An extendible plunger rod for use with a syringe is provided. Example plunger rods may include an outer plunger rod with a proximal end and a distal end, the outer plunger rod having an inner cavity an inner plunger rod, housed at least partially within the inner cavity of the outer plunger rod a compression spring disposed within the inner cavity of the outer plunger rod and between the proximal end of the outer plunger rod, and the inner plunger rod, the compression spring configured to exert force tending to push the inner plunger rod in a direction distal to the outer plunger rod and a locking mechanism, the locking mechanism configured to prevent movement of the inner plunger rod relative to the outer plunger rod while both the inner and outer plunger rods are depressed into a syringe body, until the outer plunger rod has been depressed at least a first predetermined distance into the syringe body.
