Efficacy and Safety of Yellow Light Monotherapy Versus Yellow Light Combined With 20% Vitamin C for the Treatment of Melasma: A Single‐Center, Randomized, Double‐Blind, Placebo‐Controlled Clinical Study

Background Melasma treatment remains challenging due to limited effective and safe options. This study evaluated the efficacy and safety of yellow light phototherapy alone versus combined with 20% vitamin C in treating melasma. Methods Seventy‐one patients were randomly assigned to receive twice‐weekly yellow light phototherapy combined with either 20% vitamin C solution ( n = 35) or its corresponding vehicle ( n = 36). The primary outcome was the change from baseline in modified Melasma Area and Severity Index (mMASI) score. Secondary outcomes consisted of changes relative to baseline in Physician Global Assessment (PGA), Subjective Satisfaction Assessment (SA), melanin index (MI), individual typology angle (ITA°), transepidermal water loss (TEWL), skin hydration content (SCH), skin pore and skin texture parameters. Results At Week 12, the combination group exhibited significantly greater reductions in mMASI scores (−1.95 ± 2.11 vs. −0.69 ± 2.19, p = 0.027) and lower Physician Global Assessment (PGA) scores (4.48 ± 0.80 vs. 5.06 ± 0.54, p = 0.002) compared with the monotherapy group. The combination group exhibited a more remarkable decrease in melanin index (MI) (−13.07, 95% CI: −17.82, −8.32 vs. −3.16, 95% CI: −7.86, 1.54, p = 0.005) and a more prominent elevation in individual typology angle (ITA°) (6.23, 95% CI: 4.38, 8.08 vs. 1.23, 95% CI: −0.60, 3.05) ( p < 0.001). Moreover, significant improvements in skin pore status and skin texture were noted in the combination group ( p < 0.05). No statistically significant between‐group differences were found in transepidermal water loss (TEWL) and skin hydration (SCH). No adverse events occurred throughout the trial. The participant treatment satisfaction rate reached 60.00% in the combination group, which was significantly higher than 22.55% in the control group. Conclusion Combined 20% vitamin C plus yellow light phototherapy yielded statistically superior melasma improvements relative to yellow light monotherapy, though the absolute reduction in mMASI was modest. The combination regimen preserved skin barrier function and exhibited good tolerability.