Lesch-Nyhan syndrome (LNS) is an inherited recessive X-related disorder caused by a deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase. It is characterized by dystonia and compulsive self-mutilation, in particular, biting behavior on the oral mucosa, tongue, lips, fingers, and shoulders, typically before one year of age. The majority of these patients require several procedures, including dental extractions, to prevent significant secondary lesions. This article aims to report a clinical case of a 12-year-old boy with an LNS diagnosis who was referred to the Paediatric Stomatology Department of Central Lisbon University Hospital. Since the age of eight, the patient had displayed self-harm behavior, with arm and oral injuries. On evaluation, he presented with deep ulcerated lesions on the lips and tongue, with substance loss associated with a significant decrease in food intake and consequent weight loss. The management included conservative therapy with gabapentin, lorazepam, and botulinum toxin injections. A successful reduction of self-mutilation with no signs of new lesions in the oral cavity and an improvement in nutritional status were reported. The therapeutic approach is essential to provide the best quality of life for patients and their caregivers. To delay radical treatments, multiple therapeutic options can be used. The oral pathology team considered that the most appropriate therapy was botulinum toxin A injections along with therapeutic adjustment, which was effective in wound healing and self-mutilation behavior ceasing at the two-month follow-up.