Abstract: Disclosed is a transdermal therapeutic system for administering an active ingredient through the skin comprising: a) a back layer, b) a reservoir on the back layer comprising b1) a first layer containing active ingredient, at least one gel former, at least one plasticizer, and a first polyisobutylene; and b2) a second layer containing active ingredient, at least one gel former, at least one plasticizer, and a second polyisobutylene, wherein the first polyisobutylene is different from the second polyisobutylene, wherein at least the first layer contains undissolved active ingredient in the form of active ingredient particles; and wherein the active ingredient is fentanyl or an analogue of the fentanyl.

Abstract: What is disclosed is a transdermal therapeutic system for administration of an active ingredient through the skin, comprising: (a) a back layer, (b) a reservoir present on the back layer, comprising the active ingredient, polyisobutylene, a gel former and a plasticizer, a portion of the active ingredient in the reservoir being present in the form of undissolved particles and the content of gel formers in the reservoir being at most 4% by weight (based in the total weight of the reservoir), the active ingredient being Fentanyl or an analogue thereof.