RESUMEN
INTRODUCTION: Cyclophosphamide is an alkylating agent used in routine pulmonary practice, particularly in the management of connectivitis-related diffuse interstitial lung disease. Common side effects are gastrointestinal disorders and immunosuppression, and a sudden and exceptional adverse effect is severe hyponatremia. OBSERVATION: A 78-year-old female patient treated for NSIP-OP in the context of an anti-synthetase syndrome was treated with Cyclophosphamide 15mg/kg. Twenty-four hours after the end of the infusion, she was diagnosed at home in a coma with comitial seizures. Biological assessment revealed severe hyponatremia at 109 mmol/l with blood hypo-osmolarity. The patient's condition rapidly improved following correction of the ionic disorder. CONCLUSION: Hyponatremia induced by low-dose Cyclophosphamide is a rare but serious adverse effect that requires special attention.