Fluoropyrimidines are used by approximately 2 million patients worldwide each year.
We examined the factors related to early severe fluoropyrimidine-related toxicity.
In total, 1463 patients treated in routine clinical care were analysed.
Renal function, BSA, DPD deficiency, and age were related to early severe toxicity.
These factors are associated with clinically relevant differences in risk of early severe toxicity.