Green’s Dictionary of Slang

(US drugs) one who injects into the vein, thus a veteran addict.

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 127/1: vein-shooter One who pumps narcotics directly into the blood-stream; hence a hardened addict.
1959		J.E. Schmidt Narcotics Lingo and Lore.