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. |
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 | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for FLAGYL I.V. (FLAGYL I.V.).
| Breastfeeding | Metronidazole is excreted in breast milk; M/P ratio approximately 0.9-1.0. Peak milk concentration 2-4 hours after dose. Breastfeeding not recommended during therapy and for 24 hours after the last dose due to potential carcinogenicity and adverse effects in infant. |
| 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. |
■ FDA Black Box Warning
Carcinogenicity has been observed in chronic animal studies; avoid chronic use unless necessary.
| Serious Effects |
Hypersensitivity to metronidazole or nitroimidazoles; first trimester of pregnancy; concomitant use with disulfiram or alcohol.
| 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. |
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| 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. |