A drug electrode assembly usefully employed in an electrotransport device for the delivery of drugs across a tissue surface includes an electrode, a drug reservoir which stores the drug (including an ionized (e.g., anionic) or neutrally charged drug species), and a liquid impermeable solid-state assist ion conducting barrier layer interposed between the electrode and the drug reservoir. The barrier layer can be a single-ion conductor of a specific (unique) species of ion called the assist ion. During drug delivery, the assist ion moves across the barrier layer into or out of the drug reservoir, and as the assist ion crosses the barrier layer/drug reservoir interphase, the drug species moves to the tissue surface. The assist ion can be, for example, sodium ions (Na.sup.+), and the electrode can be an electrode of the assist ion (i.e., a sodium electrode).