A diffusional drug delivery device is described which can provide for stability of the adhesive and system components, elimination of the initial burst of drug and hence irritation, and to provide for delayed onset of therapeutic effect along with delivery of a therapeutic agent at an optimum rate. The therapeutic agent in a first form which is suitable for storage, and the anhydrous activating means are inert when in an anhydrous environment. Moisture activates the system whereby the activating means provides an acidic or basic solution and the first form of the therapeutic agent is converted to a second form which is suitable for absorption through the skin or mucosa.