[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Other antifungals](https://durgical.com/en/pharmacology/classes/other-antifungals/) › Flucytosine [Pharmacology](https://durgical.com/en/pharmacology/) · [Other antifungals](https://durgical.com/en/pharmacology/classes/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 - Class: [Other antifungals](https://durgical.com/en/pharmacology/classes/other-antifungals/) - Target: Fungal RNA and thymidylate synthase, through its metabolite fluorouracil - ATC code: J02AX01 - WHO Model List of Essential Medicines 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 →](https://durgical.com/en/conditions/cryptococcal-meningitis/) - Serious Candida infections (septicaemia, endocarditis, urinary infections), combined with amphotericin B. [Candidiasis: oral, oesophageal and invasive →](https://durgical.com/en/conditions/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 ### Serious (rare, important) - Bone marrow depression: anaemia, leukopenia, thrombocytopenia, pancytopenia, even fatal aplasia - Acute liver injury, including necrosis - Kidney failure and crystalluria - Cardiac and respiratory arrest (rare) - Severe toxicity in dihydropyrimidine dehydrogenase (DPD) deficiency ## 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. ## Memory aid «Flucytosine is a Trojan horse»: the fungus lets it in and makes fluorouracil itself; humans have no cytosine deaminase. ## 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. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/flucytosine/) ## Related drugs - [Amphotericin B](https://durgical.com/en/pharmacology/amphotericin-b/) - [Fluconazole](https://durgical.com/en/pharmacology/fluconazole/) - [Fluorouracil](https://durgical.com/en/pharmacology/fluorouracil/) ## Related conditions - [Cryptococcal meningitis](https://durgical.com/en/conditions/cryptococcal-meningitis/) - [Candidiasis: oral, oesophageal and invasive](https://durgical.com/en/conditions/candidiasis-oral-oesophageal-and-invasive/) [← Back to the class: Other antifungals](https://durgical.com/en/pharmacology/classes/other-antifungals/) 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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=5721b46a-4a9f-46c2-8804-a9bad4d5f80a) (DailyMed (U.S. National Library of Medicine), 2025); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 61 (McGraw Hill, 2023). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/pharmacology/flucytosine/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/flucytosine/