A combination of survey and ethnographic research was used to investigate what injectors have labeled as the “jelling-up” of their heroin: insolubility leading to the clogging of needles/syringes. A total of 856 interviews, were conducted in two inner-city Chicago communities. Three separate samples were drawn from heroin injectors over a period of 18 months. In each sample, 85% (n = 39), 72% (n = 417) and 81% (n = 400) of the respondents reported that their heroin clogged their needles/syringes. In each of these respective samples, 55%. 28%, and 19% of the respondents reported that they had shared needles/syringes with others because their “works” had clogged. The paper explores the conditions under which inappropriate diluent/adulterants are used in the “cutting” of heroin. A complex of factors helps explain the use of improper cuts; these include: oligopolistic market conditions engendered by gangs, ignorance of the proper diluents/adulterants. and the creation of dual markets (intranasal and injecting) for heroin.