• Patients with partial or complete dihydropyrimidine dehydrogenase (DPD) deficiency are at increased risk of severe and fatal toxicity during treatment with fluoropyrimidines (5-FU, capecitabine, tegafur). • Phenotype and/or genotype testing before initiation of treatment with fluoropyrimidines is recommended. • Treatment with 5-FU, capecitabine or tegafur-containing medicinal products is contraindicated in patients with known complete DPD deficiency. • Consider a reduced starting dose in patients with identified partial DPD deficiency. • Therapeutic drug monitoring (TDM) of fluorouracil may improve clinical outcomes in patients receiving continuous 5-FU infusions.