A transdermal patch timer comprising an indicator configured to indicate whether a predetermined period of time has elapsed； and an attachment mechanism configured to couple the transdermal patch timer to a transdermal patch. A method of manufacturing a transdermal patch comprising applying a transdermal patch timer to a transdermal patch. A further method of using a transdermal patch comprising: adhering the transdermal patch to the user； activating the transdermal patch timer on the transdermal patch； and removing the transdermal patch from the skin of a patient in response to observing that the transdermal patch timer shows a predetermined period of time has elapsed. An indicator can be arranged on the transdermal patch timer to be isolated from an active agent reservoir of the transdermal patch. The indicator can be a visual indicator e.g. a dye configured to change appearance over time or after the predetermined time has elapsed. The transdermal patch timer can be a carbon dioxide indicator.
