The ability of a uracil breath test and a thymine loading test to correctly identify patients who have experienced severe 5-Fluorouracil (5FU) toxicity.

Interventions: uracil breath test (once off test), thymine assay of saliva, blood and urine, genetic analysis of blood. The testing will be completed over 2 days once informed consent is signed. These tests are not standard practice. The testing will be done on patient who exeprienced 5FU toxicity up to 1 year ago.
Primary outcome(s): To determine the proportion of patients with severe 5FU toxicity have (a) abnormal Uracil breath test results and (b) abnormal thymine loading tests in saliva and urine[12 months post testing]
Study Design: Purpose: Screening;Duration: Cross-sectional;Selection: Case control;Timing: Retrospective