Erythema multiforme(EM), a blistering and ulcerative inflammatory disorder, affect skins or mucosa, and is thought to be triggered mainly by preceding infection, such as Herpes simplex virus, or exposure to drugs and medication, particular antibiotics or analgesics. Symptoms include typical cutaneous target lesions on skins and in case of oral manifestation, erosive and ulcerative lesions on lips, buccal mucosa, and tongue are known to occur, which needs differential diagnosis with other intraoral lesions. In this case, EM assumed that it is occurred by giving Trichomonas infection or Metronidazole in oral region is introduced with a review of diagnosis and treatment of EM.