Abstract: The present invention provides a drug delivery device (1) comprising a drug delivery unit (10) comprising a dose expelling mechanism operable to expel a dose of drug from a drug reservoir (11), a cap (20) configured for dismountable mounting over a portion of the drug delivery unit (10), the cap (20) comprising a cavity (25, 26) for reception of the portion of the drug delivery unit (10), a seal (40) adapted to provide a sealing in an interface between the cap (20) and the drug delivery unit (10), a sensor system (30) configured to monitor internal pressure in the cavity (25, 26) and to register an event in response to sensing a change in the internal pressure which numerically exceeds a specified threshold level over a period of less than 5 seconds, and energy means (50) for powering the sensor system (30).