Green’s Dictionary of Slang

(drugs) to draw up blood into an eyedropper or syringe while injecting a vein; thus as n., the mix of heroin/water/blood drawn up into the syringe.

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 108/1: To register. (tr. or intr.) To apply a very slight suction to the hypodermic needle before injecting the narcotic; by this method the addict assures himself by the showing of blood in the glass that the needle has struck the vein and avoids blowing his shot.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.
1968		‘Sl. of Watts’ in Current Sl. III:2.
1971		E.E. Landy Underground Dict. (1972).
1992		Bentley & Corbett Prison Sl. 82: To ‘register’ is to ensure the needle is in the vein. This is accomplished by drawing a small amount of blood into the syringe prior to injecting the drug.
1995		J. Stahl Permanent Midnight 104: I slipped in the needle [...] got a nice red register.
2001		(con. 1975–6) E. Little Steel Toes 104: After countless tries and much bleeding she gets a register and sends salvation up a vein in her leg.