not-yet-known not-yet-known not-yet-known unknown In 2015, the United States Food and Drug Administration (FDA) approved uridine triacetate to treat overdose and severe toxicity of the fluoropyrimidine chemotherapy agents 5-fluorouracil (5-FU) and its oral prodrug capecitabine. Uridine triacetate is as an oral prodrug of uridine which competes with cytotoxic fluoropyrimidine metabolites for incorporation into nucleotides. Two million people worldwide start fluoropyrimidine chemotherapy each year with 20-30% developing severe or life-threatening adverse effects, often attributable to a genetic predisposition such as dihydropyrimidine dehydrogenase deficiency. Whilst genetic pre-screening is recommended prior to starting fluoropyrimidine agents, this only prevents 20-30% of early-onset life-threatening toxicity and so does not obviate the need for an antidote. Initial in-human studies established that uridine triacetate more than doubles the maximum tolerated weekly 5-FU bolus dose . A lack of clinical equipoise meant a placebo-controlled phase-III trial was not ethical and so the phase-III trials used historical controls. These found that uridine triacetate improved survival in those with fluoropyrimidine overdose and severe toxicity from 16% to 94%, with 34% able to resume chemotherapy within 30days. Five case reports of delayed fluoropyrimidine toxicity demonstrate improvement following uridine triacetate treatment 120-504 hours after last fluoropyrimidine administration suggesting efficacy beyond the FDA licensing indications. Mechanistically uridine triacetate would be expected to be effective for overdose and severe toxicity of Tegafur (a 5-FU prodrug), but there are no published case reports describing this. Uridine triacetate is available internationally through an expanded access scheme and has been available in the UK since 2019 on a named patient basis.