Background: Adverse toxicities to chemotherapy remain a limiting factor for patients receiving treatment for malignancies. In gastrointestinal cancers, particularly metastatic colorectal cancer, the majority of front line treatment remains chemotherapy containing 5-fluorouracil and/or irinotecan. One particular cause that can predispose a patient to increased toxicity from 5-fluorouracil is genetic variation in the DPYD gene, as well as TYMS gene. Similarly, genetic variation in the UGT1A1 gene can increase the risk of irinotecan toxicity. These gene variations may have major implications when deciding on treatment regimens for these types of patients; however, they are not routinely checked. Methods: In Mayo Clinic Florida, patients with metastatic colorectal cancer receiving 5-fluorouracil and/or irinotecan chemotherapy during the period of July 2017-July 2018 were tested for genetic variants in DPYD/TYMS and UGT1A1. This testing was done during routine clinic visit either by buccal swab or blood test. The DPYD/TYMS and UGT1A1 variant results were evaluated together with a pharmacist for recommendations regarding dose reductions on 5-fluorouracil and irinotecan respectively. Results: A total of 292 patients were tested. Total number of patients who had a variation in the DPYD/TYMS and/or UGT1A1 gene was 169 (57.9%). The UGT1A1 variation was the most prevalent with 154 (52.7%) patients having a variant (115 heterozygous variant, 34 homozygous variant and 5 double heterozygous variant). Specifically there were 3 patients with both UGT1A1 and DPYD variants and 3 patients with both UGT1A1 and TYMS variants. One patient had both DPYD and TYMS variations. Conclusions: Routine pharmacogenomic testing for UGT1A1 and DPYD/TYMS genetic variants should be done in all patients receiving irinotecan and 5-fluorouracil chemotherapy respectively. Avoiding or preemptively dose reducing these agents in patients with gene variations may help prevent toxic adverse events and make treatment more tolerable.