Background: Transcranial photobiomodulation (t-PBM) consists in the delivery of near-infrared light (NIR) to the scalp, directed to cortical areas of the brain. NIR t-PBM recently emerged as a potential therapy for depression, although safety of repeated treatments has not been adequately explored. Objective: This study assessed incidence of side effects, including weight and blood pressure changes, during repeated sessions of NIR t-PBM using a light-emitting diode source. Methods: We performed a secondary analysis of a double-blind clinical trial on t-PBM for major depressive disorder. Eighteen individuals received NIR t-PBM (n = 9) or sham (n = 9) twice weekly for 8 weeks. Side effects were assessed using the Systematic Assessment for Treatment-Emergent Effects-Specific Inquiry. In 14 individuals (n_NIR = 6 vs. n_sham = 8), body weight and systemic blood pressure were recorded at baseline and end-point. Results: More subjects in the NIR t-PBM group experienced side effects compared to sham, but only a trend for statistical significance was observed (χ² = 3.60; df = 1; p = 0.058). The rate of side effects described by participants as "severe" in intensity was low and similar between the treatment groups (χ² = 0.4; df = 1; p = 0.53), with no serious adverse events. Most side effects resolved during the study and treatment interruption were not required. Changes in weight and systolic blood pressure across groups were neither significant nor approached significance. In the NIR t-PBM group, diastolic blood pressure increased and reached statistical-however not clinical-significance (5.67 ± 7.26 vs. -6.13 ± 6.88; z = -2.40, p = 0.016). Conclusions: This small-sample, exploratory study indicates repeated sessions of NIR t-PBM might be associated with treatment-emergent side effects. The systemic metabolic and hemodynamic profile of repeated t-PBM appeared benign. Future studies with larger samples and longer follow-up are needed to more accurately determine the side-effect profile and safety of NIR t-PBM.

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