Effectiveness and adverse effects of the use of mirtazapine as compared to duloxetine for fibromyalgia: real-life data from a retrospective cohort

On the background of a restricted armamentarium of drugs available for the management of fibromyalgia (FM), we aimed to compare the real-world effectiveness of two serotonin-norepinephrine reuptake inhibitors (SNRIs), mirtazapine (MTZ) and duloxetine (DLX) in FM. A medical records review was done to identify patients diagnosed with FM and prescribed a stable dose of either MTZ or DLX for more than 6 months. Their present status was determined by a telephonic interview which included a subjective assessment of improvement (Likert scale), FIQR (Revised Fibromyalgia Impact Questionnaire), adverse drug effects and compliance. One-fifty-eight patients were screened to include 81 patients [mean age 46.7 (± 13.0) years, 64 (79%) females]. Sixty (79%) had primary fibromyalgia and 66 (81.5%) were on DLX (20–40 mg) while 15(18.5%) were on MTZ (7.5 mg). In addition to the drugs, lifestyle modification was followed by 57 (70.3%). A moderate-to-good improvement was seen in 66 (81.5%), while 15 (18.5%) reported poor to no improvement overall. In the DLX group, a majority (59, 89.4%) showed moderate-to-good improvement compared to 7(46.7%) on MTZ [p = 0.001, 9.6(2.6–34)]. However, FIQR was similar for those on DLX (3.6 ± 0.9) and MTZ (3.8 ± 0.7). Adverse effects were reported for 51 (77%) of patients on DLX and all (100%) on MTZ with a poorer compliance with MTZ 5 (33.3%) compared to DLX 47 (71.2%) [p = 0.008, OR 0.1(0.03–0.4)]. On multivariate analysis, DLX use [OR 16.7 (95% CI 2.7–100); p = 0.008] and lifestyle modification [p = 0.002; OR 11.2(1.5–83.3)] were associated with better subjective outcomes. Low-dose MTZ appears to be inferior to DLX in the management of FM in this real-world cohort.