L-glutathione and L-cysteine have been hypothesized to brighten skin tone by controlling the ratio of pheomelanin (a “light” pigment) and eumelanin (a “dark” pigment) in the skin.

The study

This randomized controlled trial evaluated whether supplementing with L-glutathione and L-cysteine could brighten skin. For 12 weeks, 124 women 30–50 years old with at least one facial dark spot and skin phototype III or IV were randomized into one of the following supplementation groups:

  • 500 mg of oral L-cysteine plus 250 mg of oral L-glutathione (GC) per day
  • 250 mg of oral reduced L-glutathione (LG) per day
  • 500 mg of oral L-cysteine (LC) per day
  • placebo

The investigators assessed skin color at baseline, 6 weeks, and 12 weeks via spectrophotometry on the cheek, upper forearm, and lower forearm. They evaluated the size and color of dark spots on the face from photographs, and the participants evaluated the subjective efficacy of the supplements by responding to a questionnaire.

The results

GC lightened skin on the cheeks and lower forearm, with no effect observed on the upper forearms. LC and LG both lightened skin on the upper forearm. No effect was observed for placebo. GC and LC both reduced the color of dark spots, but only GC reduced the size of dark spots. Subjective satisfaction was similar in all four groups.


Several of the study authors worked for BLF Life Sciences and Spincontrol Asia, which are companies involved in the formulation of the GC product used in this study.

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This Study Summary was published on June 4, 2021.