Introduction

Xylazine, an α₂‑adrenoceptor agonist and veterinary sedative, has increasingly appeared as an adulterant in illicit fentanyl supplies, raising significant public health concerns regarding its effects on ventilatory control. While fentanyl is well known to cause profound ventilatory depression in humans; xylazine’s effect is less well studied. 

Aims and Objectives

In this study, we aimed to determine how xylazine and fentanyl, individually and in combination, affect respiratory regulation in adult mice. We also evaluated the ability of the antagonists atipamezole (for xylazine) and naloxone (for fentanyl) to reverse these effects. We hypothesised that non‑toxic doses of xylazine would exacerbate fentanyl‑induced ventilatory depression, and that a combined atipamezole-naloxone intervention would be required to reverse respiratory impairment following co‑drug exposure, reflecting emerging clinical reports of xylazine attenuating naloxone’s effectiveness¹.

Methods
Unrestrained whole‑body plethysmography² was used to record ventilatory parameters including breathing frequency, tidal volume, minute ventilation, inspiratory and expiratory times (Ti and Te), and peak inspiratory and expiratory flows (PIF and PEF). Adult male and female CD1 mice received intraperitoneal injections of either xylazine (3 or 10 mg/kg) or fentanyl (0.05, 0.15, or 1.35 mg/kg), and their respective antagonists (1 mg/kg naloxone; 3 mg/kg atipamezole), the latter given 20 minutes post‑drug. N was 6 per experimental group. Co‑administration experiments used 0.15 mg/kg fentanyl with 3 mg/kg xylazine. All experiments were carried out in accordance with the UK Animals (Scientific Procedures) Act 1986 and were approved by the University of Bristol’s Animal Welfare Review Body. Data were normalised to baseline (first 10 minutes), area under the curve calculated and these processed to generate mean parameter-per-minute values. Statistical significance was assessed using one‑way ANOVA with Šídák’s post‑hoc test.

Results
Either fentanyl or xylazine alone produced pronounced ventilatory depression through distinct alterations of the ventilatory cycle. High‑dose fentanyl (1.35 mg/kg) markedly prolonged Ti (167% of baseline), whereas xylazine (10 mg/kg) significantly increased Te (197%), with both changes reducing ventilation rate (p < 0.05). Minute ventilation decreased in a dose‑dependent manner for both drugs, with xylazine producing slightly greater suppression at the highest dose (60% vs. 50%). Fentanyl increased the Ti/Te ratio (+65%), while xylazine reduced it (-30.5%). Co‑administration produced similar effect sizes on Ti and Te to the individual drugs but caused a significantly greater reduction in the PIF/PEF ratio (-34%, p < 0.01), exceeding that induced by fentanyl alone (-24%), indicating a stronger combined impairment of inspiratory function. Only the combined naloxone-atipamezole treatment fully restored ventilation rate to baseline after co‑administration (p < 0.01). All antagonist treatments improved Ti/Te and PIF/PEF ratios, with the strongest improvements observed following naloxone alone and the combined therapy (p < 0.001).

Conclusion
These findings underscore the clinical complexity of xylazine-fentanyl co‑use and the heightened risk of overdose, emphasising the need for updated treatment strategies. Ongoing work using the working heart-brain preparation³ aims to identify the central mechanisms underlying the drugs’ divergent ventilatory effects.