Fialuridine, or 1-(2-deoxy-2-fluoro-1-D-arabinofuranosyl)-5-iodouracil (FIAU), is a nucleoside analogue that was investigated as a potential therapy for hepatitis B virus infection. In a 1993 clinical study at the NIH, unexpected toxicity led to the death of 5 out of 15 patients from liver failure alongside lactic acidosis; two further participants required liver transplantation. This toxicity was unusual in that it was not predicted by animal studies. It is suspected that the drug's toxicity was due to its ability to damage mitochondria.