The effects of the oral supplementation of L-Cystine associated with reduced L-Glutathione-GSH on human skin pigmentation: a randomized, double-blinded, benchmark- and placebo-controlled clinical trial
Duperray J, Sergheraert R, Chalothorn K, et al. (2022). Journal of Cosmetic Dermatology.
Summary
A 12-week trial in 124 women testing oral glutathione for skin lightening, one of the uses it is most often sold for. Participants were randomised to a cystine and glutathione combination, glutathione alone, cystine alone, or placebo, with skin colour measured at baseline, six and twelve weeks. The proposed mechanism the authors describe is shifting melanin production toward the lighter form. Note that the trial's headline comparison is the combination product rather than glutathione on its own.