Acetyl-L-Carnitine as an Add-On Treatment in Fibromyalgia Syndrome: A Retrospective Analysis on 183 Patients, According to the Generalized Linear Mixed Model for Longitudinal Data

<b>Background</b>. Fibromyalgia Syndrome (FMS) is characterized by chronic widespread pain, sleep disturbances, fatigue and cognitive impairment. <b>Methods</b>. In this retrospective study, we analyzed data collected between 2017 and 2022 regarding Acetyl-L-Carnitine (ALC) as an add-on treatment in 183 adult patients with FMS according to the 2016 ACR (American College of Rheumatology) criteria and patients' pain lasting for over three months. Patients with prior exposure to ALC or without informed consent were excluded. <b>Results</b>. Regarding efficacy, in the 137 analyzed patients, the change from baseline to the end of observation in Visual Analogue Scale score (VAS) was statistically significant, ranging from 75.9 ± 1.56 to 51.9 ± 1.99 (<i>p</i> < 0.001). Patients without FMS concomitant drug treatments achieved better VAS reduction than patients who were not drug-free at baseline. Regarding quality of life, a significant improvement in the Revised Fibromyalgia Impact Questionnaire (FIQ-R) score was evidenced, ranging from 75.1 ± 1.13 to 53.5 ± 1.34 (<i>p</i> < 0.001). The Short Form 12 Health Survey (SF12) scores showed a statistically significant improvement in both physical and mental components. Finally, the Pittsburgh Sleep Quality Index (PSQI) did not show a statistically significant difference from baseline. In the whole population, 23 patients (16.7%) reported Adverse Events (AEs), predominantly insomnia, shivering, headaches, and nausea. Only six patients reporting AEs discontinued the ALC treatment. <b>Conclusions</b>. This retrospective study evidenced the efficacy and safety of ALC in FMS patients. This may represent a useful approach, particularly for long-term treatments. Methodologically stronger studies will be necessary to validate our observations.