Abstract Purpose Irinotecan has considerable importance in the treatment of metastatic colorectal cancer (mCRC). UDP-glucoronyltransferase (UGT) 1A1 is responsible for the inactivation of SN-38, a metabolite of irinotecan. Depending on UGT1A1 polymorphism, the activity of the UGT enzyme can be reduced leading to more frequent occurrence of adverse events related to irinotecan. The present study aimed to assess the safety and efficacy of different doses of irinotecan adjusted according to UGT1A1 polymorphism. Methods Thirty-four patients treated with FOLFIRI as first-line treatment for mCRC were included in this study. The irinotecan dosage was adapted on the basis of UGT1A1 polymorphisms: *1/*1 (370 mg/m 2 ); *1/*28 (310 mg/m 2 ), and *28/*28 (180 mg/m 2 ). The incidence of grades 3 and 4 toxicities (neutropenia, febrile neutropenia, and diarrhoea) was recorded. Response was assessed according to the RECIST 1.1 criteria. Results On the basis of UGT1A1 genotyping, 20 patients were *1/*1 (58.8%), 12 were *1/*28 (35.3%) and 2 were *28/*28 (5.9%). Seven patients experienced at least one severe toxicity, i.e., 21% of the population, amounting to eleven adverse events. Concerning the response rate, 15 patients (44%) had partial or complete response. Conclusion This study demonstrates that mCRC patients treated with FOLFIRI can tolerate a higher dose of irinotecan than the standard dose, i.e., > 180 mg/m 2 , on the basis of their UGT1A1 genotype, without increased toxicities. Trial registration NCT01963182 (registered on 16/10/2013, Clermont-Ferrand, France).