One-third of people who injected drugs at baseline transitioned to smoking only.

The risk of smoking opioids more often than injecting them increased over time.

Smoking methamphetamine more often than injecting it markedly increased over time.

Unregulated injection drug use is likely being supplanted by smoking drug use.

Amidst an increasingly toxic drug supply in North America, people who inject drugs may be transitioning to smoking them. We aimed to assess changes in injecting and smoking opioids and methamphetamine among a cohort of people who inject drugs from San Diego, California.

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.

Of 362 participants, median age was 40 years; a minority were female (29%), Hispanic/Latinx/Mexican (45%), and housed (33%). Among this cohort, of whom 100% injected (and 84% injected and smoked) in period one (October 2020-April 2021), by period five (November 2022-April 2023), 34% only smoked, 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) 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) compared to period one. Risks for smoking heroin and methamphetamine more frequently than injecting these drugs increased across all periods.

Opioid and methamphetamine injection declined precipitously, with notable increases in smoking these drugs. Research is needed to understand the health consequences of these trends.

Injection drug use

Epidemiology

Fentanyl

Smoking

Methamphetamine

Heroin

Data is available upon reasonable request to the Principal Investigator of La Frontera, Dr. Steffanie Strathdee ([email protected]).

© 2024 The Author(s). Published by Elsevier B.V.