Antibiotic (Nitroimidazole) 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

Metronidazole is a nitroimidazole antimicrobial active against anaerobic bacteria and certain protozoa, widely used for anaerobic infections, Clostridioides difficile infection, dental and intra-abdominal infections, and protozoal infections such as giardiasis and trichomoniasis.

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

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: 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) — one-day treatment: 2 g given either as a single dose or as two divided doses of 1 g each on the same day; or a seven-day course: 250 mg three times daily for seven consecutive days; treatment in the male should be individualised as for the female. When repeat courses are required, allow an interval of four to six weeks between courses, reconfirm the trichomonad, and check total and differential leukocyte counts before and after re-treatment. 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. 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

After reductive activation in anaerobic organisms it generates reactive intermediates that damage microbial DNA, leading to strand breakage and cell death.

Prescribing in practice

A disulfiram-like reaction can occur with alcohol, so patients should avoid alcohol during treatment and for a short period afterwards per the SPC.
Prolonged or high-dose courses carry a risk of peripheral neuropathy and, rarely, central nervous system effects, so duration should be no longer than necessary.
It interacts with warfarin and can enhance its anticoagulant effect, so the INR should be monitored when they are used together.

Monitoring

Monitor for neurological symptoms with prolonged courses and check the INR in patients taking warfarin.

Counselling the patient

Do not drink alcohol while taking this medicine or for a short time after finishing it.
Complete the full course even if you feel better.
Report any numbness, tingling or persistent unsteadiness.

Evidence & guidelines

Metronidazole is an established first-line agent for anaerobic and several protozoal infections, supported by long-standing clinical use and guideline recommendation.

Reference: PHE Antibiotic 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

Pathways