An injection device delivers a preselected dose of a drug, such as insulin, from a conventional syringe. The device includes a sleeve having a first opening and a cavity sufficiently large to house a barrel of a conventional syringe. The sleeve further includes a second opening through which a cannula of the syringe extends and which is sufficiently small to retain the barrel of the syringe. A plunger-engaging cap is positioned within the sleeve in sliding relationship and engages a plunger of the syringe. The sleeve includes a slot preferably extending longitudinally along the sleeve and the plunger-engaging cap includes a pin extending outwardly from the cap's surface and positioned within the slot. A stop is positionable at a predetermined location along the slot defining a preselected dosage such that when the plunger-engaging cap is pushed into the sleeve, the plunger of the syringe is pushed within the barrel of the syringe with the cap and plunger stopping when the pin of the cap engages the stop thereby expelling the preselected dose of the drug.