days after starting
Busulfan — metronidazole has been reported to increase plasma concentrations of busulfan, increasing the risk of serious busulfan toxicity

Clinical monograph

How it works

It is reduced intracellularly by anaerobic organisms to reactive nitroso intermediates that cause DNA strand breakage and bacterial cell death, giving it selective activity against anaerobes and microaerophiles.

Prescribing in practice

Warn patients to avoid alcohol during treatment and for a short period afterwards because of a potential disulfiram-like reaction with flushing, nausea and vomiting.
It potentiates warfarin and other coumarin anticoagulants, so monitor INR closely when co-prescribed.
Prolonged or high-dose courses carry a risk of peripheral and central neuropathy, so avoid unnecessarily long treatment.

Monitoring

For short courses no routine monitoring is needed, but check for neurological symptoms during prolonged therapy and monitor INR in patients taking anticoagulants.

Counselling the patient

Do not drink alcohol while taking this medicine and avoid it for a couple of days after finishing the course.
Complete the full course even if you feel better.
Report any numbness, tingling or unsteadiness, which should be reviewed.

Evidence & guidelines

Metronidazole is established UK standard-of-care for anaerobic and protozoal infections and features in NICE and national antimicrobial guidance.

Reference: NICE NG199 (C. difficile); BASHH Bacterial Vaginosis Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Drugs

Metronidazole · Nitroimidazole antibiotic / antiprotozoal

Pathways