A transdermal drug delivery system which enhances the delivery of the drug comprises a composition containing, as an enhancer, one or more C.sub.6 to C.sub.22 fatty acid esters of a lactic acid salt. These compositions are made up of a safe and effective amount of an active pharmaceutical permeant contained in a penetration-enhancing vehicle comprising, 0.25 to 50% w. of the fatty acid ester of a lactic acid salt enhancer in a suitable pressure sensitive adhesive carrier vehicle formed from and aqueous emulsion based pressure sensitive adhesive.