Green’s Dictionary of Slang

(drugs) benzocaine and mannitol, chemicals used in the manufacture of crack cocaine.

1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 17: Max’s recipe calls for [...] 40 grams of ‘comeback,’ an adulterant that has allowed Max to double his profits from crack.
1994		R.P. McNamara Times Square Hustler 82: The processing also allows for additional adulterants to be added. Ingredients such as lidocaine or benzocaine, known as ‘comeback’ look and taste like cocaine but produce no high.
2001		ONDCP Street Terms 6: Comeback — Benzocaine and mannitol used to adulterate cocaine for conversion to crack.