Antifungal (pyrimidine analogue) Pregnancy: Teratogenic in rats (vertebral fusions; cleft lip/palate and micrognathia at higher doses). No adequate and well-controlled studies in pregnant women. Use during pregnancy only if potential benefit justifies the potential risk to the fetus.

Flucytosine

Brand names: Ancotil

Flucytosine is an antifungal agent used, usually in combination with amphotericin B, for serious systemic fungal infections such as cryptococcal meningitis and severe candidiasis.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 50 to 150 mg/kg/day

Route: Oral (capsules)

Frequency: Administered in divided doses at 6-hour intervals

US labelling: usual dosage 50 to 150 mg/kg/day in divided doses at 6-hour intervals. Nausea/vomiting may be reduced or avoided if capsules given a few at a time over a 15-minute period. If BUN or serum creatinine is elevated, or other signs of renal impairment, the initial dose should be at the lower level. Should be used in combination with amphotericin B for systemic candidiasis and cryptococcosis because of emergence of resistance. Paediatric use: efficacy and safety not systematically studied in children; a small number of neonates have been treated with 25 to 200 mg/kg/day (with/without amphotericin B) for systemic candidiasis - clinician to source paediatric dose from a children's formulary.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Known hypersensitivity to the drug
Known complete dihydropyrimidine dehydrogenase (DPD) enzyme deficiency

Side effects

Gastrointestinal: nausea, emesis, abdominal pain, diarrhoea, anorexia; acute hepatic injury including hepatic necrosis (possible fatal outcome in debilitated patients)
Haematologic: anaemia, agranulocytosis, aplastic anaemia, leukopenia, pancytopenia, thrombocytopenia, fatal cases of bone marrow aplasia
Neurologic: ataxia, headache, paraesthesia, peripheral neuropathy, convulsions, vertigo, sedation
Genitourinary: azotaemia, creatinine and BUN elevation, crystalluria, renal failure
Dermatologic: rash, pruritus, urticaria, photosensitivity

Interactions

Cytosine arabinoside - reported to inactivate the antifungal activity of flucytosine by competitive inhibition
Drugs which impair glomerular filtration - may prolong the biological half-life of flucytosine

Clinical monograph

How it works

It is taken up by fungal cells and converted to 5-fluorouracil, which disrupts fungal RNA and DNA synthesis; it is given with another antifungal to enhance efficacy and limit resistance.

Prescribing in practice

Dose-related bone marrow suppression (leucopenia and thrombocytopenia) is the principal toxicity, so blood counts and drug levels must be monitored closely.
It is almost always combined with another antifungal because resistance develops rapidly when used alone.
It is largely renally excreted, so renal impairment markedly increases the risk of toxicity and requires dose adjustment and level monitoring.

Monitoring

Monitor full blood count, renal and hepatic function, and plasma flucytosine concentrations to avoid toxic accumulation.

Counselling the patient

Regular blood tests are needed to check your blood counts and drug levels.
Report any signs of infection, unusual bruising or bleeding.
This is given alongside another antifungal medicine as part of your treatment.

Evidence & guidelines

Flucytosine combined with amphotericin B is standard induction therapy for cryptococcal meningitis in UK and international guidance.

Reference: BHIVA opportunistic infection guidance; SmPC; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Pathways