Thymine test for 5-fluorouracil side effects

Interventions: Participants are administered a single dose of thymine (250 mg, orally) at any time prior to, or after completion of, a cycle of 5-fluorouracil/capecitabine treatment course.
A cumulative 4 h urine sample is collected for detemination of the amount of thymine and metabolites excreted
Primary outcome(s): To determine whether the ratio of thymine to metabolites detected in the urine sample after the thymine test dose can discriminate between patients who tolerate 5-FU/capecitabine treatment and those who experience severe toxicity.[6 months]
Study Design: Purpose: Natural history;Duration: Cross-sectional;Selection: Case control;Timing: Both