Section 15 · Monograph
Flucytosine
Reviewed Nov 2026FDA label
AncobonAntifungal
- Forms
- Cap: 250, 500mg
- Dose
- 100mg/kg/day PO divided q6h with amphotericin (adjust for renal function; monitor levels and CBC)
- MOA
- Converted to 5-FU, inhibits fungal DNA/RNA synthesis
Black Box Warning
- Use with extreme caution in patients with impaired renal function; close monitoring of hematologic, renal and hepatic status is required
Indications
- Serious infections caused by susceptible Candida or Cryptococcus, including cryptococcal meningitis (used with amphotericin B)FDA-labeled
Mechanism of Action
- Converted by fungal cytosine deaminase to 5-fluorouracil, inhibiting fungal DNA and RNA synthesis
Full Dosing
Adult / usual
- 50-150mg/kg/day orally in divided doses every 6 hours; cryptococcal meningitis commonly 100mg/kg/day
- Give capsules a few at a time over 15 minutes to reduce nausea
Renal
- Reduce dose and extend the interval in renal impairment
Contraindications
- Known hypersensitivity to flucytosine
- Dihydropyrimidine dehydrogenase (DPD) deficiency — avoid (risk of severe toxicity)
Drug Interactions
- Cytarabine antagonizes antifungal activity
- Nephrotoxic drugs (amphotericin B) raise flucytosine levels and toxicity
- Additive marrow suppression with other myelosuppressive drugs
Adverse Effects
- Bone marrow suppression (leukopenia, thrombocytopenia)
- Hepatotoxicity
- Nausea, diarrhea, enterocolitis
- Accumulation toxicity with renal dysfunction
Monitoring Parameters
- CBC and liver tests frequently
- Renal function
- Serum flucytosine levels (peak about 2 hours post-dose; avoid levels above about 100mcg/mL)
Pregnancy & Lactation
- Fetal harm in animal studies; use only if benefit justifies risk
- Breastfeeding not recommended