This study is a multicenter randomized controlled trial designed to investigate the effects of Clostridium butyricum on adverse events during adjuvant treatment for colorectal cancer.

A total of 238 participants who have undergone radical resection for colorectal cancer and are scheduled to initiate adjuvant chemotherapy will be enrolled and randomly assigned in a 1:1 ratio to the intervention group or the control group. During adjuvant chemotherapy, participants in the intervention group will receive Clostridium butyricum capsules, while those in the control group will receive a placebo. The primary outcome is the incidence of CTCAE grade 3-4 diarrhea during adjuvant chemotherapy.

Inclusion Criteria:

• Aged 18-70 years;
• No restriction on gender;
• Have completed radical resection for colorectal cancer (including open, laparoscopic, or robotic surgery), and assessed by the MDT as requiring adjuvant therapy primarily based on 5-FU and its derivatives or platinum-based regimens (including chemotherapy, targeted therapy, or radiotherapy);
• ECOG performance status score of 0-2;
• Signed informed consent

Exclusion Criteria:

• Use of probiotics, prebiotics, synbiotics, or antibiotics within 2 weeks prior to enrollment;
• Presence of psychiatric disorders or other conditions that prevent cooperation with the intervention;
• Dysfunction of vital organs such as the liver, kidneys, or heart that renders the individual unsuitable for clinical research upon assessment, or inadequate bone marrow, liver, or renal function to undergo adjuvant therapy;
• Participation in other clinical studies within 3 months prior to enrollment;
• History of inflammatory bowel disease;
• History of autoimmune diseases;
• Pregnancy or breastfeeding;
• Receipt of neoadjuvant therapy (including chemotherapy, radiotherapy, or targeted therapy) prior to surgery;
• Underwent ostomy surgery during the operation (including temporary or permanent ostomy).

Eligible participants who meet the inclusion criteria after screening will receive either the investigational product or placebo, packaged and labeled with a unique randomization number, as assigned by an independent investigator. The participants, as well as the researchers involved in recruitment, screening, and follow-up, and the nursing staff, will all be blinded to the group assignments. Unblinding and data analysis will be conducted only after all participants have completed adjuvant chemotherapy.