Background Oral mucositis (OM) is a side effect of chemotherapy in head and neck cancer. Severe OM often has a large impact on quality of life. Therefore, the treatment of OM during chemotherapy is very important. It was recently reported that Hangeshashinto (TJ-14), a Japanese traditional medicine (Kampo), is effective for OM caused by fluorinated pyrimidinebased agents used in colon cancer. We investigated the efficacy of TJ-14 for OM. Methods We enrolled patients with head and neck cancer who were treated with induction chemotherapy between September 2014 and March 2016. In this double-blind trial, patients were randomly assigned to the TJ-14 group or placebo group. Patients were instructed to dissolve 2.5 g of TJ-14 or placebo in 100 ml of drinking water, rinse their mouths with the solution for 30 s and then spit it out. They were not allowed to eat anything for 30 minutes before or after using the mouthwash. Results The incidence of ≥ grade 2 OM was 37.5% (three patients) in the TJ-14 group and 50.0% (four patients) in the placebo group, with no significant difference between the two groups. The mean day of onset was 9.7 in the TJ-14 group and 6.7 in the placebo group. The mean duration of ≥ grade 2 OM was 1.3 days in the TJ-14 group and 3.7 days in the placebo group. Thus TJ-14 significantly reduced the duration of ≥ grade 2 OM. Conclusion Treatment of OM with TJ-14 was associated with a statistically significant reduction in the duration of ≥ grade 2 OM compared to placebo. Gargling with TJ-14 is a safe and effective method of administering the drug to patients with head and neck cancer.