Prazosin is formulated in solution with a akin permeation enhancer composition of: a sulfhydryl-containing compound, a fatty acid eater, and a polar solvent for transdermal administration to treat hypertension or benign prostatic hypertrophy. The formulation is employed in the form of a skin patch comprising (a) an impermeable backing layer, (b) a reservoir of the prazosin-enhancer solution, (c) a porous support member that retains the solution but is not a permeation barrier to the solution, a peripheral adhesive layer for affixing the patch to the skin, and a release liner layer.