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Antibiotic / Antiprotozoal Pregnancy: There is inadequate evidence of the safety of metronidazole in pregnancy but it has been in wide use for many years without apparent ill consequence. Nevertheless Metronidazole, like other medicines, should not be given during pregnancy or during lactation unless the physician considers it essential; in these circumstances the short, high-dosage regimens are not recommended.

Metronidazole

Brand names: Flagyl, Metrogel, Zidoval

Metronidazole is a nitroimidazole antimicrobial used in obstetrics and gynaecology to treat anaerobic bacterial and protozoal infections such as pelvic inflammatory disease, trichomoniasis and bacterial vaginosis.

Adult dose

Dose: Adults, by mouth (400 mg tablets). Bacterial vaginosis: 400 mg twice daily for 5-7 days, or 2000 mg as a single dose. Urogenital trichomoniasis: 2000 mg as a single dose, or 200 mg three times daily, or 400 mg twice daily (the table's duration column reads "7 Or 5 - 7" days). Where re-infection is likely the consort should receive a similar course of treatment concurrently. Treatment of established anaerobic infection (the label's list includes puerperal sepsis, pelvic abscess, pelvic cellulitis and post-operative wound infections from which pathogenic anaerobes have been isolated): 800 mg followed by 400 mg 8 hourly; a course is about 7 days. Prophylaxis against anaerobic infection, chiefly in abdominal (especially colorectal) and gynaecological surgery: 400 mg 8 hourly during the 24 hours immediately preceding operation, followed by post-operative intravenous or rectal administration until the patient is able to take tablets.
Route: Oral (tablets, 400 mg)
Frequency: Bacterial vaginosis: twice daily for 5-7 days, or one single dose. Trichomoniasis: single dose, or three times daily, or twice daily. Anaerobic infection: 8 hourly. Surgical prophylaxis: 8 hourly for the 24 hours before operation.
The pinned product 'Metronidazole 400 mg Tablets' §4.1 lists prevention of post-operative anaerobic infections (§4.2: chiefly gynaecological surgery), puerperal sepsis, pelvic abscess, pelvic cellulitis, urogenital trichomoniasis in the female and bacterial vaginosis, so the page's obstetric and gynaecological uses are covered and a dose is written. PELVIC INFLAMMATORY DISEASE is not named in §4.1 (pelvic abscess and pelvic cellulitis are), so no PID-specific regimen is written. ROUTE: this card covers the ORAL tablet only. The page's intravenous route and its vaginal gel presentations (brands Metrogel, Zidoval) are NOT covered by this product, and no IV, rectal or gel dose is written; the post-operative IV or rectal step of the prophylaxis line is the label's wording and its dose is not given. PREGNANCY (§4.6): metronidazole "should not be given during pregnancy or during lactation unless the physician considers it essential; in these circumstances the short, high-dosage regimens are not recommended". The single 2000 mg regimens above are such short, high-dose regimens. TABLE READING: the dosage table is flattened in the retrieved text. Bacterial vaginosis (adults and children over 10 years) prints "5 - 7 Or 1" days against "400 mg twice daily Or 2000 mg as a single dose", read here in the order printed (400 mg twice daily for 5-7 days; 2000 mg single dose). For trichomoniasis the three regimens are printed with only two durations ("7 Or 5 - 7"), so which duration belongs to which multi-dose regimen cannot be recovered from the text and none is paired here. NO MAXIMUM DOSE is written: none is printed for these adult regimens (the "not to exceed 2400 mg/day" is for the weight-based amoebiasis alternative, not this indication). ELDERLY: "a pharmacokinetic study suggests cautious use of high dosage regimens in this age group". paedDose is null: this is an obstetric and gynaecological page; the label's paediatric regimens (children's dosing for other indications) are not carried. Use a children's formulary for under-18s.

Dose adjustments

Renal

The retrieved SPC §4.2 contains no renal-impairment dosing instruction. §4.4 says "For information on renal and hepatic insufficiency, please see section 4.2", but the retrieved §4.2 has no such text, so no adjustment is stated here.

Hepatic

The retrieved SPC §4.2 contains no hepatic-impairment dosing instruction (see the §4.4 cross-reference above). §4.4: in patients with Cockayne Syndrome "metronidazole should not be used unless the benefit is considered to outweigh the risk and if no alternative treatment is available"; liver function tests must be done before, during and after treatment, and the drug stopped if they become markedly elevated.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Known hypersensitivity to nitroimidazoles, metronidazole or any of the excipients listed in section 6.1 (§4.3).
  • Pregnancy and lactation: not to be given unless the physician considers it essential, and then short high-dosage regimens are not recommended (§4.6; a warning, not a §4.3 contraindication).
  • Cockayne Syndrome: not to be used unless the benefit outweighs the risk and no alternative exists, because of severe, sometimes fatal hepatotoxicity (§4.4).
  • Caution, not a contraindication: active or chronic severe peripheral and central nervous system disease, because of the risk of neurological aggravation (§4.4).

Side effects

  • Blood (very rare): agranulocytosis, neutropenia, thrombocytopenia, pancytopenia; (not known): leucopenia (§4.8).
  • Immune: anaphylaxis (rare); angioedema, urticaria, fever (not known) (§4.8).
  • Metabolism: anorexia (not known). Psychiatric: psychotic disorders including confusion and hallucinations (very rare); depressed mood (not known) (§4.8).
  • Nervous system (very rare): encephalopathy and subacute cerebellar syndrome (may resolve on discontinuation); drowsiness, dizziness, convulsions, headaches. (Not known): peripheral sensory neuropathy or transient epileptiform seizures during intensive or prolonged therapy, aseptic meningitis, vertigo (§4.8).
  • Eye: diplopia and myopia, usually transient (very rare); optic neuropathy/neuritis (not known). Ear: hearing impaired/hearing loss, tinnitus (not known) (§4.8).
  • Cardiac: QT prolongation (not known), particularly with drugs that prolong the QT interval (§4.8).
  • Gastrointestinal (not known): taste disorders, oral mucositis, furred tongue, nausea, vomiting, epigastric pain and diarrhoea (§4.8).
  • Hepatobiliary (very rare): increased liver enzymes, cholestatic or mixed hepatitis, hepatocellular liver injury, jaundice, pancreatitis; liver failure requiring transplant reported in combination with other antibiotics (§4.8).
  • Skin (very rare): skin rashes, pustular eruptions, AGEP, pruritus, flushing; (not known): erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis, fixed drug eruption (§4.8).
  • Urine may darken (§4.4).
  • §4.8 is truncated at the source-fetch limit at the musculoskeletal heading (later system-organ classes not retrieved).

Interactions

  • Alcohol: avoid during therapy and for at least 48 hours afterwards (disulfiram-like reaction) (§4.5).
  • Disulfiram: psychotic reactions reported with concurrent use (§4.5).
  • Warfarin-type oral anticoagulants: potentiation reported; the anticoagulant dose may need reducing; monitor prothrombin times; no interaction with heparin (§4.5).
  • Lithium: retention with possible renal damage; taper or withdraw lithium before metronidazole; monitor lithium, creatinine and electrolytes (§4.5).
  • Phenytoin or phenobarbital: metronidazole is metabolised much faster, half-life reduced to about 3 hours (§4.5).
  • 5-fluorouracil: reduced clearance, increased toxicity (§4.5).
  • Ciclosporin: risk of elevated ciclosporin levels; monitor serum ciclosporin and creatinine (§4.5).
  • Busulfan: plasma levels may increase, which may lead to severe busulfan toxicity (§4.5).
  • Drugs that prolong the QT interval: QT prolongation reported (§4.5).
  • Laboratory tests: may interfere with ALT, AST, LDH, triglyceride and glucose determinations, giving falsely low or negative results (§4.4).

Monitoring

  • If treatment for more than 10 days is considered necessary: regular clinical and laboratory monitoring (especially leucocyte count) and watch for peripheral or central neuropathy (paraesthesia, ataxia, dizziness, vertigo, convulsive seizures) (§4.4).
  • Liver function tests just before, throughout and after treatment in Cockayne Syndrome, until within normal range or baseline; stop if markedly elevated (§4.4).
  • Prothrombin time with warfarin-type anticoagulants (§4.5).
  • Plasma lithium, creatinine and electrolytes in patients on lithium (§4.5); serum ciclosporin and creatinine in patients on ciclosporin (§4.5).
  • After Trichomonas vaginalis is eliminated, a gonococcal infection might persist (§4.4).
  • Severe bullous skin reactions (SJS, TEN, AGEP): stop metronidazole immediately if signs are present (§4.4).

Clinical monograph

How it works

After reduction in anaerobic and protozoal organisms it generates reactive intermediates that damage microbial DNA, producing a bactericidal and antiprotozoal effect.

Prescribing in practice

  • Warn patients to avoid alcohol during treatment and for a short period afterwards because of a disulfiram-like reaction; it also potentiates warfarin.
  • A metallic taste, nausea and gastrointestinal upset are common, and prolonged or high-dose courses carry a risk of peripheral neuropathy.
  • It is generally regarded as acceptable in pregnancy when indicated, though high single-dose regimens are usually avoided in favour of standard courses.

Monitoring

Routine laboratory monitoring is not required for short courses; review for neuropathy with prolonged use and INR if co-prescribed with warfarin.

Counselling the patient

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

Evidence & guidelines

Metronidazole is a standard component of UK and BASHH treatment regimens for anaerobic and protozoal genital tract infections.

Reference: BASHH BV Guidelines (2023); BASHH PID Guidelines (2019); NICE CG190; 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).

Related

Curated clinical cross-links plus same-class fallbacks.

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