Venlafaxine for Non-specific Low Back Pain

• Control Group (Drug): Participants in the control group will receive routine clinical care for chronic nonspecific low back pain.
• Venlafaxine group (Drug): Venlafaxine is administered orally at a starting dose of 75 mg once daily. In patients with insufficient pain response who can tolerate the adverse effects, the dose may be carefully titrated up to 225 mg once daily. The maintenance period is 3 months.

Primary outcomes

• Change From Baseline in Average Low Back Pain Intensity at Month 3 [3 months after randomization]: Average low back pain intensity during the previous week will be assessed using an 11-point Numerical Rating Scale, ranging from 0 to 10, where 0 indicates no pain and 10 indicates the worst imaginable pain.

Secondary outcomes

• Change From Baseline in Average Low Back Pain Intensity [Baseline, 1 month, and 6 months after randomization]: Average low back pain intensity during the previous week will be assessed using an 11-point Numerical Rating Scale, ranging from 0 to 10, where 0 indicates no pain and 10 indicates the worst imaginable pain.
• Change From Baseline in Functional Disability Assessed by the Roland-Morris Disability Questionnaire [Baseline, 1 month, 3 months, and 6 months after randomization]: Functional disability related to low back pain will be assessed using the Roland-Morris Disability Questionnaire. Higher scores indicate greater functional disability.
• Global Perceived Recovery [1 month, 3 months, and 6 months after randomization]: Overall improvement will be assessed using a 6-point Likert scale ranging from "much worse" to "completely recovered."
• Change From Baseline in Health-related Quality of Life Assessed by EQ-5D-5L [Baseline, 1 month, 3 months, and 6 months after randomization]: Health-related quality of life will be assessed using the EQ-5D-5L. The EQ-5D-5L includes five health dimensions, and each dimension has five severity levels. Higher health utility scores indicate better health status.
• Change From Baseline in Sleep Quality Assessed by the Insomnia Severity Index [Baseline, 1 month, 3 months, and 6 months after randomization]: Sleep quality and insomnia symptoms will be assessed using the patient-reported version of the Insomnia Severity Index. The scale contains 7 items evaluating sleep onset difficulty, sleep maintenance difficulty, early morning awakening, satisfaction with sleep, interference with daytime functioning, noticeability of sleep problems, and distress caused by sleep difficulties. Each item is scored from 0 to 4, and the total score ranges from 0 to 28. Scores are interpreted as follows: 0-7, no clinically significant insomnia; 8-14, subthreshold insomnia; 15-21, moderate insomnia; and 22-28, severe insomnia.
• Change From Baseline in Back Pain Beliefs Assessed by the Back Beliefs Questionnaire [Baseline, 1 month, 3 months, and 6 months after randomization]: Beliefs about back pain will be assessed using the Back Beliefs Questionnaire. Scores range from 9 to 45, with lower scores indicating more pessimistic beliefs about the consequences of back pain.
• Change From Baseline in Neuropathic Pain Features Assessed by the painDETECT Questionnaire [Baseline, 1 month, 3 months, and 6 months after randomization]: Neuropathic pain features will be assessed using the painDETECT questionnaire. A score of 19 or higher suggests the presence of a neuropathic pain component.
• Use of Nonsteroidal Anti-inflammatory Drugs [Baseline, 1 month, 3 months, and 6 months after randomization]: The amount and frequency of nonsteroidal anti-inflammatory drug use during the study period will be recorded.
• Incidence of Adverse Events [From randomization to 6 months after randomization]: Adverse events will be recorded throughout the study

Sex: All, ages: 18 Years to 75 Years

Inclusion Criteria:

* Adults aged 18 to 75 years.
* Diagnosis of chronic nonspecific low back pain, defined as pain located below the costal margin and above the inferior gluteal folds, lasting for more than 3 months, without a specific pathological cause.
* Moderate to severe pain, defined as a Numerical Rating Scale score of 3 or higher.
* Able to understand the study procedures and sign written informed consent.

Exclusion Criteria:

* Low back pain caused by a known specific pathological condition, including but not limited to fracture, malignancy, infection, inflammatory disease, or other identifiable structural or systemic causes.
* Major comorbidities that may interfere with study outcomes or represent contraindications to the study medication.
* Current or previous diagnosis of depression, regardless of whether treatment was received.
* Current or previous use of antidepressant medication.
* Current use of opioid analgesics.
* Pregnancy, breastfeeding, or planned pregnancy during the study period.
* Known allergy, hypersensitivity, or contraindication to toludesvenlafaxine.
* History of psychosis or other major psychiatric disorders.
* Any condition that, in the investigator's judgment, makes the participant unsuitable for the study.

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