BACKGROUND: Peripheral sensory neurotoxicity is a frequent adverse effect of oxaliplatin therapy. Calcium and magnesium (Ca/Mg) infusions are frequently used as preventatives, but a recent phase III trial failed to show that they prevent neurotoxicity. We therefore conducted a multicenter randomized phase III trial to compare fluorouracil, leucovorin, and oxaliplatin (mFOLFOX6) with and without Goshajinkigan (GJG), a traditional Japanese herbal medicine (Kampo), to determine GJG's potential for reducing peripheral neuropathy in patients with colorectal cancer. METHODS: Patients with colon cancer who were undergoing adjuvant therapy with infusional mFOLFOX6 were randomly assigned to GJG (7.5 mg three times daily) or placebo in a double-blind manner. The primary endpoint was the time to grade 2 or greater neuropathy, which was determined at any point during or after oxaliplatin-based therapy using version 3 of the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE). FINDINGS: An interim analysis was performed when 142 of the planned 310 patients had been enrolled and the safety assessment committee recommended that the study be discontinued. One hundred eighty-two patients were evaluable for response. They included 89 patients in the GJG group and 93 patients in the placebo group. The incidence of grade 2 or greater neurotoxicity was 50.6 % in the GJG group and 31.2 % in the placebo group. A Cox proportional hazards analysis indicated that the use of GJG was significantly associated with the incidence of neuropathy (hazard ratio, 1.908; p = 0.007). CONCLUSION: Goshajinkigan did not prevent oxaliplatin-associated peripheral neuropathy in this clinical trial. The clinical study was therefore terminated.