Lenvatinib hasapplicationsin thyroid and hepatocellular carcinomas. When lenvatinib wasapproved for hepatocellular carcinoma in Japan, the manufacturer noted an increase of 5% in the incidence of side effects than those observed in thyroid cancer, based on the clinical-trial data. The monitoring of side effectsisimportant during chemotherapy. It isdifficult to confirm all the side effects within a single practice, and it is important to consider the incidence and severity of side effects before prescribing a particular treatment regimen. An antineoplastic agent is often used for different diseases, and it may be difficult to confirm a specific side effect. Because clinical conditions vary among different diseases, it is likely that the onset of side effects also differs. We investigated the difference between the onset of side effects in thyroid and hepatocellular carcinomas using the Japanese Adverse Drug Event Report database. The main side effects reported for thyroid cancer included bleeding, hypertension, cardiac disorders, myelosuppression,acute cholecystitis, delayed wound healing, infection, gastrointestinal perforation, fistula formation, and pneumothorax. In addition,patients with hepatocellular carcinoma experienced liver damage and hepatic encephalopathy. A significant strong correlation was observed between the drug dose and number of reports of the varied side effects. We compared the side effects in patients with thyroid cancer to those with hepatocellular carcinoma; although the same drug was used, there was varied expression of the side effects. Thisneedsto be taken into account when determining which drugsare to be used for the treatment of a particular cancer type.