Heroin can be taken into the human body in a wide variety of ways, including snorting or sniffing powder or heroin solution (intranasal use), inhalation of the heated vapors (“chasing”), orally and as anal suppositories (“plugging”). Injecting, whether intravenous, subcutaneous or intramuscular, is the method of administration carrying the highest risk for multiple types of infections, overdoses and their complications [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. However, despite these risks, injection appeals to users because it is the most efficient, cost-effective method of use, and intravenous injection in particular has the most intense onset of effect (“rush”). These can be important considerations when users face rising tolerance to heroin’s effects [16, 17].

Heroin “source form”—its chemical characteristics based on country of origin—also influences its mode of use [18, 19]. Bioavailability, or the measure of a drug dose that achieves circulation in the bloodstream, in turn reflects the mode of use. The three main source forms of heroin in the US—Colombian-sourced powdered heroin and both Mexican-sourced black tar heroin and powdered heroin—are all hydrochloride salts in contrast with the Afghanistan-sourced base heroin, aka “smoking heroin,” found in Europe [20]. The hydrochloride (HCL) form has relatively low bioavailability when smoked compared with base heroin [21]. Heroin HCL may be potentiated when smoked by the addition of caffeine [22], but research indicates no significant culture of heroin smoking in the US [23, 24] likely due to available source forms. Use outside of controlled conditions [25] as well as poor chasing technique [26] may also negatively affect the bioavailability of smoked heroin.

Another non-injection option to chasing/smoking is intranasal use. Intranasal heroin administration has been found to be an effective drug delivery route. It has been theorized that the lower price of US heroin in the early 1990s rather than its higher purity—as well as fears of contracting HIV—may have led to increased usage of intranasal administration [27]. US data on hospitalizations in the 1990s suggests an increase in heroin inhalation and smoking, primarily in the northeast during this period [28].

For many decades, overdose has been the primary cause of deaths among people injecting drugs [6, 9, 29,30,31,32], but since 2001, heroin-related overdose deaths have risen sixfold in the United States [33]. Heroin-related overdose intensified after 2010, with overdose mortality rates tripling between 2010 and 2014 from 1.0 to 3.4 per 100,000 [34]. The increase in heroin-related deaths has been paralleled by a rise in the death rate attributed to synthetic opioids other than methadone. The age-adjusted rate of overdose deaths attributed to synthetic opioids other than methadone, which includes fentanyl and its analogs, doubled between 2015 and 2016, rising to 6.2 per 100,000 [35]. Evidence from the US Drug Enforcement Agency indicates this increase is being primarily driven by illicitly manufactured fentanyl rather than diverted pharmaceutical fentanyl [36, 37]. While some have focused on the potency of fentanyl [38, 39] in increasing the risk for overdose, others have highlighted the risk of vicissitudes in the purity of fentanyl and its analogs in combination with heroin [40, 41].

In the face of overdose-related morbidity and mortality, numerous public health interventions have tried to address the risks of overdose over the last 20 years. These strategies include overdose education and peer naloxone distribution [42,43,44]. Promoting what has been termed “reverse transition” from injecting to a non-injecting route of administration constitutes another approach to reducing overdose risk [45].

One study of UK-based “chasers” and injectors found that while preferred administration routes were likely to persist for years and multiple route transitions were uncommon, 16% of their sample had reverse transitioned [46]. Another study of reverse transitions among heroin and cocaine users in New York City found that transition from injection to non-injection use appeared to be a relatively stable, long-term behavior change [47]. Among other transitions, several harm reduction organizations in the UK advocated for switching to rectal administration ("plugging"), the so-called “up your bum” campaign, but it is unclear how widespread this practice actually is [48, 49].

Given the low rate of “chasing” among US heroin users, one practice some injectors have adopted to reduce the risk of overdose is the use of “tester shots” or “test shots.” This entails injecting a small quantity of a drug sample in order to assess its potency qualitatively before deciding whether to inject the remainder of the dose [50,51,52]. A tester shot requires practitioners to perform two injections, a factor that may act as a barrier to the widespread adoption of this behavior, as venous access can become challenging for long-term injectors and higher injection frequency increases the risk of injection-related complications including viral transmission [2, 53].

Circumventing the multiple injection problem of the tester shot, “slow shots” allow the injector to insert the needle, release the tourniquet, and very slowly inject while assessing the embodied effect with the needle still in the vein [54]. A further refinement of this is the “graduated” or “controlled” shot where the needle remains in the vein, but the dose is divided into three with a pause for assessment of its effects between each third [55]. The term “tester shot” is used in this paper to encompass all forms of partial dose injection, while “drug sampling” refers to any method of administration involving a partial dose.

There is a dearth of qualitative research on behavioral adaptions that current heroin injectors are making with respect to the ongoing fentanyl adulteration crisis in the US. In this paper, we present findings from ethnographic fieldwork trips in 2015 and 2016 to Baltimore, Maryland; Worcester, Lowell, and Lawrence, Massachusetts; Nashua, New Hampshire; San Francisco, California; and Chicago, Illinois on embodied methods of gauging opioid strength that injection drug users in these areas are taking to prevent overdose. With the exception of California, where solid black tar heroin dominates, all these states have powder sourced from Mexico or Colombia and are suffering rising heroin- and fentanyl-related deaths.

In 2016, Baltimore lost 454 people to heroin-related overdoses, up from 260 the previous year, and 419 people to fentanyl-related overdoses, up from 120 in 2015 [56, 57]. The figures available for Massachusetts do not distinguish between heroin and prescription opioids. In 2016, among the 1374 individuals whose deaths were opioid-related (including heroin) and a toxicology screen was also available, 1031 of them (75%) had a positive screen result for fentanyl, an increase from 754 (57%) in 2015, although this may depend on the frequency of toxicological screening [58, 59]. Drug overdose deaths in New Hampshire increased by 1629% between 2010 and 2015, largely as a result of fentanyl. Hillsborough County, the location of Nashua, where 43.6% of the fentanyl deaths occurred, was most affected by these overdose deaths. The rate of death caused by fentanyl, heroin, and other opioids rose sharply between 2015 and 2016 [60]. In Chicago in 2016, there were 487 overdose deaths involving heroin and 420 involving fentanyl, both rising from the previous year [61]. In San Francisco in 2016, 41 deaths were attributed to heroin overdose and 22 attributed to fentanyl, doubling from the previous year [62, 63]. Data for 2017 are not available for all sites, but Baltimore showed a small decline in heroin-related deaths (from 334 in January to September 2016 to 305 in the same period of 2017) but a much larger increase in fentanyl-related deaths (from 276 January to September 2016 to 427 in the same period in 2017) [64]. Massachusetts experienced a modest decline in overall opioid deaths in 2017 but an increase in the proportion screening positive for fentanyl (to 83%) [65].