Purpose <p>Metformin hydrochloride (Metformin HCl) is commonly used for the treatment of non-insulin-dependent diabetes. It has low bioavailability and many side effects such as gastric irritation after oral administration. This study aims to formulate new microemulsions containing metformin for transdermal application and to assess their in vitro and in vivo bioavailability.</p> Methods <p>Non-ionic surfactants were used to develop seven microemulsions (MEs) containing metformin HCl. The MEs were characterized by their droplet size, polydispersity index, and rheological properties. Furthermore, the flux of metformin was estimated through rat’s skin using Franz diffusion cells. In addition, the oral and transdermal in vivo bioavailability was evaluated in rats.</p> Results <p>The results showed that the developed MEs had colloidal properties. In vitro bioavailability using Franz diffusion cells through the skin over 24&#xa0;h showed a flux as high as 81.78&#xa0;µg/cm².hr. Moreover, the in vivo transdermal bioavailability of metformin in the developed MEs in rats was slightly higher than the oral solution.</p> Conclusion <p>The developed nonionic metformin-loaded microemulsions were found to be ideal carriers and promising formulations for the transdermal administration of metformin HCl.</p>