He's a 'fiend' for physical fitness. [=he is very devoted to physical fitness]
She's been working 'like a fiend'. [=working very hard]
a dope/drug 'fiend' [='addict']
(..) You think I don't know how to read between the lines? That was your man Thackery, wasn't it? Your top doctor's turned out to be a fucking cocaine fiend. You see what dire straits we're in when a man of Dr. Thackery's reputation must resort to burglary for the sake of his patients. (..)2014 TvShow: The Knick Title: The Golden Lotus Season: 1 Episode 9