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INTRODUCTION Chemotherapy induced toxicities can generate changes in prescribing and relative dose intensity which have an impact on therapeutic efficacy. METHOD This is a prospective observational study performed in hepato-gastroenterology department for 6 months. All patients treated for colorectal cancer and beginning a protocol with at least one parenteral drug have been included. RESULTS Among the 48 patients enrolled, 85.4% of them had at least one prescription change, which concerned 30.3% of 238 cycles. Of the 766 analyzed prescription lines, 16.6% of them were postponed and/or 6.7% had modified dosage and/or 5.6% were stopped prematurely. Grades 2 to 4 adverse reactions were responsible for at least one change prescribing to 64.6% of patients and 17.6% of cycles. Toxicity induced prescription changes were mainly due to clinical toxicities (79.3%). The rate of patients with a relative dose intensity greater than 70% was 92.9% in adjuvant state, 66.7% and 62.5% in metastatic state first line and second and subsequent line. CONCLUSION High-grade clinical toxicities are the main chemotherapy prescription change pattern in colorectal cancer. Knowledge of toxicities before the patient's arrival is expected to target patients for which the drug preparation can be anticipated and for which a cycle postponement, dose adjustment or discontinuation is necessary.