This case report describes a rare episode of allergic reaction to the topical application of chlorhexidine gel. The patient, following the prescription from her private dentist, presented to the dental emergency unit, referring to the onset of an increasing swelling of the upper lip a few hours after the application of chlorhexidine gel. After the exclusion of a possible infective process starting from the maxillary incisors, the patient was prescribed prednisone and exhibited a quick remission of the swelling. This report aims to emphasize the importance of a correct differential diagnosis to warrant an optimal therapeutic approach to the patient, since swellings of the upper lip could be related to different conditions.
