Abstract: The present invention provides a device (1) for monitoring the application of a drug to a patient by means of a drug pen (100), whereby the drug pen comprises a front end provided with an injection needle and a rear end provided with an actuation pushbutton, the device comprising a body (2) which can be dismountably coupled to the pen; an injection detection means determining when a drug injection is carried out; and a processing means configured for storing the date and time of the injection when the injection detection means detects that a drug injection is carried out, whereby the body is configured to be coupled to the pushbutton of the pen such that the pushbutton is actuated by pushing directly on the body, whereby the injection detection means is implemented as actuation detector configured for detecting said pushing action.