A nebulizer comprising an aerosol generator and a titration mechanism comprising a dosing mechanism, a screw assembly, a plunger and an actuator, wherein the dosing mechanism is rotatably mounted in the nebulizer and the plunger is mounted in the nebulizer to travel along a longitudinal axis substantially perpendicular to the plane of rotation of the dosing mechanism. The actuator is mounted to the nebulizer to travel in the direction of the longitudinal axis. The screw mechanism allows the rotation of the dosing mechanism to fix a variable distance of travel of the plunger based on the selected dose, without moving the plunger longitudinally. This allows the user to verify and if necessary correct the selected dose prior to any drug being released from the vial. Operation of the actuator causes longitudinal movement, through the screw mechanism, to the plunger. The actuator always moves a fixed distance along the longitudinal axis, while the plunger always moves a variable distance depending on rotation of the dosing mechanism to a selected dose.