A method and system are disclosed for driving a plunger in a drug reservoir. In some embodiments, a variable friction element interconnects between a pushing shaft and a wall of the reservoir. For example, rotation of a driving element in a first direction relative to the pushing shaft may advance the pushing shaft in the reservoir. The friction element may have increased friction resistance to rotation of the pushing shaft in the first direction in comparison to frictional resistance to advancing the pushing shaft. For example, the increased friction may result from wedging a radial element between the pushing shaft and the wall of the reservoir.