Thiazide diuretics are of proven efficacy and substantial benefits for the treatment of uncomplicated hypertension. But the hyponatremia caused by thiazides are often overlooked. This is a case report of 70 year old female patient who was taking chlorthalidone since two months and developed acute sodium deficiency. Factors that influence risk of hyponatremia include advanced age, body weight, renal function, dietary intake and drug interactions. Therefore patients who are at increased likelihood should be identified earlier. Timely intervention by a clinical pharmacist to an extend, can deter these adverse drug events and utilise the health care facility in a cost effective manner.