BACKGROUND: Kampo medicines are often used in Japan as therapy for the side effects induced by oral kinase inhibitors. However, the pharmacokinetic interactions between Kampo medicines and oral kinase inhibitors such as lenvatinib have not been studied. OBJECTIVE: We investigated the effects of Kampo medicines (rikkunshito, shakuyakukanzoto and goreisan) on the steady-state plasma trough concentration (C METHODS: Thirty-nine patients receiving lenvatinib therapy at Ito Hospital between May 2015 and December 2019 were enrolled. The mean C RESULTS: After the repeated administration of rikkunshito (n = 21), shakuyakukanzoto (n = 10) or goreisan (n = 8), the mean C CONCLUSIONS: Although these Kampo medicines are reported to inhibit drug-metabolizing enzymes and drug transporters, the risk of drug interactions for patients receiving lenvatinib therapy is low. Patients should feel confident that they can receive Kampo medicines as supportive care for lenvatinib therapy without a risk of drug interactions that could affect treatment efficacy.