In a multicentre, randomized, double-blind trial, women self-treated at home with a device delivering 655 nm light for 25 minutes every other day for 16 weeks (60 sessions). The active group showed a significantly larger increase in terminal hair counts than the sham group, described as about 37% higher hair growth vs placebo, and no adverse events or side effects were reported in participants who completed the study.

This medically reviewed overview pulls together results from multiple studies and reports that red light (low-level laser therapy) has been associated with about 35% to 51% more hair growth than placebo in controlled trials when used consistently. It also summarizes a proposed biological mechanism involving mitochondrial activity and ATP production that may support follicles staying in the growth phase longer. 

According to the abstract on PubMed, this multicentre randomized controlled trial evaluated a device using an array of 650 nm laser diodes. With use every other day for 17 weeks, participants receiving active light treatment achieved a 51% increase in hair counts compared with sham-treated controls in women aged 18 to 60 with androgenetic alopecia.