Various embodiments of a "smart" drug delivery system are provided which includes an add-on module and a reusable or disposable drug pen. Upon attachment to the pen, the add-on module may: determine dosage selected, injection of selected dosage, duration of injection, time of injection, whether the pen has been primed or shaken to thoroughly mix up insulin mixtures, transmit information relating to insulin dosage and injection to a data management unit, provide reminders, error warning or messages on improper usage or reusage of needles, track amount of drug remaining on board the pen or duration of usage of pen with respect to expiry of the drug on board, or provide an audible alarm for locating misplaced pen and module.
