Changes in injecting versus smoking heroin, fentanyl, and methamphetamine among people who inject drugs in San Diego, California, 2020 to 2023.

Background: Amidst a rapidly evolving drug supply in North America, people who inject drugs may be transitioning to smoking them. We aimed to assess changes in injecting and smoking heroin, fentanyl and methamphetamine among a cohort of people who injected drugs at baseline from San Diego, California. Methods: Over five six-month periods spanning October 2020-April 2023, we assessed prevalence of injecting and smoking opioids or methamphetamine and whether participants used these drugs more frequently by smoking than injecting. Multivariable Poisson regression via Generalized Estimating Equations was used to examine time trends. Results: Of 362 participants, median age was 40 years; most were male (72%), non-Hispanic (55%), and unhoused (67%). Among this cohort, of whom 100% injected (or injected and smoked) at baseline, by period five (two years later), 34% reported only smoking, while 59% injected and smoked, and 7% only injected. By period five, the adjusted relative risk (aRR) of injecting opioids was 0.41 (95% Confidence Interval [CI]: 0.33, 0.51) compared to period one, and the aRR for injecting methamphetamine was 0.50 (95% CI: 0.39, 0.63). Compared to period one, risks for smoking fentanyl rose significantly during period three (aRR=1.44, 95% CI: 1.06, 1.94), four (aRR=1.65, 95% CI: 1.24, 2.20) and five (aRR=1.90, 95% CI: 1.43, 2.53). Risks for smoking heroin and methamphetamine more frequently than injecting these drugs increased across all periods. Conclusions: Opioid and methamphetamine injection declined precipitously, with notable increases in smoking these drugs. Research is urgently needed to understand the health consequences of these trends.