Sentence:  Then I saw the more advanced narcotic addicts, who shot unbelievable doses of powerful heroin in the main line – the vein of their arms; the hysien users; chloroform sniffers, who belonged to the riff-raff element of the dope chippeys, who mingled freely with others of their kind; canned heat stiffs, paragoric hounds, laudanum fiends, and last but not least, the veronal <WSD>heads</WSD>. 
Meaning of the word marked with <WSD> tags:  A heavy or habitual user of illicit drugs.