A method of amplifying an injection force of a drug delivery device, comprising the steps of storing energy in an additive force device, connecting the additive force device to the drug delivery device, setting a dose of medicament to be administered, and selectively releasing the stored energy of the additive force device to amplify an injection force of the drug delivery device. The first circuit generates a second DC voltage from a first DC voltage and includes a transformer. The carboxylate nucleating agent is selected from the group consisting of sodium benzoate and hydroxyl aluminum para-tertiary butyl benzoate, and the phosphate nucleating agent is selected from any one of sodium 2,2\u2032-methylene-di phosphate and aluminum bis2,2\u2032-methylene-di phosphate. A controller in the drive unit 11 includes a motor monitoring unit for monitoring an operational status of the motor, and a motor control unit for limiting a drive signal supplied to the motor based on a monitoring result provided by the motor monitoring unit, in order to prevent an overload state of the motor.