© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd. Aim: Allergy to materials used during root canal treatment is well recognized in the endodontic literature. However, allergy to Ledermix paste, a compound containing triamcinolone (a corticosteroid) and demeclocycline (a tetracycline antibiotic), has not been reported apart from one very recent case in a Letter to the Editor of a journal. The aim of this report is to describe a proven allergic reaction to tetracycline following the use of Ledermix paste as a root canal medicament. Summary: A 33-year-old female patient undergoing root canal retreatment of her right mandibular second premolar tooth experienced symptoms of type 1 allergy following the placement of an intracanal medicament containing a mixture of Ledermix paste and calcium hydroxide [Ca(OH)2]. Signs of the type 1 allergy included urticaria, pruritus all over the body, general malaise and fever. These all subsided after removing and flushing out the Ledermix paste-Ca(OH)2 mixture and redressing the root canal with Ca(OH)2 alone. Allergic scratch tests were performed, and these confirmed that the patient was allergic to tetracycline, one of the components of Ledermix paste.