Xylazine Reshapes the Sleep and Respiratory Consequences of Chronic Fentanyl Exposure in a Sex-Dependent Manner

Recent shifts in the illicit fentanyl supply have led to widespread exposure to xylazine, yet the biological consequences of chronic fentanyl-xylazine co-exposure remain poorly understood. Here, we compared the behavioral and molecular effects of chronic exposures to fentanyl and xylazine both alone and in combination in male and female mice. Combined fentanyl-xylazine produced distinct, sex-dependent alterations in sleep architecture, including changes in NREMS recovery, REMS dynamics, and reduced sleep bout duration compared with fentanyl alone. The 2-adrenergic receptor antagonist yohimbine partially reversed these sleep alterations in a sex-dependent manner. Fentanyl-xylazine also exacerbated respiratory depression relative to fentanyl alone, particularly in males. In contrast, despite its prevalence in humans no skin lesions were observed. Molecular analyses revealed that fentanyl-xylazine co-administration leads to increased peripheral Fibroblast growth factor 21, a hepatokine linked to reduced opioid preference and alcohol consumption. Together, these findings demonstrate that xylazine substantially alters the physiological and neurobiological consequences of chronic fentanyl exposure in a sex-dependent manner, providing a preclinical framework for understanding the growing public health impact of fentanyl-xylazine co-use.