Background: This study was carried out to compare the levels of inflammatory markers in the complete blood count before and after they began receiving duloxetine in patients with fibromyalgia syndrome (FMS). Methods: The patient and control groups were composed of 40 patients diagnosed with FMS in accordance with the 2016 American College of Rheumatology (ACR) criteria and 40 healthy volunteers, respectively. The data collection tools comprised the sociodemographic information form, the fibromyalgia impact questionnaire (FIQ), and the sleep hygiene index (SHI), which were used to assess patients’ sociodemographic characteristics, FMS disease activity, and sleep quality, respectively. The inflammatory markers of the patient group were assessed by complete blood count before and after the duloxetine treatment and compared with those of the control group. Results: The white blood cell (WBC), neutrophil, and lymphocyte counts were significantly higher in the patient group than in the control group (p 0.001, p = 0.036 and p = 0.004, respectively). Moreover, platelet distribution width (PDW) was significantly lower, whereas mean platelet volume (MPV) was significantly higher in the patient group than in the control group (p 0.001 for both cases). In addition to patients’ platelet-to-lymphocyte ratio (PLR) values, C-reactive protein (CRP) levels, and white blood cell (WBC) counts decreasing but not significantly (p = 0.083, p = 0.068, and p = 0.065, respectively), their neutrophil-to-lymphocyte ratio (NLR), hemoglobin (Hgb), and hematocrit (Hct) values declined substantially after commencing duloxetine treatment (p = 0.001, p = 0.008, and p = 0.001, respectively). Conclusions: The significant reduction in NLR, Hgb, and Hct levels following duloxetine treatment may indicate that these parameters can be utilized as biomarkers in determining the efficacy of treatment and in the follow-up of the treatment in FMS patients.