This document discusses study design considerations for comparing topical dosage forms in vitro and in vivo. It outlines three methods approved by the Thai FDA to check bioequivalence of topical drugs: in vitro drug release testing using Franz cell diffusion, pharmacodynamic studies measuring physiological effects, and clinical trials. The US FDA only approves clinical trials. In vitro methods are useful but human studies with 100-4000 subjects are ultimately needed to adequately assess bioequivalence of topical dosage forms.