An adjustment mechanism for adjusting the amount of a substance to be expelled by a substance delivery device, the adjustment mechanism comprising a plunger (140) a stop member (144) mounted with respect to the plunger for limiting the substantially unobstructed travel of the plunger and means for rotating the plunger about its longitudinal axis, wherein the stop member is mounted with respect to the plunger and arranged to cooperate with a lateral guide means such that it travels longitudinally along the axis of the plunger on rotation of the plunger.
