We report a case of an accidental injection of benzalkonium chloride (zephiran) instead of a local anesthetic agent during a tooth extraction. The sudden development of chin and neck swelling led to dyspnea and the patient lost consciousness. She was sent to an emergency clinic by her dentist immediately. After medical treatment for 20 days, the necrotic tissue was debrided and a gingival sulcoplasty was performed. Healing was uneventful.