Dihydropyrimidine Dehydrogenase (DPD) for Fluorouracil (5-FU). 5-FU (fluorouracil) is metabolized by dihydropyrimidine dehydrogenase (DPD), which is highly polymorphic. An estimated 3-6% of patients have a genetic variation in DPD that can result in severe toxic reactions that may be fatal with even small doses and often the very first dose of 5-FU.