Repetitive Transcranial Photobiomodulation Improves Working Memory and Attention in Adults with ADHD: A 4-Week Follow-Up Study

Background: Working memory (WM) impairments and inattention symptoms are prevalent among adults with attention-deficit/hyperactivity disorder (ADHD). Transcranial photobiomodulation (tPBM) is a promising brain stimulation technique that may enhance cognitive function among adults with ADHD. Objectives: We aimed to explore the effects of tPBM on improving N-back WM and Continuous Performance Test-Identical Pairs (CPT-IP) attention tasks in adults with ADHD, as well as its baseline predictive factors. Methods: Forty-eight adults with ADHD underwent a 7-day tPBM intervention (720 s daily, 1064-nm wavelength, 250 mW/cm2 irradiance). Participants completed the N-back (1-back, 2-back, 3-back) WM, and CPT-IP (cpt-2, cpt-3, cpt-4) attention tasks at baseline (T1), after the first (T2), and seventh (T3) interventions, and during four weekly follow-ups (T4-T7). Safety was assessed using the Treatment Emergent Symptom Scale (TESS). Results: The participants showed significant improvements in the 2-back, 3-back, cpt-3, and cpt-4 tasks (all p < 0.001), with peak effect sizes observed at 2-3 weeks post-intervention (Cohen's d = 0.84-1.26). Lower baseline performance predicted greater improvement. The intervention was well-tolerated; three (6.3%) participants reported mild adverse events (TESS scores ≤2), all of which resolved spontaneously. Conclusions: tPBM is effective and well-tolerated for improving WM and attention in adults with ADHD, suggesting its potential use as a non-pharmacological approach for ADHD management.