A definite hypotensive effect from chlorothiazide was observed in 80 % of 65 patients studied. This was more pronounced in those with severe hypertension and in those receiving ganglionic blocking agents. Of the 23 patients who had formerly required ganglionic blocking agents, 12 were able to discontinue that therapy. It would seem that periodic determinations of the serum potassium level are necessary in any patient who is receiving chlorothiazide on a long-term basis. Twenty-eight patients (43%) in this group showed a drop in potassium level to 3.5 mEq. per liter or below. This occurred in 12 despite the concurrent administration of 2 to 3 Gm. of potassium chloride daily. It is now well recognized that renal lesions may accompany potassium deficiency in man. Symptoms of this may be minimal and their onset insidious. There is also danger of hypokalemia potentiating digitalis poisoning.
Bartels CC, Evans JA, Townley RG. CHLOROTHIAZIDE—A SURVEY OF ITS EFFECTS IN HYPERTENSIVE PATIENTS. JAMA. 1959;170(15):1796–1802. doi:10.1001/jama.1959.03010150040011
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