Cyclophosphamide-Induced SIADH in a Breast Cancer Patient: A Serious Complication

Abstract Cyclophosphamide is an antineoplastic agent commonly used to treat several malignancies. While it can cause a range of common side effects, including immunosuppression and alopecia, in rare instances, it has been found to cause severe hyponatremia. The probable mechanism by which it causes hyponatremia is by inducing the syndrome of inappropriate antidiuretic hormone secretion. This case report discusses the case of a 66-year-old female diagnosed with grade 3 breast cancer, who showed drastic improvement in her sodium levels with fluid restriction. Her subsequent chemotherapy regimen was changed to doxorubicin monotherapy while cyclophosphamide was omitted, to which she responded favorably without any hyponatremia. Early recognition and prompt treatment of this life-threatening complication are important.