Disposable safety syringe

A safety syringe 10 includes a generally tubular body 14 having a needle end 18 and plunger end 22, needle 38 and retractable needle seat 30, two-way valve 36, plunger 42, stopper 62, piston 52, and rear plunger seal 44. During use, the plunger may be moved to create a vacuum in the plunger. The user then moves the plunger toward the needle end, pressurizing a vial, then pulls backward to a selected position corresponding to the desired volume of fluid to be withdrawn. At the conclusion of the injection stroke, the piston is disengaged from the plunger. The vacuum within the plunger retracts the needle safely into the tubular body.