Multilevel Drivers and Constraints of Shifts from Injecting to Smoking Opioids and Stimulants: A Qualitative Study from the San Diego-Tijuana Border Region

Background Amidst declines in injecting opioids and stimulants across North America, we sought to explore multilevel drivers and constraints of shifts from injecting to smoking these substances among people with a history of injection drug use. Methods From October-December 2024, we conducted semi-structured qualitative interviews with individuals with histories of injection drug use in San Diego, California, and Tijuana, Baja California. Eligibility included past-month heroin, fentanyl, or methamphetamine use by injection and/or smoking. Informed by the social ecological model, we used thematic analysis to examine the multilevel determinants of shifts in routes of drug use. Results Among 36 participants (50% in San Diego), median age was 40 years, 61% were male and 58% were unsheltered. In the past month, 81% both injected and smoked heroin, fentanyl, and/or methamphetamine, 17% smoked only and 3% injected only. Rather than complete substitution, most participants described smoking as their primary route alongside intermittent or situational injection, driven by: (1) desires to avoid fentanyl-related injection harms; (2) greater ease, discretion, and lower stigma of smoking; and (3) drug-supply conditions and fentanyl–methamphetamine co-use. Sustained movement toward smoking was constrained by (4) embodied injection practices and beliefs about desired effects or withdrawal relief and (5) inconsistent access to smoking equipment. Conclusion Individual shifts toward smoking were often intentional (e.g., to reduce health harms) but depended on drug market conditions and material environments. Expanding access to smoking equipment while maintaining syringe coverage for ongoing injection may reduce harms in the context of fentanyl–methamphetamine co-use.