Provided herein are drug delivery systems, such as self-nanoemulsifying drug delivery systems, self-emulsifying drug delivery systems and parenteral microemulsion formulations, suitable for parenteral or oral delivery to a subject. The drug delivery systems may comprise a benzimidazole derivative, e.g., mebendazole, an oil, a surfactant, a cosurfactant and a dipolar aprotic solvent in a microemulsion formulation. Also provided are methods for improving the bioavailability of a benzimidazole derivative during treatment of a pathophysiological condition by using a formulation combining a particular emulsion droplet diameter and ratio of the surfactant:cosurfactant therein, for increasing concentration and retention of a benzimidazole derivative in the lung via a parenterally administerable microemulsion with droplet size of about 35 nm to less than 100 nm and for defining hemolytically safe microemulsions of a benzimidazole derivative during a therapeutic treatment via a parenterally administerable microemulsion with a surfactant:cosurfactant content by weight of about 6% to 48%.