Influence of the administration time on the antihypertensive and hypokalaemic effects of chlorthalidone in patients with primary hypertension.

A double-blind crossover study was designed to compare the antihypertensive and hypokalaemic effects of chlorthalidone administered at 10h00 and 22h00 to sixteen patients with primary hypertension. Blood pressure values during chlorthalidone treatment at 10h00 did not differ significantly from those at 22h00. Significant correlation (r = 0.8987) was seen, however, only between the patients' blood pressure during the placebo period and the fall in blood pressure obtained by administration of chlorthalidone at 22h00. The serum potassium level fell significantly (p less than 0.01) during morning treatment while a slight, insignificant fall was found during evening treatment. No correlation was seen between decrease in blood pressure and decrease in serum potassium level.