Flucytosine, also known as 5-fluorocytosine (5-FC), is an antifungal medication. It is specifically used, together with amphotericin B, for serious Candida infections and cryptococcosis. It may be used by itself or with other antifungals for chromomycosis. Flucytosine is used by mouth and by injection into a vein.

Common side effects include bone marrow suppression, loss of appetite, diarrhea, vomiting, and psychosis. Anaphylaxis and other allergic reactions occasionally occur. It is unclear if use in pregnancy is safe for the baby. Flucytosine is in the fluorinated pyrimidine analogue family of medications. It works by being converted into fluorouracil inside the fungus, which impairs its ability to make protein.

Flucytosine was first made in 1957. It is on the World Health Organization's List of Essential Medicines. As of 2016, in the United States the medication cost about US$2,000 per day while in the United Kingdom it is about US$22 per day. It is not available in much of the developing world.