Effect of Naltrexone on Spinal and Supraspinal Pain Mechanisms and Functional Capacity in Women with Fibromyalgia: Exploratory Outcomes from the Randomized Placebo-Controlled FINAL Trial.

Background: The widespread pain and muscular fatigue characteristics of fibromyalgia are believed to be mediated by central mechanisms. Low-dose naltrexone (LDN) has emerged as a new treatment option for fibromyalgia, possibly modulating central mechanisms via glial or opioid receptors.

Methods: In the randomized, placebo-controlled FINAL trial, 99 women with fibromyalgia were treated with LDN or placebo for 12 weeks. In this secondary analysis, we examined the potential effects of LDN versus placebo on changes from baseline in pain tolerance, temporal summation of pain, and conditioned pain modulation (CPM) in the complete case population (45 versus 47 participants). Measures of muscular fatigue were evaluated using the 30-s chair stand test and a shoulder abduction test.

Results: Of the five examined outcomes, only change in CPM showed a significant between-group difference with greater enhancement following LDN treatment (2.0 kPa; 95% confidence interval (CI) 0.4-3.7 kPa) compared with placebo. Within-group changes in CPM showed an increase of 1.2 kPa (95% CI 0.05-2.4 kPa) in the LDN group and a possible decrease of 0.8 kPa (95% CI - 1.9 to 0.4 kPa) in the placebo group.

Conclusions: We found a significant between-group difference in CPM change in favor of LDN. However, this difference was partly explained by a decrease in CPM in the placebo group. Sensitivity analyses showed no association between changes in CPM and clinical pain improvement, suggesting that the group-difference in CPM is a random finding.

Study registration: ClinicalTrials.gov (NCT0427877; registered on 30 January 2020).