Background: Fluoropyrimidines (FPs) are an essential part of the majority of systemic regimens in the treatment of metastatic colorectal cancer (CRC). The use of the oral FP S-1 has been approved by the European Medicines Agency as monotherapy or in combination with oxaliplatin or irinotecan, with or without bevacizumab, for the treatment of patients with metastatic CRC in whom it is not possible to continue treatment with another FP due to hand-foot syndrome (HFS) or cardiovascular toxicity (CVT). Subsequently, this indication has been included in the 2022 ESMO guidelines for metastatic CRC. Recommendations for use in daily practice are not available., Patients and Methods: Based on peer-reviewed published data on the use of S-1 in Western patients with metastatic CRC who switched from infusional 5-fluorouracil (5-FU) or capecitabine to S-1 for reasons of HFS or CVT, recommendations for its use were formulated by an international group of medical oncologists with expertise in the treatment of metastatic CRC and a cardio-oncologist., Results: In patients who experience pain and/or functional impairment due to HFS during treatment with capecitabine or infusional 5-FU, a switch to S-1 is recommended without prior dose reduction of capecitabine/5-FU. S-1 should preferably be initiated at full dose when HFS has decreased to grade ≤1. In patients with cardiac complaints, in whom an association with capecitabine or infusional 5-FU treatment cannot be excluded, capecitabine/5-FU should be discontinued and a switch to S-1 is recommended., Conclusions: These recommendations should guide clinicians in daily practice in the treatment of patients with metastatic CRC with FP-containing regimens., Competing Interests: Disclosure CJAP reports an advisory role for Nordic Pharma. VH reports honoraria from Merck, Amgen, Roche, Sanofi, Servier, Pfizer, Pierre-Fabre, AstraZeneca, BMS, MSD, Novartis, Boehringer Ingelheim, Celgene, SIRTEX, Terumo, OncoSil, Nordic Pharma, Seagen; and research funding from Merck, Amgen, Roche, Sanofi, Boehringer-Ingelheim, SIRTEX, and Servier. TM reports an advisory role for Nordic Pharma, Perspectum, and AstraZeneca. CC reports honoraria from Amgen, Bayer, Servier, Merck-Serono, MSD, Organon, Pierre-Fabre, Roche, and Nordic Pharma; research grants from Merck-Serono, Bayer, and Servier. EVC reports participation in advisory boards for AbbVie, ALX, Amgen, Array, Astellas, AstraZeneca, Bayer, BeiGene, Boehringer Ingelheim, Bristol-Myers Squibb, Daiichi, GSK, Incyte, Ipsen, Lilly, Merck Sharp & Dohme, Merck KGaA, Mirati, Novartis, Nordic, Pierre Fabre, Pfizer, Roche, Seattle Genetics, Servier, Takeda, Terumo, Taiho, and Zymeworks; and research grants from Amgen, Bayer, Boehringer Ingelheim, Bristol-Myers Squibb, Ipsen, Lilly, Merck Sharp & Dohme, Merck KGaA, Novartis, Roche, and Servier paid to his institution. RM reports advisory board fees from Astellas Pharma, Bayer, Bristol Myers Squibb, Clovis Oncology, F. Hoffmann–La Roche, Ipsen Biopharm, Janssen Biotech, Merck, and Pfizer; and clinical trial fees from Regeneron Pharmaceuticals. GB reports advisory role for Amgen, Astellas, Bayer, Janssen, Lilly, MSD, Merck, Nordic Pharma, Novartis, Pfizer, Roche, and Ipsen; and lecture fees from Amgen, Bayer, GSK, Merck, Nordic Pharma, Novartis, and Roche. TA reports honoraria from Amgen, AstraZeneca, Bristol-Myers Squibb, GlaxoSmithKline, Merck Sharp & Dohme, Pierre Fabre, Roche/Vantana, Sanofi, and Servier; consulting or advisory roles for Amgen, Astellas Pharma, Bristol-Myers Squibb, GamaMabs Pharma, Gritstone Oncology, Merck Sharp & Dohme, Nordic Pharma, Pierre Fabre, Seagen, and Servier; and Transgène speaker’s bureaus for Bristol-Myers Squibb, Merck Sharp & Dohme, Seagen, and Servier; travel and accommodation expenses from Merck Sharp & Dohme, and Bristol-Myers Squibb; and nonremunerated activities for the ARCAD Foundation and GERCOR Group. PO reports honoraria from/advisory role for Amgen, Astra Zeneca/Daiichi Sankyo, Bristol-Myers Squibb, Eisai/Ewopharma, Fresenius, Incyte, Janssen, Merck, Merck, Sharp & Dome, Nordic Drugs/Pharma, Nutricia, Pierre Fabre, Roche, Sanofi, and Servier; research support to institution from Amgen, Eli Lilly, Merck, Roche, Sanofi, and Servier; and nonremunerated activities with Colores, Duodecim, ESMO faculty, ESMO scientific committee, ESMO guidelines committee, ASCO/ESMO global curriculum, and the ESMO Lines of therapy project. PP reports advisory role for Servier, Nordic Pharma, and MSD; and research funding for institution from Servier, Nordic Drugs, Shire, Merck, Egetis, Isofol, Lilly, Roche, Merck-Serono, Amgen, and Celgene. AJT has declared no conflicts of interest., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)