Since 2001, heroin-related overdose deaths in the United States have risen six-fold, a rise unaccounted for by the expanding user population. Has heroin become a more dangerous drug? Reports of fentanyl and its analogs, often concealed in or sold as heroin, have also increased sharply. This article investigates heroin injectors' perceptions and experiences of changes in the heroin supply in the East Coast city of Baltimore, Maryland, currently facing an epidemic in heroin- and fentanyl-related overdose deaths. Unusually, Baltimore's heroin market is divided between two types: "Raw," believed to be Colombian in origin and relatively pure, and the more adulterated "Scramble" (raw heroin traditionally blended with quinine and lactose). Users reported that Scramble heroin, while gaining market share, has become a highly unstable product, varying dramatically in appearance, intensity of onset, duration of action, and effect. Some considered that Scramble was no longer "heroin," but was heavily adulterated or even replaced, mentioning fentanyl, benzodiazepines, and crushed opioid pills as additives. There was intense awareness of overdose as a present danger in users' lives, which they linked to the recent adulteration of the heroin supply. Responses to this perceived adulteration varied, including information gathering, attraction, avoidance, taking precautions, and acceptance.