Track Runner
Pretty much the same thing as a heroin shooter. The arm is the 'track', the addict is the 'runner'.
Nurse: He has some small injections between his junk and legs.
DEA agent: He is a track runner.
Nurse: A what?
A: He is a heroin addict.
DEA agent: He is a track runner.
Nurse: A what?
A: He is a heroin addict.