Kinetics of Transdermal Clonidine

To the Editor.  — We read with interest the article by Popli et al entitled "Transdermal Clonidine in Mild Hypertension" in the November 1986 issue of the Archives.¹ However, we have the following comments on the pharmacokinetic aspects of transdermally administered clonidine, as presented in their article.During week 10, plasma clonidine levels were determined at 0,24, and 48 hours after the placebo patch had been applied to estimate the rate of decline of plasma clonidine concentration after discontinuation of transdermally administered clonidine. The authors estimated the plasma half-life during the first day after discontinuation of clonidine therapy from the slope of the line connecting the zero-hour and 24-hour values in the semilog plot of plasma clonidine concentration vs time. In the same manner, the authors estimated the plasma half-life of clonidine during the second day after stopping therapy. The plasma half-lives obtained by the authors averaged 34 and