Sildenafil citrate has low oral bioavailability, systemic adverse effects, and a relatively delayed action. These issues may be addressed through direct transdermal delivery to the penis. This study aims to investigate the microemulsion formulation of the drug for effective transdermal delivery. Sildenafil citrate was formulated as a microemulsion using clove oil, dimethyl sulphoxide, phosphate buffer (pH 7), propylene glycol, Tween^®80, and distilled water. Different proportions of these components were used to create six formulations of the microemulsion (F1–F6), which were then characterised by their physical appearance and clarity, pH, viscosity, conductivity, percent transmission, and droplet size. Furthermore, the stability, content analysis, in-vitro drug release, and transdermal permeation of sildenafil citrate from the generated drug-loaded microemulsions were studied. All prepared formulas contained nano-sized oil droplets (less than 20 nm), and the pH values were within the range of skin pH; however, two formulas were not transparent. Additionally, all formulations were thermodynamically stable, passing freeze-thaw, heating-cooling, and centrifugation tests. Next, the formulas demonstrated zero-order release kinetics, indicating that they can provide a sustained release profile for sildenafil citrate. Finally, the microemulsion formulation exhibited a 2.8-fold enhancement in skin permeation compared with that of the sildenafil citrate suspension. The prepared microemulsions demonstrated beneficial physical properties and skin permeation profiles that are promising for the local administration of sildenafil citrate.

Keywords: Microemulsion; sildenafil citrate; transdermal delivery; permeation enhancer