FLAGYL I.V.
Clinical safety rating: caution
Comprehensive clinical and safety monograph for FLAGYL I.V. (FLAGYL I.V.).
Metronidazole, a nitroimidazole antibiotic, is reduced by bacterial nitroreductases to form reactive intermediates that disrupt bacterial DNA and inhibit nucleic acid synthesis.
| Metabolism | Hepatic metabolism via oxidation and glucuronidation; major metabolites: hydroxy metabolite (active) and acetic acid metabolite; CYP450 involvement primarily CYP2A6 and CYP3A4. |
| Excretion | Renal (60-80% unchanged), fecal (6-15% as metabolites), biliary (minor). |
| Half-life | 8 hours (range 6-12 hours) in adults; prolonged in hepatic impairment (up to 20 hours) and neonates. |
| Protein binding | Less than 20%, primarily bound to albumin. |
| Volume of Distribution | 0.5-0.8 L/kg; indicates extensive tissue distribution, including CNS, bone, and abscesses. |
| Bioavailability | Intravenous: 100%. |
| Onset of Action | Intravenous: immediate (peak concentration at end of infusion). |
| Duration of Action | 8-12 hours; bactericidal effect persists for at least 12 hours after a single dose. |
| Molecular Weight | 171.15 |
500 mg IV every 6 hours. For severe infection, 750 mg IV every 6 hours.
| Dosage form | INJECTABLE |
| Renal impairment | No dose adjustment for CrCl >10 mL/min. For CrCl <10 mL/min, extend interval to every 12 hours. For hemodialysis, administer dose post-dialysis. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: reduce dose by 50%. Child-Pugh C: reduce dose by 75%. |
| Pediatric use | Loading dose: 15 mg/kg IV. Maintenance: 7.5 mg/kg IV every 6 hours. Maximum single dose: 750 mg. |
| Geriatric use | Monitor renal function; adjust dose based on CrCl. No specific age-related dose reduction. |
| 1st trimester | Avoid use in first trimester unless essential; metronidazole crosses placenta and is associated with potential risk of cleft lip/palate in animal studies, but human data do not show consistent teratogenicity. Use only if benefit outweighs risk. |
| 2nd trimester | Use with caution; no known teratogenic risk in second trimester, but consider alternatives if possible. |
| 3rd trimester | Use with caution; no known fetal risk in third trimester, but may accumulate in fetal circulation. Short-term use for trichomoniasis or bacterial vaginosis is considered acceptable. |
Clinical note
Comprehensive clinical and safety monograph for FLAGYL I.V. (FLAGYL I.V.).
| Placental transfer | Metronidazole crosses the placenta readily; fetal serum concentrations approximate maternal levels. Rapid transfer confirmed in human studies. |
| Breastfeeding | Metronidazole is excreted into breast milk (approximately 25% of maternal dose). After single IV dose, milk levels are similar to plasma. The American Academy of Pediatrics considers metronidazole compatible with breastfeeding, but some experts advise withholding breastfeeding for 12-24 hours after IV administration to reduce infant exposure. Monitor infant for diarrhea, thrush, or rash. |
■ FDA Black Box Warning
Carcinogenicity has been observed in chronic animal studies; avoid chronic use unless necessary.
| Serious Effects |
Hypersensitivity to metronidazole or other nitroimidazole derivativesFirst trimester of pregnancy (relative contraindication; use only if essential)
| Precautions | Peripheral neuropathy and central nervous system toxicity (e.g., seizures, encephalopathy) with prolonged use; discontinue if neurological symptoms appear; use with caution in hepatic impairment; may cause disulfiram-like reaction with alcohol. |
| Food/Dietary | Avoid alcohol and any products containing ethanol (e.g., cough syrups, mouthwash) during therapy and for at least 48 hours after completion. No specific food restrictions. |
Loading safety data…
| Lactation Rating | L2 (Safer) for short-term use; L3 (Moderately Safe) for prolonged or high-dose IV therapy per LactMed. |
| Teratogenic Risk | Flagyl I.V. (metronidazole) crosses the placenta. First trimester: Avoid unless essential; no clear evidence of major malformations but risk cannot be excluded (FDA category B). Second and third trimesters: Use only if clearly needed; no documented fetal toxicity at usual doses. |
| Fetal Monitoring | Monitor maternal liver function tests, CBC with differential (risk of neutropenia), and signs of peripheral neuropathy. Fetal monitoring indicated in third trimester for preterm labor or intrauterine infection; no specific fetal heart rate monitoring required. |
| Fertility Effects | No known adverse effects on fertility in humans. Animal studies show no impairment of fertility at therapeutic doses. |
| Clinical Pearls | FLAGYL I.V. (metronidazole) is a nitroimidazole antibiotic with potent anaerobic coverage. It is the drug of choice for Clostridioides difficile infection, but oral vancomycin is preferred for severe cases. IV formulation is used when oral route is not feasible. Monitor for peripheral neuropathy with prolonged use. Avoid alcohol during therapy and for 48 hours after last dose due to disulfiram-like reaction. Dose adjustment required in severe hepatic impairment (Child-Pugh C). Metronidazole can prolong QT interval; use caution with other QT-prolonging drugs. It is compatible with most IV solutions but avoid mixing with calcium-containing solutions. |
| Patient Advice | Avoid alcohol and alcohol-containing products during treatment and for 48 hours after the last dose to prevent severe nausea, vomiting, headache, and flushing. · Inform your doctor if you experience numbness or tingling in your hands or feet, as this may indicate nerve damage. · Report any new or worsening symptoms, especially if you have liver disease or are on blood thinners like warfarin (metronidazole can increase INR). · Do not take this medication if you are pregnant without consulting your doctor, especially in the first trimester. · Shake the IV bag gently before use; do not use if the solution is cloudy or contains particles. |