DPD deficiency is most often caused by inherited variants of the DYPD gene and therefore patients with a known complete DPD deficiency must not be given fluorouracil injection or infusion as this puts them at higher risk of severe and life-threatening side effects (European Medicines Agency, 2020; Medicines & Healthcare products Regulatory Agency, 2020). This evidence concerns the gene DPYD and dihydropyrimidine dehydrogenase deficiency.