The principal distinction between transdermal and topical drugs is that only the former are intended for systemic delivery. Both formulations are, however, subject to the same skin irritancy testing prior to human exposures.
The skin is biologically intended to be a barrier. Evading this barrier is not easy, because drugs must traverse dead epithelium and live dermis; the former is hydrophobic, while lipophilic drugs tends to form a reservoir in the latter. As in oral transmucosal administration, potent drugs, with modest requirements for mass absorbed and reasonable lipophilicity, are the best candidates for transdermal delivery. Fentanyl, nicotine, and sco-polamine are good examples.
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