Metronidazole
Brands (AU): Flagyl, Flagyl S, METRONIDAMED, Metrogyl, Metrogyl 200, Metrogyl 400, Metronidazole IV, Metronide…
Also known as: Anabact, Apo-Metronidazole, Elyzol Flagyl, Femazole, Flagyl 375, Flagyl ER, Flagyl I.V., Flagyl I.V. RTU…
Item code, max quantity & repeats below. Open the prescriber to generate a printable script — you write the directions.
AU-authoritative guidance for dental use, from Therapeutic Guidelines: Oral and Dental (accessed 2026-06-24, licensed). This leads the record; the reference sections below are US/UK-sourced and superseded for AU practice where they differ.
Necrotising gingivitis — drug of choice
Metronidazole is the drug of choice for necrotising gingivitis (effective against the anaerobes that are the main pathogens), as an adjunct to debridement.
Dose: metronidazole 400 mg orally, 12-hourly for 3 to 5 days. If metronidazole is unsuitable: amoxicillin 500 mg orally, 8-hourly for 3 to 5 days. Debridement is essential — poor response is usually due to inadequate debridement, not the antibiotic.
Spreading odontogenic infection — anaerobic cover
For a spreading odontogenic infection when a dental procedure is delayed >24 h or incomplete (or penicillin monotherapy has not responded), add metronidazole 400 mg orally, 12-hourly for 5 days (child: 10 mg/kg up to 400 mg) to a penicillin, for broader anaerobic activity. An adjunct to drainage, not a substitute.
Course length — stewardship
If improving 48–72 h after source control, advise the patient to stop — completing the full course is not necessary.
Dental-specific drug data from MIMS Drugs4dent® (MIMS Australia × University of Melbourne). View the full MIMS entry ↗
- Drug class
- Other antibiotics and anti-infectives
- Indications
- gingivitis, gum inflammation, systemic bacterial infection, giardiasis, trichomoniasis, amoebiasis, dysentery, nonspecific vaginitis
- Dental procedural considerations
- No known dental procedural considerations
- Oral adverse effects
- Drug-associated black hairy tongue; Drug-associated taste disturbance ← MIMS CONFIRMS these for metronidazole (the classic metallic taste). Contrast amoxicillin = none. Use to keep these flags, drop the unconfirmed ones.
imidazole hypersensitivity; blood dyscrasia incl history; active organic CNS disease; Cockayne syndrome.
Precautions
ineffective against aerobic/facultative anaerobes; prolonged use >10 days → monitor FBC (esp leucocytes) for neuropathy; renal (incl dialysis) and hepatic impairment; hepatic encephalopathy; active chronic severe neurological disease; monitor for PRES (MRI); psychiatric symptoms; IBD/Crohn's (↑ cancer incidence); possible mutagen/carcinogen; ensure surgical drainage; latex condom/diaphragm rupture risk (suppos); elderly (consider monitoring drug levels); pregnancy — should not use in 1st trimester (2nd/3rd only if local palliative trichomoniasis treatment inadequate); lactation not recommended.
Drug & allergy interactions — 41 total (severe shown)
SEVERE (16) — chairside-critical:
- Ethanol / alcohol (Good) — disulfiram-like reaction (flushing, nausea, vomiting, tachycardia). The metronidazole counselling point: no alcohol during + ≥24–48 h after.
- Warfarin (Good) — potentiates anticoagulation → bleeding.
- Disulfiram (Good) — acute psychosis/confusion.
- Ergot derivatives — dihydroergotamine, ergometrine, ergotamine, methylergometrine, methysergide (Not Established) — ergotism.
- Fluorouracil, Tegafur (Limited) — ↑ 5-FU toxicity.
- Amiodarone (Limited), Busulfan (Limited), Mebendazole (Limited — SJS/TEN).
MODERATE (23): barbiturates (phenobarbital, amobarbital, etc. — ↓ metronidazole efficacy); carbamazepine, phenytoin/fosphenytoin (↑ levels); ciclosporin, tacrolimus (↑ levels); lithium (toxicity); mycophenolate; ritonavir; thalidomide; primidone. CAUTION (2): colestipol, colestyramine (↓ absorption).
(Full 41-row table held in MIMS; above = all Severe + key Moderate. Link to MIMS for exhaustive detail.)
Official AU regulatory prescribing information from the TGA (METROGYL). General — not dental-specific; the guidance above takes precedence for dental decisions. View the official PI on TGA ↗
DosingPI §4.2
Summarised in Table 1 below. A maximum of 4 g should not be exceeded during 24 hour period. Oral The tablets should be swallowed, without chewing, with half a glass of water. Treatment for seven days should be satisfactory for most patients but, depending on clinical and bacteriological assessment, the clinician might decide to prolong treatment. In patients with impaired liver function, dosage should be reduced or dosage intervals increased. Plasma metronidazole levels should be monitored (see Section 4.4 SPECIAL WARNINGS AND PRECAUTIONS FOR USE). In elderly patients, the pharmacokinetics of metronidazole may be altered and therefore monitoring of serum levels may be necessary to adjust the metronidazole dosage accordingly. Plasma metronidazole levels should be monitored. (see Section 4.4 SPECIAL WARNINGS AND PRECAUTIONS FOR USE). Table 1: Dosage for different Indications Indication Duration (days) Adults and children over 12 years Children 7-12 years 3-7 years 1-3 years Dose Doses …
ContraindicationsPI §4.3
- Patients with active organic disease of the central nervous system.
- Patients with evidence of, or history of blood dyscrasias should not receive the drug since upon occasion a mild leucopenia has been observed during its administration. However, no persistent haematological abnormalities have been observed in animals or clinical studies.
- Hypersensitivity to metronidazole and other imidazoles.
- Metronidazole is contraindicated in patients with Cockayne syndrome. Severe irreversible hepatotoxicity/acute liver failure with fatal outcomes have been reported after initiation of metronidazole in patients with Cockayne syndrome (see Section 4.8 Adverse effects (undesirable
Precautions & warningsPI §4.4
Alcohol Alcoholic beverages and drugs containing alcohol, should not be consumed by patients being treated with metronidazole, or for at least 24 hours afterwards, as nausea, vomiting, abdominal cramps, headaches, tachycardia and flushing may occur. There is the possibility of a disulfiram-like (Antabuse) effect reaction. Candidiasis Candida overgrowth in the gastrointestinal or genital tract may occur during metronidazole therapy and require treatment with a candidacidal drug. Posterior Reversible Encephalopathy Syndrome (PRES) Patients treated with metronidazole have been reported to develop posterior reversible encephalopathy syndrome (PRES). If patients taking metronidazole present with symptoms indicating PRES such as headache, altered mental status, seizures, and visual disturbances, a radiological procedure (e.g. MRI) should be performed. If PRES is diagnosed, adequate blood pressure control and immediate discontinuation of metronidazole is advised. Most patients completely …
InteractionsPI §4.5
Warfarin Metronidazole enhances the activity of warfarin, therefore if METROGYL is to be given to patients receiving this or other anticoagulants, the dosages of anticoagulants should be recalibrated. There is an increased haemorrhagic risk caused by decreased hepatic metabolism. Prothrombin times and anticoagulant activity should be monitored. Alcohol Alcoholic beverages and drugs containing alcohol should not be consumed during metronidazole therapy and for at least one day afterwards because the possibility of a disulfiram-like (antabuse effect) reaction (vomiting, tachycardia, and flushing) Carmustine (BCNU) or cyclophosphamide Metronidazole should be used with caution in patients receiving these drugs. Lithium Plasma levels of lithium may be increased by metronidazole. Lithium retention accompanied by evidence of possible renal damage has been reported in patients treated simultaneously with lithium and metronidazole. Lithium treatment should be tapered or withdrawn before …
Adverse effectsPI §4.8
Frequency, type and severity of adverse reactions in children are the same as in adults. Gastrointestinal Metronidazole when given orally is well tolerated. Common adverse reactions refer to the gastrointestinal tract, particularly nausea, sometimes accompanied by headache, anorexia and occasionally vomiting, diarrhoea, epigastric pain or distress, dyspepsia abdominal cramping and constipation. Oral mucositis, dry mouth and taste disorders have also been reported. A metallic, sharp, unpleasant taste is not unusual. Cases of pancreatitis which abated on withdrawal of the drug, have been reported. Patients with Crohn's disease are known to have an increased incidence of gastrointestinal and certain extraintestinal cancers. If patients receiving metronidazole drink alcoholic beverages, they may experience abdominal distress, nausea, vomiting, flushing or headache. A modification of the taste of alcoholic beverages has also been reported. Furry tongue, tongue discolouration, coated …
OverdosePI §4.9
Symptoms Overdosage with metronidazole appears to be associated with very few abnormal signs or symptoms. Disorientation, ataxia and vomiting may occur, especially after ingestion of large amounts. In case of suspected massive overdosages, a symptomatic and supportive treatment should be instituted. Single oral doses of metronidazole, up to 12 g, have been reported in suicide attempts and accidental overdoses. Treatment There is no specific antidote for metronidazole overdosage. In cases of suspected overdosage, a symptomatic and supportive treatment should be instituted. For information on the management of overdose, contact the Poisons Information Centre on 13 11 26
Pregnancy & lactationPI §4.6
Effects on Fertility No data available. Use in Pregnancy Pregnancy Category: B2 The safety of use of metronidazole in pregnancy has not been established and its use should be avoided; if essential, short high-dose regimes should not be used . Metronidazole should not be given in the first trimester of pregnancy as it crosses the placenta and enters fetal circulation rapidly. As its effects on human fetal organogenesis are not known, its use in pregnancy should be carefully evaluated. Although it has not been shown to be teratogenic in either human or animal studies, such a possibility cannot be excluded. Use of metronidazole for trichomoniasis in the second and third trimesters should be restricted to those in whom local palliative treatment has been inadequate to control symptoms. If a patient is treated during the 2nd or 3rd trimesters of pregnancy for urogenital trichomoniasis, the 2 g stat dose therapy should not be used as it results in higher serum levels which reach the fetal …
Faded = flagged by a single source only (lower confidence).
US/UK reference sections (Lexicomp, Mosby's) — superseded by the Australian guidance and Product Information above. Retained for audit; click to expand.
Effects on dental treatment
Dental use for oral soft tissue infections caused by anaerobic bacteria and for periodontitis associated with Actinobacillus actinomycetemcomitans, alone or with agents such as amoxicillin, amoxicillin/clavulanate, or ciprofloxacin. Key dental-related adverse events include unusual or metallic taste, glossitis, stomatitis, xerostomia, and furry tongue. (lexicomp-2260 p.1132)
LA / vasoconstrictor precautions
No information available to require special precautions. (lexicomp-2260 p.1132)
Drug interactions of concern in dentistry
• Disulfiram-like reaction: alcohol, alcohol-containing products • Decreased action: phenobarbital • Possible increase in blood levels of tacrolimus • Enhanced effects of warfarin, carbamazepine (mosbys-2015 p.891)
Oral adverse effects
unusual taste; metallic taste; glossitis; stomatitis; xerostomia; furry tongue; nausea; anorexia; abdominal cramps; constipation; diarrhea; epigastric distress; pancreatitis; proctitis; vomiting; neutropenia; thrombocytopenia; moniliasis (lexicomp-2260 p.1132)
Dental considerations
General: • Patients on chronic drug therapy may rarely have symptoms of blood dyscrasias, which can include infection, bleeding, and poor healing. • Assess salivary flow as a factor in caries, periodontal disease, and candidiasis. • Determine why the patient is taking the drug. Consultations: • In a patient with symptoms of blood dyscrasias, request a medical consultation for blood studies and postpone dental treatment until normal values are reestablished. • Medical consultation may be required to assess disease control. Teach Patient/Family to: • Avoid alcoholic beverages and mouth rinses. • Report taste alterations. • Encourage effective oral hygiene to prevent soft tissue inflammation. • Use caution to prevent injury when using oral hygiene aids. • When chronic dry mouth occurs, advise patient to: • Avoid mouth rinses with high alcohol content because of drying effects. • Use sugarless gum, frequent \sips of water, or saliva substitutes. • Use daily home fluoride products for anticaries effect. (mosbys-2015 p.891)
Dental dosing
Anaerobic infections/abscess: adults oral or IV 500 mg every 6-8 hours, maximum 4 g/day. Periodontitis associated with A. actinomycetemcomitans: adults oral 500 mg every 8 hours for 8 days. (lexicomp-2260 p.1132)
Precautions / contraindications
Hypersensitivity to metronidazole or other nitroimidazole derivatives (also parabens with topical application) Caution: Candida infections; avoid unnecessary use because shown to be carcinogenic in rodents (mosbys-2015 p.891)
Serious reactions
• Oral therapy may result in furry tongue, glossitis, cystitis, dysuria, pancreatitis, and flattening ofT waves on ECG readings. • Peripheral neuropathy, manifested as numbness and tingling in hands or feet, is usually reversible if treatment is stopped immediately after neurologic symptoms appear. • Seizures occur occasionally. (mosbys-2015 p.891)