Green’s Dictionary of Slang

1. a poisoner.

1913

J. London John Barleycorn (1989) 78: Oh! John Barleycorn is a wizard dopester. Brain and body, scorched and jangled and poisoned, return to be tuned up by the very poison that caused the damage.

2. a drug user or seller.

1925		O.O. McIntyre New York Day by Day 1 Oct. [synd. col.] Overdose dopesters who suddenly go berserk and in their frenzy kill.
1938		J. Rice Man Who Insulted Somersville 45: A dopester seldom drinks and most drinkers have not yet taken to dope .
1942		Berrey & Van den Bark Amer. Thes. Sl.
1959		J.E. Schmidt Narcotics Lingo and Lore.