Fibromyalgia is a complex generalized and diffuse musculoskeletal chronic pain; and pharmacological approaches are widely used to relieve pain and increase life quality. In this context, low-dose naltrexone (LDN) was able to increase nociceptive threshold in patients with fibromyalgia. Moreover, non-pharmacological techniques, like Transcranial Direct Current Stimulation (tDCS), have been shown effective for pain management. This study aims to evaluate the analgesic and neuromodulatory effect of combined LDN followed by tDCS in fibromyalgia patients. This is a randomized, double-blinded, parallel, placebo/sham-controlled trial, in which 92 (10% loss) women with fibromyalgia will be included included and signed the informed consent. Patients will be allocated into 4 groups: tDCS+LDN (n=21), Sham-tDCS+LDN (n=22), tDCS+Placebo (n=22), and Sham-tDCS+Placebo (n=21). LDN or placebo (p.o.) intervention lasts 26 days, in the last five, tDCS will be applied (sham or active, 20min, 2mA). Questionnaires assessed are: Sociodemographic, Visual Analog Pain Scale (VAS), Pain Catastrophizing Scale (PCS), State-Trait Anxiety Inventory (STAI), Fibromyalgia Impact Questionnaire (FIQ), Beck Depression Inventory (BDI-II), Chronic Pain Profile Scale (CPP). Also, pain measures were taken: Pain Pressure Threshold (PPT) and Conditioned Pain Modulation (CPM). Blood samples will be collected to analyze Brain Derived Neurotrophic Factor (BDNF) serum levels.