Dimethyl sulfoxide (DMSO) is a molecule with a long history in pharmaceutics and is now well established as a penetration enhancer in topical pharmaceutical formulations. It is currently used for this purpose in diclofenac sodium topical solution (approved in the United States to treat signs and symptoms of osteoarthritis) and idoxuridine topical solution (approved in Europe for the treatment of herpes zoster). This article reviews the mechanism of action of DMSO as a pharmaceutical penetration enhancer, the characteristics of the molecule that facilitate transdermal drug delivery, and studies of efficacy and safety. The clinical use of pharmaceutical-grade DMSO as a penetration enhancer is supported by the robust data that have accumulated over the past 3 decades demonstrating the favorable safety and tolerability profile. Dimethyl sulfoxide is a safe and effective mechanism for facilitating the transdermal delivery of both hydrophilic and lipophilic medications to provide localized drug delivery.