Abstract: The present invention generally relates to compositions and methods for transdermal drug delivery. The compositions can be used in a variety of applications, including treating erectile dysfunction or sexual dysfunction, treating wounds, or causing or promoting hair growth. For example, in one aspect, the present invention is generally directed to compositions for delivery of nitric oxide, transdermally and/or to a mucosal surface. The composition may include nitric oxide. The nitric oxide may be present within a first phase comprising a lecithin, such as phosphatidylcholine. In certain embodiments, the lecithin is present in liposomes, micelles, or other vesicles containing nitric oxide. The composition can take the form of a gel, a cream, a lotion, an ointment, a solution, a solid “stick,” etc., that can be rubbed or sprayed onto the skin, e.g., onto the penis, vulva, or other suitable portion of the skin. The composition can also be applied to a mucosal surface in some instances.