Background: Colon cancer is the third leading cause of death globally, and mucositis is one of the complications of cancer treatment following drug therapy. Objectives: This study investigated the effect of licorice root extract mouthwash with combined mouthwash on the incidence and severity of chemotherapy-induced mucositis symptoms in colon cancer patients admitted to intensive care units (ICUs). Methods: In this clinical trial, 72 colon cancer patients were treated with chemotherapy. A purposive random sample of patients was divided into two groups of intervention (n = 36) and control (n = 36). Patients in the control group received routinely used combined mouthwash. However, the intervention group received licorice root extract 5% from the beginning of the treatment according to the researcher's plan. Then, the degree of mucositis and ulcer area were recorded on the first, third, and seventh days of treatment based on the WHO standard tool for measuring mucositis severity. Results: The intervention and control groups had no significant difference on the first, third, and seventh days of treatment in the incidence of mucositis (P = 0.554, P = 0.308, and P = 0.601, respectively) and the severity of mucositis (P = 0.357, P = 0.857, P = 0.607, respectively). Conclusions: There was no difference in the efficacy of combined mouthwash and licorice root extract mouthwash in the incidence and severity of mucositis. Due to the interest of many patients in the use of herbal compounds, licorice root extract mouthwash can be used as an alternative to combined mouthwash.