A transdermal drug delivery device (20) is provided having both an active drug reservoir (24) and a passive drug reservoir (26). Drug is actively delivered by iontophoresis from the active drug reservoir (24) by an electric field generated by a power source (21). Simultaneously, drug is delivered from passive reservoir (26) by passive (i.e., non-electrically assisted) diffusion. In one embodiment, the passive drug reservoir (26) is electrically insulated from the active drug reservoir (24). In a second embodiment, both the active and the passive drug are contained in the same reservoir (34). In the second embodiment, the active drug is ionizable while the passive drug is non-ionizable. Most preferably, the active drug is an ionizable form (i.e., a salt form) of the passive drug.