Abstract: A protective device for a removable cannula used with a hypodermic syringe includes an elongated sheath open at one end. A piston in the sheath is slidable longitudinally between an extended position and a retracted position. The removable cannula is on one side of the piston, and the hypodermic syringe on the outer side. Means are provided for connecting the cannula to the syringe through a bore in the piston so the piston is between the cannula and the syringe barrel. With the piston in the extended position, the cannula extends from one end of the sheath, and the hypodermic syringe plunger extends from the other end so the assembly is ready to use in the same manner as a conventional hypodermic syringe. After use, the piston is moved to the retracted position so that the scarf end of the cannula is safely enclosed within the sheath.