PURPOSE: Many cancer patients express interest in using herbal medicine during chemotherapy, but little is known about its benefits and risks. This study aimed to evaluate the effects of the Chinese herbal medicine JianPi-BuShen formula (JPBS) on adjuvant chemotherapy completion in colon cancer patients. PATIENTS AND METHODS: This multi-center, phase III, randomized, placebo-controlled trial included patients with stage II (high risk for recurrence) and stage III colon cancer following surgery, planning to receive CAPOX (capecitabine and oxaliplatin) chemotherapy. Patients were randomized 1:1 to receive either JPBS or a placebo. The primary outcome was the completion rate of planned chemotherapy cycles. Secondary outcomes included relative dose intensity (RDI), chemotherapy-induced toxicities, quality of life (measured by the Edmonton Symptom Assessment System - ESAS), adverse events (AEs), and serious AEs (SAEs). Predefined subgroup analyses were performed by age (>65/≤65) and TNM stage (II/III). RESULTS: A total of 376 participants were analyzed, with a median age of 60.3 years; 56.9 % were male, and 67.6 % had stage III disease. Chemotherapy completion was significantly higher in the JPBS group than in the placebo group (63.0 % vs. 47.6 %, P = 0.003). Oxaliplatin RDI was also higher in the JPBS group (P = 0.049). Subgroup analyses showed JPBS significantly improved completion rates for stage II patients (73.0 % vs. 42.4 %, P = 0.001) and younger patients (66.9 % vs. 48.8 %, P = 0.004). JPBS reduced grade ≥ 2 vomiting (3.8 % vs. 6.4 %, P = 0.007) but increased grade ≥ 2 thrombocytopenia (16.2 % vs. 12.4 %, P = 0.012). Quality of life improved in stage II and younger patients. CONCLUSION: JPBS improved chemotherapy completion rates in stage II and younger colon cancer patients without compromising tolerability. Further research is needed to explore its mechanisms and long-term effects.