## 3 Results
### Characteristics of Included Studies  
The 17 included studies represented a variety of clinical trial designs [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51]. Among them, three were single-arm trials, including one feasibility trial with an embedded qualitative component [36] and two open-label longitudinal studies [46; 47]. Six studies were randomized, triple-blinded, placebo-controlled trials [38; 40; 41; 42; 43; 48], one of which was a multicenter study [40]. Two were randomized, double-blinded, controlled trials [35; 49]; one was a randomized, crossover, double-blinded trial [45]; one was a randomized, single-blinded clinical trial [47]; and one was an observational retrospective study [44]. Several studies incorporated comparative interventions, including local anesthetic injections [47], functional exercise [48], and green light therapy [39; 43].  
All studies focused on Fibromyalgia and chronic pain syndromes, comprising a total of 857 patients (843 women, 98.4%; 14 men, 1.6%), aged 18-75 years. PBMT was the primary intervention across all the studies, with variations in wavelengths, dosage regimens, and application techniques. Several studies evaluated whole-body PBMT using NovoTHOR(r) beds (THOR Photomedicine Ltd., Amersham, UK) [38; 41; 42], while others employed high-output laser systems, such as the FibroLux(tm) System (FibroLux GmbH, Hofheim, Germany) [40],  
Figure 1: Flow diagram of study selection (PRISMA 2020).  
the LCT-1000 Class IV laser (LiteCure LLC, Newark, DE, USA) [50], or Class IIIb lasers like Frank Line IR 30 (Fysiomed, Edegem, Belgium) [51]. Localized photobiomodulating targeting specific anatomical regions, such as tender points, muscles, or painful areas, was administered using devices like Antares(IRAMED, Amparo, Brazil) [35], LasoRehab Advanced LS2200(Nu-Tek Health Inc., Hong Kong, China) [47], and Photon Laser III(r) (DMC Equipamentos, Sao Carlos, Brazil) [48]. In addition, multimodal interventions were explored, such as the combination of photobiomodulation and therapeutic ultrasound using a patented prototype device (MMOptics BR102014007397-3 A2, Sao Carlos, Brazil) [46], or a super-pulsed laser combined with LEDs and magnetic fields, as in Pain Away/PainCure(tm) (Multi Radiance Medical(r), Solon, OH, USA) [49].  
One study also tested a whole-body thermal therapy via water-filtered infrared A (wIRA-WBH), using IRATHERM1000(r) (Von Ardenne Institut fur Angewandte Medizinische Forschung GmbH, Dresden, Germany) [37]. For non-invasive visual interventions, green light-emitting diodes (GLEDs) were applied via ambient exposure (Led Supply(r), Randolph, VT, USA) [43], and green light-filtering eyeglasses were tested as a passive visual neuromodulation strategy, without the use of LEDs or lasers [39]. Three studies compared PBMT to placebo treatments [38; 40; 49], while others combined PBM with exercise therapy [35; 40; 48], therapeutic ultrasound [46], or alternative treatments like green light exposure [39; 43].  
The primary outcome variables assessed included QoL, pain intensity, functionality, and sleep disturbances [35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51]. Several secondary variables were also measured, such as the pressure pain threshold [38; 42], morning stiffness [35; 47], depression and anxiety [36; 39; 43], cognitive impairment [36; 47], central sensitization [41; 42], muscle spasms [47], and opioid medication reduction [39]. Study durations ranged from four weeks [38] to seven months [40], reflecting a broad range of treatment protocols.  
The studies were conducted across multiple countries. Brazil accounted for five studies [35; 40; 45; 46; 49], followed by Spain with three [38; 41; 42], and the United States with three [36; 43; 50]. Two studies were performed in Turkey [47; 51], while Germany [37], Italy [39], South Korea [44], and the United Kingdom [36] each contributed one study. Further details can be found in Table S2: Characteristics of the included studies.