Reviewed on 3/25/2022

What Is Flucytosine and How Does It Work?

Flucytosine is a prescription medicine used to treat serious fungal infections of the blood, lungs, heart, central nervous system, and urinary tract.

  • Flucytosine is available under the following different brand names: Ancobon, 5-FC, 5-fluorocytosine

What Are Dosages of Flucytosine?

Adult and pediatric dosage


  • 250mg
  • 500mg

Candidiasis/Cryptococcus Infection

Adult dosage

  • 50-150 mg/kg/day divided every 6 hours orally 

Pediatric dosage

  • Child: same as adult dosing; 50-150 mg/kg/day divided every 6 hours orally  
  • Neonates (less than 28 days old): 80-160 mg/kg/day divided every 6 hours orally

Dosage Considerations – Should be Given as Follows: 

  • See “Dosages”

What Are Side Effects Associated with Using Flucytosine?

Common side effects of Flucytosine include:

  • nausea,
  • vomiting,
  • diarrhea,
  • decreased appetite,
  • dry mouth,
  • headache,
  • drowsiness,
  • weakness,
  • poor coordination,
  • numbness, tingling, or burning pain in hands or feet,
  • skin rash.

Serious side effects of Flucytosine include:

  • slow heart rate, 
  • weak pulse
  • fainting
  • slow breathing,
  • chest pain,
  • confusion, 
  • hallucinations,
  • seizure (convulsions),
  • pale skin, 
  • easy bruising, 
  • unusual bleeding: nose, mouth, vagina, or rectum,
  • sudden weakness or ill feeling, 
  • fever, chills, 
  • sore throat
  • mouth sores, 
  • red or swollen gums, 
  • trouble swallowing,
  • problems with hearing,
  • low potassium--leg cramps, constipation, irregular heartbeats, fluttering in the chest, increased thirst or urination, numbness or tingling, muscle weakness or limp feeling,
  • kidney problems--little or no urination, painful or difficult urination, swelling in feet or ankles, feeling tired or short of breath, 
  • liver problems--nausea, upper stomach pain, itching, tiredness, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes), and
  • signs of an allergic reaction: hives, difficult breathing, swelling of face, lips, tongue, or throat.

Rare side effects of Flucytosine include:

  • none 
This is not a complete list of side effects and other serious side effects or health problems that may occur as a result of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.


Bowel regularity means a bowel movement every day. See Answer

What Other Drugs Interact with Flucytosine?

If your medical doctor is using this medicine to treat your pain, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

This information does not contain all possible interactions or adverse effects. Visit the RxList Drug Interaction Checker for any drug interactions. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions, concerns.

What Are Warnings and Precautions for Flucytosine?


  • Hypersensitivity
  • Patients with known complete dihydropyrimidine dehydrogenase (DPD) enzyme deficiency

Effects of drug abuse

  • None

Short-Term Effects

  • See “What Are Side Effects Associated with Using Flucytosine?”

Long-Term Effects

  • See “What Are Side Effects Associated with Using Flucytosine?”


  • Measurement of serum creatinine levels should be determined by Jaffé reaction since this drug does not interfere with the determination of creatinine values by this method; most automated equipment for measurement of creatinine makes use of the Jaffé reaction
  • Electrolytes (because of hypokalemia), hematologic, and renal status of the patient should be determined before therapy is instituted; close monitoring of the patient during therapy is essential.
  • Use extreme caution in patients with impaired renal function; monitor blood concentrations and kidney function in renal impairment; the drug can progressively accumulate; dosage adjustments should be made in patients with renal insufficiency to prevent progressive accumulation of the active drug

Bone marrow depression

  • Administer with extreme caution to patients with bone marrow depression; patients may be more prone to depression of bone marrow function if they have a hematologic disease or are being treated with radiation, treated with drugs that depress bone marrow or have a history of treatment with such drugs or radiation
  • Bone marrow toxicity can be irreversible and may lead to death in immunosuppressed patients; frequent monitoring of hepatic function and the hematopoietic system indicated during therapy

Dihydropyrimidine dehydrogenase deficiency

  • 5-Fluorouracil is a metabolite of flucytosine; dihydropyrimidine dehydrogenase is a key enzyme involved in the metabolism and elimination of 5-fluorouracil; the risk of severe drug toxicity is increased when the drug is used in individuals with a deficiency in DPD
  • Possible drug toxicities include mucositis, diarrhea, neutropenia, and neurotoxicity; determination of DPD activity may be considered where drug toxicity is confirmed or suspected; in the event of suspected drug toxicity, consider stopping treatment

Drug interaction overview

  • Cytosine arabinoside, a cytostatic agent, was reported to inactivate the antifungal activity of this drug by competitive inhibition; drugs that impair glomerular filtration may prolong the biological half-life of flucytosine

Pregnancy and Lactation

  • Flucytosine was shown to be teratogenic (vertebral fusions) in the rat at doses of 40 mg/kg/day(298 mg/m2/day or 0.051 times the human dose) administered on days 7 to 13 of gestation
  • At higher doses (700 mg/kg/day; 5208 mg/m2/day or 0.89 times the human dose administered on days 9 to 12 of gestation), cleft lip and palate and micrognathia reported
  • This drug was not teratogenic in rabbits up to a dose of 100 mg/kg/day (1423 mg/m2/day or 0.243 times human dose) administered on days 6 to 18 of gestation
  • In mice, 400 mg/kg/day of flucytosine (1380 mg/M2/day or 0.236 times the human dose) administered on days 7 to 13 of gestation was associated with a low incidence of cleft palate that was not statistically significant
  • Studies in pregnant rats have shown that flucytosine injected intraperitoneally crosses the placental barrier; there are no adequate and well-controlled studies in pregnant women; this drug should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus


  • Not known whether this drug is excreted in human milk; because many drugs are excreted in human milk and because of potential for serious adverse reactions in nursing infants, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother


Fungal Skin Infections: Types, Symptoms, and Treatments See Slideshow

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