tweaker

1 :  one who makes minor adjustments to something (such as electronic equipment) to improve its performance or effectiveness :  a person who tweaks something This set-it-and-forget-it approach may seem inconceivable to tweakers, but this isn't a piece of gear you want to be playing around with on a daily basis. — Ron Goldberg, Home Theater, May 1997

2 slang :  a person who illicitly uses methamphetamine and especially crystal meth When he got like this, his neighbors would scream at him and threaten to call the cops, but they were tweakers too. — Joseph Wambaugh, Finnegan's Week, 1994 The community, it turned out, was rife with methamphetamine users—“tweakers”—who would load up on the homemade speed and stay up for three days grinding their teeth and taking apart carburetors for fun. — Dan McGraw, U.S. News & World Report, 2 Mar. 1998