Transdermal drug delivery offers a potential method of drug administration. However, its application has been limited to a few low molecular weight compounds because of the extremely low permeability of human skin. Low-frequency ultrasound was shown to increase the permeability of human skin to many drugs, including high molecular weight proteins, by several orders of magnitude, thus making transdermal administration of these molecules potentially feasible. It was possible to deliver and control therapeutic doses of proteins such as insulin, interferon γ, and erythropoeitin across human skin. Low-frequency ultrasound is thus a potential noninvasive substitute for traditional methods of drug delivery, such as injections.