Device for transdermal electrotransport delivery of fentanyl and sufentanil

The invention provides to improved electrotransport drug delivery system for analgesic drugs, namely fentanyl and sufentanil. The fentanyl/sufentanil is provided as a water soluble salt (e.g., fentanyl hydrochloride) dispersed in a hydrogel formulation for use in an electrotransport device (10). In accordance with the invention, the concentration of fentanyl/sufentanil in the donor reservoir (26) solution is maintained above a predetermined minimum concentration, whereby the transdermal electrotransport flux of fentanyl/sufentanil is maintained independent of the concentration of fentanyl/sufentanil in solution.