Factors influencing the effect of photobiomodulation in the treatment of androgenetic alopecia: A systematic review and analyses of summary-level data.

Low-level laser therapy (LLLT) is used to treat androgenetic alopecia (AGA). The therapeutic effect of LLLT on AGA has been evaluated; however, there is a paucity of studies that investigated device- and usage-related factors that may influence the effect of LLLT on hair regrowth. The literature was systematically searched to identify eligible studies; PubMed, Scopus, EMBASE and clinicaltrials.gov databases were searched on 30 April 2020. Eligible studies were randomized trials that investigated the effect of LLLT on hair density in AGA. Robust linear regressions were used to make comparisons. An increase in the per-session energy fluence by 1 J/cm is significantly associated with an increase in hair density by 0.23 hairs/cm (95% CI: 0.21 hairs/cm , 0.25 hairs/cm ). The number of laser or light-emitting diodes is not significantly associated with change in hair density. Increasing the total duration of exposure to treatment is associated with a significant increase in hair density (β = .53, P < .05). Switching from continuous to pulse irradiation was associated with a significant increase in hair density (β = 10.11, P < .01). Energy fluence, irradiation session duration, and light pulsing have a significant therapeutic effect on AGA, while the number of diodes does not.