5-fluorouracil (5-FU) is still a cornerstone in drug treatment for cancer. Some patients starting standard dosed 5-FU will experience severe adverse events (SAEs). One mechanism behind SAEs is impaired dihydropyrimidine dehydrogenase (DPD) activity, resulting in an accumulation of cytotoxic metabolites. Pre-emptive testing of DPD enzyme activity or genetic variation in its gene, DPYD, is recommended since 2020 in Sweden. We report experience from DPYD testing in 368 patients planned for 5-FU treatment. DPYD variants associated with reduced DPD activity were observed in 28 patients (8%), which is close to the expected frequency. These patients tolerated 5-FU treatment when doses were reduced according to guidelines. However, 4 out of 5 variant allele carriers starting 5-FU at standard dose due to late arrival of test results experienced SAEs. Pre-emptive testing was calculated to be cost saving and thus beneficial from a healthcare economy perspective.