Abstract: The invention provides an adaptor for a medicament delivery device is configured to enable a set dosage of a medicament to be drawn into and/or expelled from the device, the adaptor including: a body configured to be coupled to the device; an attachment portion movable with respect to the body and configured for attachment with a first part of the device; an engaging portion movable with the attachment portion; and a receiver configured to receive a movement constraint guide (MCG) and facilitate engagement between the engaging portion and the MCG; wherein the MCG includes one or more physical features defining at least an aspect of the set dosage; and wherein activation of said first part of the device to draw and/or expel the medicament is stopped when the set dosage is reached by engagement between the engaging portion and at least one of the one or more physical features of the MCG.