Glutathione

Irritancy: unknownComedogenicity: unknown

The details

Glutathione is a tiny protein-like molecule (a "tripeptide" build from 3 amino acids: glycine , cysteine , and glutamic acid ) that your body already makes on its own. Inside your body, it acts as one of the skin's main antioxidants that help fight against free radicals. In skincare, it's best known as a brightening ingredient that slows down tyrosinase, the key enzyme that makes skin pigment. It also nudges the skin toward making a lighter type of pigment instead of a darker one. This is why you'll see it in products aimed at dark spots and uneven tone. A small number of real human trials have found a topical glutathione lotion: The honest caveat is that the current evidence is still thin (few studies, small groups, short timelines). Glutathione also doesn't absorb into skin very easily so results tend to be modest and fade if you stop using it. One thing worth clearing up: The scary side effects you may have heard about come from glutathione injected intravenously, which has real safety concerns. Applying it topically is a completely different thing and has a clean track record. Most human studies used it around 2% (as Glutathione or Glutathione Disulfide) and a 2% oxidized glutathione lotion and a 2% S-acyl glutathione cream are the concentrations with actual clinical data behind them. There's no established "ideal" percentage yet but 1-2% is the evidence-backed range. Allergy-wise, there is very low risk for this ingredient; it was well-tolerated across the topical trials. Only one participant had mild temporary redness that cleared up on its own and another study reported no adverse reactions at all. One trial had ~10% of users drop out for irritation was using a combination cream that also had 10% azelaic acid so the irritation likely wasn't from the glutathione. There's no notable contact-allergy signal for topical glutathione in the literature but patch-testing before first use is still sensible for those with sensitive skin.