come off, v.² — Green’s Dictionary of Slang

come off v.²

[one has been ‘on’ heroin, ‘on’ cocaine etc]

(drugs) to stop using a given (addictive) drug.

1952		(con. 1948) G. Mandel Flee the Angry Strangers 314: [of marijuana] But an easy charge to come off of.
1963		J. Rechy City of Night 131: You always ack like youre hyped up or comin off.
1993		I. Welsh Trainspotting 14: You’ve got tae know what it’s like tae try tae come off it before ye can actually dae it.