[An unusual presentation of syndrome of inappropriate antidiuretic hormone secretion - When things aren't what they seem].

Introduction: hyponatremia is one of the most common electrolyte disorders in clinical practice and requires accurate diagnosis to prevent potentially severe neurological complications. Case report: we report the case of an 18-year-old male with no significant medical history who presented with severe headache, nausea, vomiting, and mild neurological symptoms, and was found to have severe hyponatremia secondary to syndrome of inappropriate antidiuretic hormone secretion (SIADH). After an extensive diagnostic workup with initially negative results, treatment with hypertonic saline was initiated, followed by fluid restriction and oral urea because of the risk of overcorrection. Clinical evolution was favourable, with progressive normalization of serum sodium levels. After discharge, the patient admitted chronic MDMA (3,4-methylenedioxymethamphetamine) use, and discontinuation of the drug allowed withdrawal of treatment without recurrence, confirming MDMA-induced SIADH. Discussion: this case highlights the importance of a thorough medical history and emphasizes the usefulness of oral urea as an effective and safe therapeutic option in patients at high risk of overcorrection.