Shifting Patterns of Injection Drug Use in San Francisco, United States: implications for Getting to Zero HIV Infections

BACKGROUND: We analyzed trends in drugs most often injected and their associations with HIV risk among people who inject drugs (PWID) in San Francisco, United States from 2018 to 2024. METHODS: Data are from National HIV Behavioral Surveillance serial cross-sectional surveys in 2018, 2022, and 2024. Participants were recruited through respondent-driven sampling. Trends in drugs most often injected were assessed using multinomial logistic regression analysis, adjusting for demographic differences across survey samples. Secondary analyses examined trends by age and race/ethnicity, and associations between risk behaviors and drugs most often injected. RESULTS: A total of 1,500 PWID were surveyed. PWID who injected methamphetamine most often rose from 23.9% in 2018 to 55.7% in 2024, while those injecting heroin most often dropped from 52.5% to 15.8%, and those injecting fentanyl most often rose from 0.9% to 11.3%. These shifts occurred earlier among younger and Hispanic PWID, while later for Black/African American PWID. PWID who most often injected methamphetamine had the highest prevalence of HIV (12.7%), sexually transmitted infections (16.3%), male-male sex (28.2%), and multiple sexual partners (55.3%). PWID who injected fentanyl most often had the highest prevalence of condomless sex (64.4%) and sharing of injection equipment (54.0%). HIV pre-exposure prophylaxis use was low (1.5%-3.0%) regardless of the drug most often injected. CONCLUSIONS: Methamphetamine supplanted heroin as the drug most often injected in San Francisco, with fentanyl increasing. These drugs' associations with HIV risk may increase transmission unless effective prevention methods such as PrEP can be rapidly scaled up.