The United States (US) continues to face a decades-long drug overdose mortality epidemic that has occurred in four main waves (Ahmad et al., 2022, Ciccarone, 2019, Ciccarone, 2021, Hedegaard et al., 2021, Rudd et al., 2016). The first wave was characterized by increased prescribing of opioid pills by healthcare providers starting in the 1990s; the second wave was attributed to transitions to cheaper and more accessible heroin beginning in the mid-2000s; the third wave was attributed to the introduction of illicitly made fentanyl into illicit drug markets in the early-to-mid 2010s (Frank & Pollack, 2017); and the emerging fourth wave is characterized by increased mortality involving co-use of fentanyl and stimulants from the mid-2010s to now (Friedman & Shover, 2023). Fentanyl poses a public health concern because it is much more potent than heroin with a faster onset, shorter duration of effect, and symptoms of heavier sedation (Ciccarone et al., 2017, Duhart Clarke et al., 2022, Fairbairn et al., 2017, Mayer et al., 2018, Somerville et al., 2017). Research suggests that the shorter duration and more potent effect of fentanyl, compared to heroin, could lead to high frequency use and injection along with greater syringe sharing and overdose risk (Ciccarone et al., 2017, Lambdin et al., 2019, Mayer et al., 2018, Somerville et al., 2017). The illicit nature of fentanyl production and distribution yields variability in the strength and potential toxicity of drug samples, including fentanyl, obtained in illicit markets, which introduces further uncertainty into potential toxicity and risks related to its use.

As a result of fentanyl’s high potency and risk for unintentional overdose, fentanyl-involved overdose fatalities have been steadily increasing. Beginning in 2016, fentanyl became the most common drug detected in drug overdose decedents in the US (Hedegaard et al., 2018). According to the Centers for Disease Control and Prevention (CDC), over 75% of drug overdose deaths in 2021 involved an opioid (Drug Overdose Deaths., 2021). Synthetic opioids, specifically, accounted for nearly 88% of all opioid-involved deaths in 2021. Although there was a 22% increase in synthetic opioid-involved death rates nationally from 2020 to 2021 (Drug Overdose Deaths., 2021), there was also a shift in the geographic footprint of synthetic opioid-involved overdose deaths, from East to West. During those years, eight states west of the Mississippi, including California, reported significant increases in synthetic opioid-involved mortality (Mattson et al., 2021, Shover et al., 2020).

A large body of research shows that injection drug use can lead to an increased likelihood of bacterial and viral infections, including transmission of HIV (Des Jarlais and Friedman, 1987, Des Jarlais et al., 2020), viral hepatitis (Girardi et al., 1990, Rashti et al., 2020, Sharhani et al., 2021), skin and soft tissue infections (SSTIs) (Binswanger et al., 2000, See et al., 2020), and infective endocarditis (Kadri et al., 2019). Because of the health complications related to injection drug use, many countries (e.g., Australia, Scotland, and Spain) have encouraged people to switch from injection to other modes of administration (Bridge, 2010, Dolan et al., 2004, Wodak, 1997). These systemic public health interventions have attained minimal measurable success. Although a substantial portion of drug use risk is a result of how substances are used, modes of drug administration depend on many factors including available drug types, their cost and stability within the illicit drug market, as well as people’s preferences, motivations, and available resources (Harris et al., 2015, Swift et al., 1999, Syvertsen et al., 2016).

Although injection has historically been the predominant mode of heroin use in the US, the introduction of illicit fentanyl in West Coast drug markets engendered an organic transition from injecting heroin to smoking fentanyl (Kingston et al., 2022, Kral et al., 2021). Despite the documented risk factors inherent in illicit fentanyl use relative to heroin, the transition from injecting to smoking opioids could reduce health complications associated with injection such as HIV, viral hepatitis, and SSTIs. It remains an open question whether mode of fentanyl administration is associated with overdose risk.

As fentanyl availability and use grew in California, fentanyl smoking was increasingly reported and programs working to reduce health complications of drug use began providing sterile smoking supplies, including glass pipes and aluminum foil (Fitzpatrick et al., 2022, Singh et al., 2022). This was in part supported by studies from Canada, which found positive impacts associated with smoking supply distribution for people who use crack cocaine. One study in Vancouver found that people obtaining pipes through health service organizations (compared to those obtaining pipes only through other sources) were significantly less likely to report health problems from smoking crack such as lesions, cuts, burns, and infectious diseases (Prangnell et al., 2017). Another study explored the uptake of ‘safer crack use kits’ in Victoria and found that participants described a lesser need to share equipment or commit crimes to obtain adequate equipment as well as increased health awareness and personal/community safety (Ivsins et al., 2011). Finally, Leonard et al. (2008) evaluated the impact of smoking supply distribution in Ottawa and found that there was a significant decrease in injecting after implementation of the initiative, in addition to a reduction of HCV- and HIV-related risks.

The introduction of safer smoking supply distribution by many programs was met with opposition and legislation from the US Congress in February 2022, and led to US President Biden disallowing the purchase of safe smoking equipment with federal money. Given the attention, funding, and services that seek to reduce overdose mortality from fentanyl, it is important to understand whether a potential solution is for more people to switch from injecting to smoking fentanyl. As such, we set out to conduct a study to compare health and healthcare utilization outcomes among people who inject or smoke fentanyl in California (CA), US.