Shereen Elazzazy,1 Asmaa Elhassan Mohamed,2 Amaal Gulied1 1Pharmacy Department, 2Oncology Hematology Department, National Center for Cancer Care and Research (NCCCR), Hamad Medical Corporation, Doha, QatarAbstract: Cyclophosphamide is commonly used in the treatment of malignant diseases. Symptomatic severe hyponatremia induced by low-dose cyclophosphamide is very uncommon worldwide. We report a case of severe symptomatic hyponatremia that developed in a female breast cancer patient following the first cycle of chemotherapy containing low-dose cyclophosphamide. Her laboratory test showed serum Na of 112 mmol/L. Her hyponatremia was initially treated with sodium bicarbonate. She completely recovered without neurological deficits after slow correction of the serum Na concentration. Although hyponatremia is a rare toxicity it should always be considered during the usage of cyclophosphamide, even if the dosage is low, especially with concurrent use of other medications that impair water excretion, like chlorthalidone. This report describes the first reported case of cyclophosphamide-induced hyponatremia in Qatar.Keywords: AC protocol, adjuvant chemotherapy, breast cancer, cyclophosphamide, hyponatremia, thiazides