A medical syringe with an extendible plunger rod is provided which facilitates injection of a large volume of material, for example, a dermal filler. The rod may include an outer plunger rod, an inner plunger rod, a 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 prevents movement of the inner plunger rod relative to the outer plunger rod while both the inner and outer plunger rods are depressed, until the outer plunger rod has been depressed a first predetermined distance.
