Background & Objectives: Obsessive–compulsive disorder (OCD) typically follows a chronic and fluctuating course and can substantially impair an individual's personal, academic, occupational, and social functioning. From a neurobiological perspective, OCD is a psychiatric disorder with a biological basis, characterized by deficits in emotion regulation and executive functioning. Individuals with OCD often experience their obsessive thoughts as intolerable. The resulting severe distress drives them to engage in compulsive behaviors aimed at temporarily reducing their discomfort. Transcranial photobiomodulation (tPBM) is a non–invasive intervention that uses visible or near–infrared light (600–1100 nm) to modulate cellular activity in living tissues. Given the central role of the prefrontal cortex in the executive functioning deficits and reduced distress tolerance observed in OCD, together with evidence supporting the neurometabolic effects of tPBM on this brain region, this intervention has the potential to improve the cognitive and emotional symptoms associated with the disorder. Therefore, the present study aimed to investigate the effectiveness of tPBM on executive functioning and distress tolerance in patients with OCD.

Methods: This randomized controlled trial employed a pretest–posttest design with a control group. The study population consisted of patients with obsessive–compulsive disorder (OCD) who were referred to a specialized psychiatric clinic in Rafsanjan, Iran. The sample comprised 30 patients with OCD, aged 18–65 years. Participants were recruited using convenience sampling from eligible patients and were then randomly allocated to either the experimental group or the control group (n=15 per group). The inclusion criteria were: age between 18 and 65 years; a diagnosis of OCD according to the Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM–5) established by a psychiatrist using the Structured Clinical Interview for DSM–5 Disorders (SCID–5); no active pharmacological or psychological treatment for at least four weeks before study entry; no initiation of other treatments during the study period; a Yale–Brown Obsessive Compulsive Scale (Y–BOCS) score of 16–25, indicating mild to moderate symptom severity; and the ability to understand and sign the informed consent form. The exclusion criteria included a history or presence of neurological or neurodegenerative disorders (e.g., uncontrolled epilepsy, stroke, cerebrovascular disease, Alzheimer's disease, or Parkinson's disease), based on medical history and neurological examination; active skin lesions, inflammation, or injury at the frontal stimulation sites; pregnancy or breastfeeding, confirmed by participant self–report and urine testing; severe medical conditions such as malignancy, active systemic infection, or major organ failure; a concurrent primary psychiatric diagnosis based on the SCID–5; substance abuse or dependence within the previous six months; and the presence of metallic implants in the head or facial region, as determined by a Magnetic Resonance Imaging Safety Screening Questionnaire. The study instruments included the Yale–Brown Obsessive Compulsive Scale (Goodman et al., 1989), the Barkley Deficits in Executive Functioning Scale (Barkley, 2011), and the Distress Tolerance Scale (Simons & Gaher, 2005). The intervention consisted of 10 sessions of transcranial photobiomodulation (tPBM) administered twice weekly over five weeks. During each session, the device probes were positioned over the frontal scalp at FP1, FP2, F3, F4, F7, F8, and Fz according to the international 10–20 electroencephalography system. Treatment parameters included a wavelength of 810 nm, an output power of 100 mW, an irradiance of 10–100 mW/cm², and an energy density of 6–10 J/cm². Each treatment session lasted 20 minutes. The control group received no intervention during the study period but was monitored regularly. Data were analyzed using SPSS version 24. The chi–square test was used to compare demographic characteristics, whereas the independent–samples t–test was used to compare pretest scores between the two groups. Analysis of covariance (ANCOVA) was performed to compare posttest outcomes between the experimental and control groups while controlling for pretest scores. Statistical significance was set at 0.05.

Results: The results of the analysis of covariance (ANCOVA) showed that, after controlling for pretest scores, there were statistically significant differences between the experimental and control groups in the outcome variables. Compared with the control group, the experimental group receiving tPBM showed a significant reduction in executive functioning deficits (p = 0.002) and a significant increase in distress tolerance (p < 0.001). Based on the effect size estimates, tPBM accounted for 35% of the variance in executive functioning deficits and 41% of the variance in distress tolerance. The effect size was moderate for executive functioning deficits (η²=0.35) and moderate to large for distress tolerance (η²=0.41)

Conclusion: Transcranial photobiomodulation is effective in improving executive functioning and increasing distress tolerance in patients with OCD. Therefore, it may be considered a complementary treatment alongside standard therapies, particularly for patients with treatment–resistant OCD.