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A 300-mug oral dose of clonidine was administered to 5 normal volunteers and measurements of plasma concentration and effects upon blood pressure, heart rate, circulatory reflexes, sedation, and dry mouth were made for the following 8 hr. The plasma concentration rose to a peak of 1.02 +/- 0.52 ng/ml (SD) at 90 min and fell with a mean half-life of 12.7 hr. Blood pressure of the group fell from 111.0/77.0 to 87.2/60.4 after 3 hr and was 95.2/62.2 mm Hg at 8 hr. Heart rate in recumbency was slowed. Marked sedation and a fall in salivary flow followed the same time-course as the plasma concentration. The cold pressor response was reduced but the Valsalva overshoot was little affected.
Dollery et al. (1976) studied this question.