# The changing contribution of fentanyl use to non-fatal overdose among a cohort of people who inject drugs in San Diego, California: A longitudinal assessment

## TL;DR

This study examines how fentanyl use has contributed to non-fatal overdoses among drug users in San Diego over time.

## Contribution

The study provides a longitudinal assessment of fentanyl's role in non-fatal overdoses in a specific U.S. region.

## Key findings

- Fentanyl use increased while non-fentanyl opioid use decreased among participants over the study period.
- Fentanyl use was significantly associated with non-fatal overdose risk at multiple time points.
- Fentanyl contributed to 35% of non-fatal overdoses in the cohort.

## Abstract

Fentanyl has driven overdoses across the U.S. in heterogeneous ways, with local variation in non-fatal overdoses remaining under-described. We sought to estimate the contributions of fentanyl to non-fatal overdose among people who inject drugs (PWID) in San Diego, CA, during a period of substantial drug supply evolution.

Using convenience sampling, we recruited PWID living in San Diego into a longitudinal cohort study involving biannual assessments of opioid use and non-fatal overdose from 10/2020–05/2024. Multivariable Poisson regression assessed longitudinal associations between past six-month fentanyl use and non-fatal overdose risk. We calculated the confounder-adjusted longitudinal population attributable fraction (LPAF) to quantify fentanyl’s contribution to non-fatal overdoses over time.

Among 204 participants, the median age was 40.5 years, 73.0% were male, and 68.1% experienced homelessness. Fentanyl use increased from 51.0%(104/204) at Visit 1(10/2020–10/2021) to 62.4%(68/109) at Visit 6(10/2023–05/2024) while non-fentanyl opioid use decreased from 39.7%(81/204) to 5.5%(6/109). Past six-month non-fatal overdose decreased from 22.5%(46/204) to 15.6%(17/109) overall, and from 38.5%(40/104) to 19.1%(13/68) among those using fentanyl. Significant associations between fentanyl use and non-fatal overdose risk were observed at Visits 1 (Adjusted Relative Risk (ARR) 5.97, 95%CI: 2.37–15.00) and 5 (ARR 4.19, 95%CI: 1.17–15.00). The percentage of non-fatal overdoses contributed by fentanyl use in this cohort (LPAF) was 35%(0.35, 95%CI: 0.17–0.52).

The contribution of fentanyl to non-fatal overdoses varied over time in our cohort. Disentangling the role of contextual factors such as opioid tolerance, fentanyl potency, and behavioral changes will support intervention design amid increasingly toxic drug supplies.