Green’s Dictionary of Slang

flushing n.

(UK drugs) the act of drawing blood into the syringe when injecting a narcotic.

[UK]Glatt et al. Drug Scene in Great Britain 116: Flushing – draw up and re-inject blood.
[US] cited in Spears Sl. and Jargon of Drugs and Drink (1986).
[UK]J. Morton Lowspeak.