In the pregabalin with desvenlafaxine group, the dose titration of pregabalin is identical with pregabalin monotherapy group. Desvenlafaxine will be administered at 50 mg daily with breakfast initially. If the initial dose is well-tolerated, the dose could be further escalated by 50 mg per day after 3 days, up to a maximum of 400 mg per day after 7 days. With each dose increase, AEs will be assessed and categorized as mild, moderate, or severe. If participants tolerate the current dose for 2 to 3 days, it will be maintained, with the expectation that tolerance to AEs will develop. If AEs become intolerable, the dose will be decreased back to the previous dose, which was defined as the maximum tolerated dose. In the combination group, dose titration of both pregabalin and desvenlafaxine will be conducted simultaneously. If AEs occur, the next dose regimen should be determined in consultation with a clinical physician.

The Revised FM Impact Questionnaire assesses the severity of FM symptoms and their impact on daily functioning. It evaluates three linked domains: function, overall impact, and symptoms, using a 0 to10 NRS. The total score ranges from 0 to 100, with higher scores indicating greater disease burden.

BPI-S accesses pain intensity over the past 24 hours, calculated as the average of items 3 to 6 on the BPI, scored using a 0 to 10 NRS. In the NRS scale, 0 indicates no pain, while 10 represents the worst pain imaginable. The total score ranges from 0 to 40, with higher scores indicating greater pain severity. BPI-I evaluates the degree to which pain interferes with daily activities, including general activity, mood, mobility work, relationships, sleep, and pleasure. This subscale comprises 7 items, each scored from 0 (no interference) to 10 (complete interference), with a total score ranging from 0 to 70.

The SF-36 assesses health-related quality of life, capturing preferences across various health states. It assesses 8 dimensions: physical functioning, physical role limitations due to physical health, bodily pain, general health, vitality, social functioning, emotional role limitations due to emotional problems, and mental health. Scores range from 0 to 100 for each dimension, with higher scores indicating better health status.

The MOS is a questionnaire comprising 12 items that assess various aspects of sleep using a 6-point ordinal scale (1 indicating permanence and 6 indicating absence).

The BD-Ⅱ evaluates the severity of depressive symptoms using a 4-point scale from 0 to 3, where 0 indicates no symptom and 3 indicates severe symptomatology. It comprises 21 items, the total score ranging from 0 to 63.

The incidence and proportion of adverse events will be recorded and categorized as mild, moderate, severe, or life-threatening. AEs are defined as events that arise during treatment, were absent before treatment, or worsen relative to the pretreatment state.