Objective:Chronic pain involves alterations in brain gray matter volume (GMV). Moreover, opioid medications are known to reduce GMV in numerous brain regions involved in pain processing. However, no research has evaluated (1) chronic pain-related GMV alterations in the spinal cord or (2) the effect of opioids on spinal cord GMV. Accordingly, this study evaluated spinal cord GMV in health controls and patients with fibromyalgia who were using and not using opioids long-term.Methods:We analyzed average C5 - C7 GMV of the spinal cord dorsal and ventral horns in separate female cohorts of healthy controls (HC, n = 30), fibromyalgia patients not using opioids (FMN, n = 31), and fibromyalgia patients using opioids long-term (FMO, n = 27). To assess the effect of group on average dorsal and ventral horn GMV, we conducted a one-way multivariate analysis of covariance.Results:After controlling for age, we observed a significant effect of group on ventral horn GMV (p= 0.03, η2= 0.09), and on dorsal horn GMV (p= 0.05, η2= 0.08). Tukey’s posthoc comparisons showed that, compared to HC participants, FMOs had significantly lower ventral (p= 0.01) and dorsal (p= 0.02) GMVs. Among FMOs only, ventral horn GMV was significantly positively associated with pain severity and interference, and both dorsal and ventral GMVs were significantly positively associated with cold pain tolerance.Conclusion:Long-term opioid use may impact sensory processing in fibromyalgia via gray matter changes within the cervical spinal cord.