Antibiotic — Anaerobic Coverage 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.

Metronidazole

Brand names: Flagyl

Used in: Acute Appendicitis

Metronidazole is an antimicrobial used in ENT practice for infections involving anaerobic bacteria, such as dental and deep neck-space infections, often in combination with an agent active against aerobes.

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

ENT context — the source label gives a single anaerobic-infection regimen and does not list ENT-specific indications or doses; do not extrapolate. 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; the label notes that infections of the bone and joint, lower respiratory tract, and endocardium may require longer treatment. Other indications given in the same label: amebiasis — acute intestinal amebiasis 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 two divided 1 g doses on the same day, or 250 mg three times daily for seven consecutive days. Severe hepatic impairment (Child-Pugh C): reduce the dose by 50%.

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. No paediatric ENT dose is stated. 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

Within susceptible anaerobic organisms metronidazole is reduced to reactive intermediates that damage microbial DNA, producing a bactericidal effect; it is also active against certain protozoa.

Prescribing in practice

Avoid alcohol during treatment and for a short period afterwards because of the risk of a disulfiram-like reaction (flushing, nausea, vomiting, tachycardia).
It potentiates the effect of warfarin and can raise lithium levels, and prolonged or repeated courses carry a risk of peripheral and central neuropathy.
Dosing should be reviewed in significant hepatic impairment, where clearance is reduced.

Monitoring

Monitor INR closely if the patient is on warfarin, and watch for neurological symptoms with prolonged courses.

Counselling the patient

Do not drink any alcohol while taking this and for a couple of days after finishing.
Complete the full course even if you feel better.
Report numbness, tingling in the hands or feet, or unsteadiness.

Evidence & guidelines

Metronidazole is the long-established standard agent for anaerobic infection, with its indications, interactions and cautions set out in the SPC and current prescribing references.

Reference: NICE NG84; ENT-UK Ludwig's Angina guidance; 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

Pathways