The disclosure provides an extended release formulation of 5-fluorocytosine. In another aspect, a method of treating a fungal disease is provided. The method comprises administering to a subject in need thereof a fungus-treating effective amount of a composition comprising 5-fluorocytosine. In yet another aspect, a method of treating a cancer is provided. The method comprises administering to a subject in need thereof a sufficient amount of an expression vector to induce expression of cytosine deaminase which is capable of converting 5-fluorocytosine to 5-fluorouracil in cells of the cancer and a cancer-treating effective amount of a composition comprising 5-fluorocytosine.