Described is a drive unit (1) for a drug delivery device comprising a carrier (3), a plunger (4) arranged within the carrier (3) and slidable in a direction of a longitudinal axis (A), a drive spring (5) arranged for biasing the plunger (4) in a distal direction (D) relative the carrier (3), and a drive force adaption mechanism (22) for modifying a force exerted by the drive spring (5) on the plunger (4).