A medication delivery pen assembly having a pen body and a needle assembly with a hub connected to the pen body, a needle cannula extending through the hub and a shield having a tubular portion telescopingly disposed about at least a portion of the hub. The tubular portion includes an internal surface, at least one projection (54) extending inwardly from the internal surface, and the hub having at least one externally extending projection (52). The shield is axially displaceable relatively along the hub distally from a first position, to a second position, where the distal end of the needle cannula is enclosed by the shield. Upon the shield being displaced to the second position, the at least one projection on the hub is deflectibly urged passed the at least one projection on the tubular portion so that subsequent distal-to-proximal movement of the shield relative to the hub is hindered by interengagement of the projections.
