The present invention relates to an add-on device for a metered dose inhaler, an observance improvement system, and a method for improving observance of use in metered dose inhalers, the add-on device comprising an observance system housing component comprising an observance system with at least one pressure sensor; a mouthpiece component configured to fit, surround and removably engage with an exterior surface of a mouthpiece outlet provided on the metered dose inhaler; wherein said observance system housing is configured to fit and removably engage with said mouthpiece component; and said mouthpiece component is specifically adapted to conform to the exterior surface of the mouthpiece outlet of the metered dose inhaler without obstructing delivery of a dose of drug through said outlet.