Kinetics of Transdermal Clonidine-Reply

—Lin and Ribeiro raise the point that determination of plasma half-life from only a few data points is less than optimal; we agree, but believe that the limited data that we obtained are interpretable and can lead to some useful insights. Using our three data points, we estimated the mean plasma half-lives during days 1 and 2 after the patient had been removed from transdermal therapy to be approximately 34 and 21 hours, respectively. We surmised that the longer plasma half-life during day 1 was due to a skin-depot effect, ie, to continuing absorption of clonidine from the skin at the site of the removed patch. 1 Our data are in accord with a study in which a larger number of data points was used to determine plasma half-life after discontinuation of transdermal therapy: MacGregor et al 2 found the half-life to average 17 to 20 hours, a value in