A transdermal electrotransport drug delivery device having an anode, a cathode and a source of electrical power electrically connected to the anode and the cathode. At least one of the anode and the cathode includes an electrode and a reservoir comprised of a housing composed of a polymeric material and an aqueous medium in contact with the housing. The aqueous medium includes (i) a drug or an electrolyte salt or a mixture thereof, (ii) propylene glycol, and (iii) an antimicrobial agent in an amount sufficient to inhibit microbial growth in the aqueous medium. The propylene glycol prevents the antimicrobial agent from being adsorbed by other materials used in the construction of the delivery device. A process for preparing a transdermal electrotransport drug delivery device is also provided.