Abstract: A transdermal drug delivery device in the form of a patch having an outer wall including a piercing element on its inner face facing the drug reservoir and effective, when depressed towards the opposite side of the patch, to pierce a sealing film and thereby to permit the delivery of the drug transdermally to the user. The outer wall of the patch is of a stiff elastic material which produces an audible “click” when depressed from its outer position to its inner position piercing said sealing film. In one described embodiment, the outer wall is of a monostable construction, and in a second described embodiment, it is of a bistable construction.