Abstract: A feed drug injector comprises a nipple device, a nipple plug inserted in the nipple device, a front end of a cylinder inserted in the nipple plug, and a push rod inserted in the cylinder. The nipple device has a base and a nipple head. Two inner recesses are formed on the inner periphery of the base. A passage is formed in the nipple head. A Y-shaped hole is formed in the front of the passage. A bevel is formed on the front surface of the nipple head. Three apertures which are formed on the bevel and two front sides of the nipple head communicate with the Y-shaped hole. The nipple plug has a disk with an inner thread, a Y-shaped bar, and three circular holes formed on the disk. Two protrusions are disposed on the outer periphery of the disk. The protrusion engages with the inner recess. The cylinder the disk.