AN IMPROVED PEN-TYPE DRUG DELIVERY DEVICE IS PROVIDED HAVING A PISTON ROD (20) THAT INCORPORATES ON ITS PROXIMAL END AT LEAST ONE GUIDEPOST (24) THAT IS CONFIGURED FOR TRANSMITTING AXIAL AND ROTATIONAL FORCES FROM A DRIVE MECHANISM. THE DEVICE COMPRISES A DRUG DELIVERY DEVICE HOUSING AND A MEDICAMENT CONTAINED IN THE DRUG DELIVERY DEVICE HOUSING. THE MOST ILLUSTRATIVE DRAWING ACCOMPANYING THE ABSTRACT: FIG. 2