Injecting drugs has been a major public health problem in the US. Indeed, more than 6.6 million people ages 13 or older inject drugs in their lifetime while over 770,000 inject drugs annually (Lansky et al., 2014). Drug injection may lead to serious health consequences, such as blood-borne infections (e.g., the human immunodeficiency virus [HIV], viral hepatitis) especially when needles/syringes are reused or shared (Armstrong, 2007; Bartholomew et al., 2020; Leyva et al., 2020). A recent community-based study found that over 30% of people who injected drugs (PWID) shared syringes, raising concerns for heightened infection risks among PWID (Gicquelais et al., 2019). In fact, a sharp increase in the HIV prevalence among PWID has been reported since 2010, which may be an indication of more prevalent unsafe injection practices (Williams et al., 2020). Also, drug injection is strongly associated with exposure to fentanyl, a key driver of the current deadly drug epidemic (Hayashi et al., 2018). Thus, understanding long-term drug injection trends and clinically relevant and distinct profiles of PWID can be an essential step to addressing the HIV and fatal drug overdose problems in the US.

Evidence indicates that PWID are not a monolith and that possibly important heterogeneities exist by types of drugs injected. For instance, PWID often engage in other substance use/misuse behaviors (using different administration methods) for various reasons (e.g., managing undesirable effects of particular drugs), with risks of alcohol and other illicit drug use disorders (Gicquelais et al., 2019; Jones, 2018; Lankenau et al., 2010; Valente et al., 2020). Furthermore, PWID who initiate injection drug use with cocaine tend to have prior substance misuse history compared to those who initiate injection with heroin, although both groups are more likely to be involved in injection at an earlier age and transition to other drugs (Lankenau and Clatts, 2004; Roy et al., 2002). On the other hand, PWID who inject methamphetamine report elevated economic and behavioral health problems, including homelessness, criminal justice involvement, needle sharing, and risk-taking sexual behaviors relative to those who inject other drugs (Lankenau et al., 2010; Rezaei et al., 2020). Also, studies found that those who inject multiple types of drugs are associated with the greatest substance overdose risks (Schneider et al., 2020).

Despite its adverse consequences and complications with helping PWID, extant research provides a limited understanding of how to effectively prevent and treat drug-injecting behaviors and sequelae. In particular, few studies have investigated the recent drug injection trends and patterns since 2002 (Armstrong, 2007; Han et al., 2021). Key policy changes such as the federal funding ban on needle exchange programs as well as the fast-evolving drug epidemic are some of the factors that might have affected these trends (Des Jarlais et al., 2015; Showalter, 2018). Additionally, studies that shed light on the potential heterogeneity within PWID in terms of the types of drugs injected and associated behavioral health and treatment needs are limited especially at the national level. Most drug injection literature is based on samples from specific geographical locations, such as certain metropolitan areas (Ceasar et al., 2021; Eckhardt et al., 2017; Fong et al., 2021; Hackman et al., 2020; Williams et al., 2020; Williams et al., 2021) or rural areas where the opioid crisis is prevalent (Allen et al., 2019; Baker et al., 2020; Stone et al., 2021; Surratt et al., 2020; White et al., 2021), limiting generalizability to inform national-level responses to drug injection and associated health problems.

To fill the aforementioned gaps, the present study examines up-to-date drug injection trends/patterns, and critically needed socioeconomic and behavioral profiles of the subgroups among a nationally representative sample since 2002. Specifically, after estimating the drug injection trends (separately by type of drugs injected) among adults aged 18–64 since 2002, we identified patterns of drugs injected and other substance use/misuse behaviors among PWID and their sociodemographic profiles using a latent class analysis (LCA) method. Then we tested associations between distinctive drug injection patterns and comorbid behavioral health problems (including alcohol and illicit drug use disorders), treatment receipt, and needle use-related behaviors as reported by prior empirical studies on drug injection (Han et al., 2020; Jones, 2018; Novak and Kral, 2011; Ropelewski et al., 2011).