We evaluated testosterone-containing lecithin reverse wormlike micelles (reverse worms) composed of a polar substance/lecithin/isopropyl myristate for transdermal application. Water, D-ribose, or tetraglycerol were used as the polar substance and were key ingredients for forming the reverse worms. Using the reverse worms, 1 wt% of testosterone could be stably solubilized. When using D-ribose as polar substance, the maximum zero-shear viscosity of the reverse worms solution was higher than that of systems using water or tetraglycerol as the polar substance. The mechanism of skin permeation of testosterone from reverse worms solution was elucidated using skin permeation experiments with hairless mouse skin. When the structure of the reverse worms transitioned to lamellar liquid crystals at the skin/formulation interface, testosterone became supersaturated in the formulations. The structural transition occurred in systems using water or D-ribose as the polar substance, increasing the flux of testosterone. The flux of testosterone from reverse worms solution thus depends on the type of polar substance used.Key words reverse wormlike micelle; lecithin organogel; skin permeation; rheology; phase diagram; hairless mouse skin Recent advances in drug dosage forms have led to improved quality of life for patients. The transdermal dosage form, in particular, represents one of the most important drug delivery methods. Transdermal dosage forms enable avoidance of the hepatic first-pass effect, which can be problematic with oral administration. Another advantage of transdermal dosage forms is that they are easy to apply and remove from the skin. Based on these advantages, transdermal dosage forms have been adopted for a variety of medications.We investigated the possibility of using lecithin reverse wormlike micelle (called reverse worms) comprised of a polar substance/lecithin/oil as the vehicle for transdermal therapeutic application of testosterone (TES). Lecithin is an amphiphilic molecule composed of both hydrophilic and hydrophobic groups. In general, lecithin forms spherical or ellipsoidal reverse micelles when added alone to oil. When trace amounts of a polar substance (e.g., water) 1) are added to this solution, the polar substance can attach to the phosphate groups of the lecithin molecules via hydrogen bonding, widening the gap between the head groups of neighboring lecithin molecules. Thus, the value of the critical packing parameter decreases and the interface curvature of the molecular assembly subsequently induces the formation of reverse worms. As a result, phase transition from reverse spherical micelles to reverse worms takes place. These reverse worms become tangled in the oil and form a three-dimensional (3D) network throughout the solution, turning the solution into an gel-like solution (also called a lecithin organogel). [1][2][3][4][5][6] In this reverse micellar system the polar substance is the key ingredient for the formation of reverse worms. We have also identified other key ingredients that can be...