Injection system having an injection device, comprising - a needle protection sleeve which is displaceable in a longitudinal direction between a proximal and a distal position; - a needle protection sleeve spring which is coupled to the needle protection sleeve and which, after substance delivery has taken place, pushes the needle protection sleeve into the distal position, in which it laterally surrounds or shields the needle, the needle protection sleeve spring being tensioned during a piercing process; the injection system further comprising an add-on module for attaching to the injection device, having a longitudinal axis, the add-on module comprising - a force sensor for measuring a force component, in the direction of the longitudinal axis, that is exerted by the injection device on the attached add-on module during an injection process; - an evaluation electronics system for evaluating measurements of the force sensor for the purpose of identifying a process or a state of the injection device, characterized in that the add-on module is designed to identify a piercing process from an increase in an axial force measured by the force sensor to an at least approximately constant value caused by the compressed needle protection sleeve spring, and the add-on module having a sleeve-shaped module housing having a device receptacle for receiving the injection device and a handle for holding the injection device and the attached add-on module, a clear width of the device receptacle in the region of the handle being greater than an outer diameter of the injection device, so that, at maximum grip pressure, the device receptacle is not pressed onto the injection device and no friction or retaining forces can be transmitted in the longitudinal direction via the device receptacle between the injection device and the add-on module while bypassing the force sensor. - a needle protection sleeve which is displaceable in a longitudinal direction between a proximal and a distal position; - a needle protection sleeve spring which is coupled to the needle protection sleeve and which, after substance delivery has taken place, pushes the needle protection sleeve into the distal position, in which it laterally surrounds or shields the needle, the needle protection sleeve spring being tensioned during a piercing process; the injection system further comprising an add-on module for attaching to the injection device, having a longitudinal axis, the add-on module comprising - a force sensor for measuring a force component, in the direction of the longitudinal axis, that is exerted by the injection device on the attached add-on module during an injection process; - an evaluation electronics system for evaluating measurements of the force sensor for the purpose of identifying a process or a state of the injection device, characterized in that the add-on module is designed to identify a piercing process from an increase in an axial force measured by the force sensor to an at least approximately constant value caused by the compressed needle protection sleeve spring, and the add-on module having a sleeve-shaped module housing having a device receptacle for receiving the injection device and a handle for holding the injection device and the attached add-on module, a clear width of the device receptacle in the region of the handle being greater than an outer diameter of the injection device, so that, at maximum grip pressure, the device receptacle is not pressed onto the injection device and no friction or retaining forces can be transmitted in the longitudinal direction via the device receptacle between the injection device and the add-on module while bypassing the force sensor. characterized in that the add-on module is designed to identify a piercing process from an increase in an axial force measured by the force sensor to an at least approximately constant value caused by the compressed needle protection sleeve spring, and the add-on module having a sleeve-shaped module housing having a device receptacle for receiving the injection device and a handle for holding the injection device and the attached add-on module, a clear width of the device receptacle in the region of the handle being greater than an outer diameter of the injection device, so that, at maximum grip pressure, the device receptacle is not pressed onto the injection device and no friction or retaining forces can be transmitted in the longitudinal direction via the device receptacle between the injection device and the add-on module while bypassing the force sensor.