Antibiotic (Nitroimidazole) — BV Treatment 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 (Bacterial Vaginosis)

Brand names: Flagyl, Zidoval (vaginal gel)

This entry covers metronidazole used specifically for bacterial vaginosis, available as an oral course or as an intravaginal gel, both first-line options for symptomatic infection.

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: Bacterial vaginosis: 400 mg twice daily for 5–7 days; OR 2000 mg (2 g) as a single dose (1 day)

Route: Oral — tablets swallowed with water (not chewed); recommended to be taken during or after a meal

Frequency: Twice daily for 5 to 7 days, or a single 2 g dose

SPC §4.2 (Metronidazole 200 mg Film-Coated Tablets), 'Protozoal and other infections' table, bacterial vaginosis row — the quoted figures are from the 'Adults and children over 10 years' column. PREGNANCY CAUTION FOR THE SINGLE-DOSE OPTION: SPC §4.6 states that where metronidazole is considered essential in pregnancy or lactation, 'the short, high-dosage regimens are not recommended' — i.e. the 2 g single dose should be avoided in pregnancy/lactation. Regular clinical and laboratory monitoring (especially leucocyte count) is advised if administration for more than 10 days is considered necessary, and patients should be monitored for peripheral or central neuropathy. ELDERLY: metronidazole tablets are well tolerated by the elderly but a pharmacokinetic study suggests cautious use of high-dosage regimens in this age group. HEPATIC: substantial impairment of metronidazole clearance may occur in advanced hepatic insufficiency, with significant accumulation in hepatic encephalopathy; the US label advises reducing the dose by 50% in severe hepatic impairment (Child-Pugh C). PAEDIATRIC: this SPC gives no bacterial vaginosis dose for children under 10 years (the BV row is populated only in the adults/over-10s column) — verify any paediatric dose against a children's formulary. Interactions and the additional contraindications marked '(US labelling)' below come from the US label — UK SPC §4.5 was not captured in this bundle.

Dose adjustments

Renal

The elimination half-life of metronidazole remains unchanged in the presence of renal failure, so the dosage needs no reduction; such patients do retain metronidazole metabolites, the clinical significance of which is not known. In patients undergoing haemodialysis, metronidazole and its metabolites are efficiently removed over an eight-hour dialysis period — metronidazole should therefore be re-administered immediately after haemodialysis. No routine dosage adjustment is needed for intermittent peritoneal dialysis or continuous ambulatory peritoneal dialysis.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; 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

Known hypersensitivity to nitroimidazoles, metronidazole or any of the excipients (UK SPC §4.3)
Cockayne syndrome — contraindicated in US labelling because of severe irreversible hepatotoxicity/acute liver failure with fatal outcomes; the UK SPC §4.4 states metronidazole should not be used in this population unless the benefit outweighs the risk and no alternative is available, with liver function tests before, during and after treatment
Use of disulfiram within the last two weeks — risk of psychotic reactions (US labelling)
First trimester of pregnancy when treating trichomoniasis (US labelling)

Side effects

Gastrointestinal: nausea, vomiting, epigastric pain, diarrhoea, taste disorders (a sharp metallic taste is not unusual), furred tongue, oral mucositis
Nervous system: peripheral sensory neuropathy or transient epileptiform seizures during intensive and/or prolonged therapy (neuropathy usually resolves on stopping or reducing the dose); drowsiness, dizziness, convulsions, headache; very rarely encephalopathy and subacute cerebellar syndrome; aseptic meningitis
Hepatobiliary (very rare): increased liver enzymes, cholestatic/mixed hepatitis, hepatocellular liver injury, jaundice and pancreatitis, reversible on drug withdrawal; liver failure requiring transplant reported in combination with other antibiotics
Skin: rashes, pustular eruptions, acute generalised exanthematous pustulosis, pruritus, flushing; erythema multiforme, Stevens-Johnson syndrome and toxic epidermal necrolysis (discontinue immediately if these occur)
Blood (very rare): agranulocytosis, neutropenia, thrombocytopenia, pancytopenia; leucopenia (frequency not known)
Renal/urinary (very rare): darkening of urine due to a metronidazole metabolite

Interactions

Alcohol and products containing propylene glycol — disulfiram-like reaction (abdominal cramps, nausea, vomiting, headaches, flushing); avoid during and for at least three days after therapy
Disulfiram — psychotic reactions reported; do not give metronidazole to patients who have taken disulfiram within the last two weeks
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 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 metronidazole
Busulfan — metronidazole increases plasma busulfan concentrations, increasing the risk of serious busulfan toxicity

Clinical monograph

How it works

It is reduced within anaerobic organisms to reactive intermediates that damage microbial DNA, eradicating the anaerobic overgrowth (including Gardnerella) that characterises bacterial vaginosis.

Prescribing in practice

Counsel that systemic exposure from oral therapy (and to a lesser extent the gel) produces a disulfiram-like reaction with alcohol, so alcohol should be avoided during and just after treatment.
Recurrence is common, and the intravaginal gel offers an option with fewer systemic side effects for women who do not tolerate oral therapy.
Vaginal preparations may weaken latex condoms and diaphragms, reducing their contraceptive reliability.

Monitoring

No routine laboratory monitoring is needed; assess symptomatic response and consider testing for co-existing sexually transmitted infections.

Counselling the patient

Avoid alcohol during the course and for a short time afterwards.
Latex condoms and diaphragms may be damaged by vaginal preparations, so they cannot be relied on for contraception during use.
Return if symptoms persist or recur, as bacterial vaginosis can come back.

Evidence & guidelines

BASHH guidance recommends oral or intravaginal metronidazole as first-line treatment for symptomatic bacterial vaginosis.

Reference: BASHH BV Guideline 2021; NICE CKS Bacterial Vaginosis; 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