Pharmacology · Other antifungals
Flucytosine !
Oral antifungal turned into fluorouracil inside the fungus. Given with amphotericin B for cryptococcal meningitis; it can suppress the bone marrow.
Updated 2026-11-01
Also called: 5-fluorocytosine (5-FC)
Mechanism of action
Flucytosine enters the fungus through cytosine permease and is converted by cytosine deaminase into fluorouracil. Its metabolites are falsely built into fungal RNA, disrupting protein synthesis, and inhibit DNA synthesis. Human cells lack cytosine deaminase, which explains the selectivity. Resistance appears quickly when it is used alone.
- Flucytosine enters the fungus through cytosine permease
- Fungal cytosine deaminase turns it into fluorouracil
- False RNA and blocked DNA synthesis stop the fungus
Pharmacokinetics
- Absorption
- Rapid and nearly complete oral absorption.
- Distribution
- Low protein binding; crosses the blood-brain barrier and reaches useful CSF levels.
- Elimination
- Mostly unchanged by the kidneys; it accumulates in kidney failure, where its half-life becomes very long.
Indications
- Cryptococcal meningitis, combined with amphotericin B. Cryptococcal meningitis →
- Serious Candida infections (septicaemia, endocarditis, urinary infections), combined with amphotericin B. Candidiasis: oral, oesophageal and invasive →
Adverse effects
Common
- Nausea, vomiting, diarrhoea, abdominal pain and loss of appetite
- Rash, itching and photosensitivity
- Confusion, headache, vertigo and sedation
- Low potassium
Contraindications
- Hypersensitivity to flucytosine
- Complete DPD deficiency
Key interactions
- Amphotericin B
- Synergistic, but amphotericin-related kidney injury raises flucytosine levels and toxicity.
- Cytarabine
- May inactivate flucytosine's antifungal activity by competitive inhibition.
- Drugs that reduce glomerular filtration
- Prolong its half-life and favour accumulation.
- Bone marrow suppressants and radiotherapy
- Raise the risk of marrow depression.
What is monitored
- Kidney function and flucytosine blood levels
- Leukocyte and platelet counts, often
- Liver tests (alkaline phosphatase, AST, ALT)
- Electrolytes, especially potassium, before starting
Administration and safety
- Oral capsules; given with amphotericin B, not as monotherapy for systemic infection.
- The dose depends on kidney function: check creatinine and drug levels before each adjustment.
- Laboratory note: creatinine should be measured by the Jaffé method, which flucytosine does not disturb.
Nursing care
- Baseline blood count, creatinine, liver tests and potassium before the first dose.
- Watch for fever, sore throat, bleeding or bruising (marrow suppression).
- Report jaundice, dark urine, confusion or reduced urine output.
Pregnancy and breastfeeding
Teratogenic in rats and not well studied in humans; used in pregnancy only when the benefit clearly outweighs the risk.
Test yourself
Think of the answer, then tap the question to see it.
Why is flucytosine not used alone?
Resistance appears quickly during monotherapy; it is combined with amphotericin B, with which it is synergistic.
What makes flucytosine selective for fungi?
Fungi convert it to fluorouracil with cytosine deaminase, an enzyme human cells lack.
What is the main dose-related toxicity, and what raises its risk?
Bone marrow suppression; it is more likely when kidney function falls (for example from amphotericin B) and levels rise.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - FLUCYTOSINE capsule (DailyMed (U.S. National Library of Medicine), 2025); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 61 (McGraw Hill, 2023).
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