Device for monitoring the application of a drug to a patient by means of a drug pen, whereby the drug pen comprises a front end provided with an injection needle and a rear end provided with an actuation pushbutton, which comprises: - a body which can be dismountably coupled to the pen; - an injection detection means implemented as actuation detector, whereby the actuation detector is a mechanical button provided in the cover portion, configured for detecting said pushing action such that the injection detection means is configured to determine when a drug injection is carried out by detecting the pushing action; and - a processing means configured for storing the date and time of the injection when the injection detection means (3, 3') detects that a drug injection is carried out, whereby the processing means is adapted to store a dataset including the time and date of the injection, whereby the device comprises means for detecting, at the time of application, the temperature of the drug, preferably the insulin, and the processing means is adapted to store this temperature into the dataset and whereby the device further comprises alarm means adapted to warn the user of the pen in the case that the detected drug temperature is above or below a preset threshold and if no or a wrong dose is applied or the injection is uncompleted and when the injection is completed to confirm such correct application, whereby the device further comprises one or more accelerometer/s adapted to detect the orientation angle of the pen at the time of drug application, whereby the processing means is adapted to store this orientation angle of the device into the data set, whereby the device further comprises a drug kind detection means comprising a color sensor and being adapted to automatically detect the kind of applied drug when a drug injection is detected, whereby the kind of drug is identified via a color label provided at the pushbutton (101 of the pen, and whereby the processing means is adapted to store the detection result of the drug kind detection means into the dataset; and whereby the pushbutton to which the body of the device is coupled, before being actuated, is retracted by turning it backwards a distance in proportion to the insulin dose to be injected, as the user turns the pushbutton, an indication window shows the user the number of insulin units that is being charged for injection, wherein the device is characterized by the body comprising: - an essentially frustoconical gasket having an orifice configured for housing therein the pushbutton of the drug pen; - a coupling portion having a through orifice configured for housing the gasket therein such that a compression of said gasket takes place for compressing it firmly around the pushbutton; and - a cover portion configured for coupling to the coupling portion, whereby the injection detection means (3, 3') is provided in said cover portion; and - whereby the body being configured to be coupled to the pushbutton of the pen such that the pushbutton is actuated by pushing directly on the body. - a body which can be dismountably coupled to the pen; - an injection detection means implemented as actuation detector, whereby the actuation detector is a mechanical button provided in the cover portion, configured for detecting said pushing action such that the injection detection means is configured to determine when a drug injection is carried out by detecting the pushing action; and - a processing means configured for storing the date and time of the injection when the injection detection means (3, 3') detects that a drug injection is carried out, - an essentially frustoconical gasket having an orifice configured for housing therein the pushbutton of the drug pen; - a coupling portion having a through orifice configured for housing the gasket therein such that a compression of said gasket takes place for compressing it firmly around the pushbutton; and - a cover portion configured for coupling to the coupling portion, whereby the injection detection means (3, 3') is provided in said cover portion; and - whereby the body being configured to be coupled to the pushbutton of the pen such that the pushbutton is actuated by pushing directly on the body.