Abstract
Erythema multiforme is a skin disorder caused by immunity and allergic reactions caused by medicines and infectious diseases, and is divided into a relatively mild type （EM minor） without mucosal rash and a type with mucosal rash （EM major）. We experienced a case of EM major during adjuvant chemoradiotherapy （CRT） for a patient at high risk of postoperative recurrence of mouth floor cancer. In cases with mucosal and skin symptoms from previous treatment such as CRT and cetuximab combination radiation therapy （bioradiotherapy: BRT）, it is more difficult to distinguish between EM major and Stevens-Johnson syndrome （SJS）. In patients with CRT and BRT, it is important to consult a dermatologist promptly and treat the patient jointly, if a severe drug eruption accompanied by rapid skin eruption and systemic symptoms and mucosal symptoms is observed.
