Comparison of the effect of duloxetine at doses of 20, 30, and 60 mg on pain intensity in patients with chronic low back pain aged 40 to 65 years

To compare the effect of duloxetine at daily doses of 20, 30, and 60 mg on pain intensity in patients with chronic low back pain

Randomized, parallel-group, single-blind (assessor-blind), phase 3 clinical trial with 1:1:1 allocation using permuted block randomization. Allocation concealed in sequentially numbered, opaque, sealed envelopes

The study is conducted at Imam Ali Clinic, Shahrekord, started in 2026. Participants and the treating physician are not blinded due to identifiable drug doses. The outcome assessor, who measures pain via Visual Analogue Scale (VAS) and records adverse events, remains blinded. Pain is assessed at baseline and every 2 weeks through week 6

Inclusion criteria: chronic low back pain diagnosed by a rheumatologist; discopathy; age 40–65 years; non-pregnant and non-lactating; no psychiatric medication use. Exclusion criteria: confirmed aortic aneurysm; unavailability for follow-up; known hypersensitivity to duloxetine; unwillingness to participate.

Three groups of 45 patients: Group 1 receives duloxetine 20 mg daily for 6 weeks. Group 2 receives duloxetine 30 mg daily for 6 weeks. Group 3 starts with 20 mg daily in week 1, increasing by 20 mg weekly to reach 60 mg daily by week 3, then continues 60 mg daily through week 6

Pain intensity measured by Visual Analogue Scale (VAS, 0–10) at weeks 2, 4, and 6; adverse events.

Permuted block randomization with a 1:1:1 allocation ratio. Block sizes of 6 and 9 are used randomly. The unit of randomization is the individual patient. No stratification is applied. The random sequence is generated using Random Allocation Software (version 1.0). For allocation concealment, the assignment sequence is prepared by an independent colleague not involved in the intervention or outcome assessment, and placed in sequentially numbered, opaque, sealed envelopes (SNOSE). After obtaining informed consent and completing baseline assessments, the corresponding envelope is opened to reveal the group assignment.

Due to the nature of the intervention, with the drug dose clearly identifiable on the medication packaging, participants and the treating physician (principal investigator responsible for prescribing and patient visits) are not blinded to group assignment. All patients are informed that they are participating in a clinical trial and will receive one of three doses of duloxetine, and written informed consent is obtained. The outcome assessor (measuring pain intensity via VAS and recording adverse events) is a trained independent colleague not involved in patient care, who remains blinded to treatment allocation. To maintain assessor blinding, patients are instructed not to disclose their dose during assessment sessions. Group allocation is not recorded on assessment forms or case report files used by the assessor; groups are identified only by a unique code that does not reveal the dose. After data collection, the database is anonymized by another person and handed over to the statistician for analysis without group identifiers. No formal Data Safety and Monitoring Board (DSMB) has been established, but study progress and serious adverse events are continuously monitored by the supervising professors. The authors drafting the manuscript will remain blinded to group allocation until the database is locked and the code is broken.