Glutathione (GSH for short) is a small peptide of three amino acids that every cell in the body makes for itself. It is the most abundant antioxidant in the organism — a molecule that “collects” free radicals and protects proteins, fats and DNA from damage. Unlike the other compounds in our encyclopaedia it is not a signalling peptide: it acts on no receptor, it simply takes part in chemical reactions. As a supplement it has provoked the same dispute for years: does swallowed glutathione reach the blood and the cells at all? In this review we explain in plain terms what GSH is, how it works and what the human studies have shown — and those are few, small and partly contradictory.
What glutathione is
Glutathione is a tripeptide built from glutamic acid, cysteine and glycine. It exists in two forms: reduced (GSH), which is “charged” and ready to donate an electron, and oxidised (GSSG), which has already donated it. The ratio of GSH to GSSG in cells and blood is one of the classic markers of oxidative stress — the more of the oxidised form, the greater the burden. It is precisely this ratio that supplementation studies measure (Allen 2011; Richie 2015).
How it works
The cysteine in the middle of the molecule carries a thiol group (–SH) that reacts readily with free radicals and peroxides — GSH neutralises them and is itself converted to GSSG. Enzymes in the cell (among them glutathione peroxidase) use GSH as “fuel”, and other enzymes regenerate it from GSSG. Glutathione also attaches itself in the liver to many foreign substances and their breakdown products so that they can be excreted — part of so-called phase-two detoxification. Importantly, this is the action of a substrate, not a signal. Glutathione does not “switch on” anything in the cell; it is consumed and regenerated.
Structure and origin — why absorption is the problem
The body does not rely on glutathione from food — it synthesises it inside cells from the three amino acids, cysteine usually being the limiting ingredient. Swallowed glutathione meets an enzyme in the gut and liver (gamma-glutamyltransferase) that breaks it into pieces. Witschi and colleagues (1992) gave seven healthy volunteers a single 3 g dose of glutathione and measured GSH, cysteine and glutamate in plasma for 270 minutes: none of these concentrations rose significantly. The authors concluded that the systemic availability of oral glutathione is “negligible”. This is where the belief that oral GSH “does not work” came from, together with the idea of giving precursors instead: N-acetylcysteine (NAC, a source of cysteine) and glycine.
Oral glutathione — the studies contradict each other
Two randomised placebo-controlled trials gave different results, and they cannot be averaged. Allen and Bradley (2011) gave 40 healthy adults 500 mg of glutathione twice a day for 4 weeks. Neither the urinary markers of oxidative stress (F2-isoprostanes, 8-OHdG) nor GSH, GSSG and their ratio in red blood cells changed — a null result. Richie and colleagues (2015) ran their trial for longer: 54 non-smoking adults received placebo or glutathione at a daily dose of 250 or 1,000 mg for 6 months. Here blood GSH rose after 1, 3 and 6 months; after half a year it was 30–35% higher in the high-dose group in red cells, plasma and lymphocytes, and as much as 260% higher in cheek epithelial cells. In the low-dose group the rise was 17% (whole blood) and 29% (red cells). After a one-month washout the values returned to baseline. The GSSG-to-GSH ratio also fell, and the activity of NK cells (part of innate immunity) more than doubled in the high-dose group at 3 months. The authors write that the effectiveness of oral GSH in raising body stores was shown “for the first time”.
How can these results be reconciled? The simplest explanation is time: a single dose (Witschi) and 4 weeks (Allen) may have been too short, while 6 months (Richie) was long enough for cellular stores to build up slowly. That is a hypothesis, though; the studies also differed in the markers they measured. The honest conclusion is: ordinary oral glutathione is not “useless” by definition, but the evidence that it works is limited to a single 6-month trial in 54 people.
Liposomal and intravenous forms
Liposomal. Sinha and colleagues (2018) — a team from the same centre as Richie — gave 12 healthy adults liposomal glutathione (500 or 1,000 mg per day) for a month. GSH rose after just one week, and the maximum increases at 2 weeks were 40% in whole blood, 25% in red cells, 28% in plasma and 100% in blood mononuclear cells; plasma 8-isoprostane fell by 35%, and NK-cell activity rose by up to 400%. It sounds impressive, but the limitations matter: the study was open-label, without a placebo group, in 12 people, and it was funded by the manufacturer of the product tested (which the authors disclosed). There were no differences between the doses. It is a signal worth checking, not proof that the liposomal form is better than the ordinary one — the two forms have never been compared directly in a single study.
Intravenous. Hauser and colleagues (2009), in a pilot placebo-controlled trial, gave 21 patients with Parkinson's disease 1,400 mg of intravenous glutathione three times a week for 4 weeks. The treatment was well tolerated, but on the UPDRS symptom scale there were no significant differences from placebo (improvement greater by 2.8 points; p = 0.32). The systematic review by Sarkar and colleagues (2025) found only one placebo-controlled trial of intravenous GSH for skin lightening (response in 6 of 16 versus 3 of 16; p = 0.054) and judged the intravenous route inadvisable “due to lack of efficacy and side effects”.
Precursors: glycine and NAC
Since the body assembles glutathione itself, supplying it with the building blocks is logical. Kumar and colleagues (2023), in a placebo-controlled trial, gave 24 older adults a mixture of glycine and N-acetylcysteine (GlyNAC; 12 people) or alanine as a placebo with the same nitrogen content (12 people) for 16 weeks; 12 young adults received GlyNAC for 2 weeks as a reference. At the outset the older adults, compared with the young, had GSH deficiency, higher oxidative stress, poorer mitochondrial function, more inflammation and insulin resistance. GlyNAC — and not placebo — improved or corrected these markers, including physical function (gait speed, muscle strength and the distance in the 6-minute walk test were measured). It is a small single-centre study (12 people per group), but methodologically sound and with an interesting conclusion: the best-documented way to raise GSH in humans is not glutathione itself but its precursors.
Skin lightening
Glutathione is used as a skin-lightening agent because it inhibits melanin synthesis. Sarkar and colleagues (2025) systematically reviewed the studies of the past decade: five randomised trials and one open-label study of oral GSH (250–500 mg daily dose) showed a significant reduction in the melanin index compared with placebo; topical 0.5% glutathione worked better than 0.1% and placebo, and the combination of a 2% topical preparation with oral GSH better than either form alone. The authors rate the effect as moderate, unsustainable (it wears off after stopping) and subject to a mixed risk of bias in the studies; for the oral form they describe the adverse effects as “substantial”, and they consider the intravenous form contraindicated.
Safety and the limits of the evidence
In the studies cited, oral and intravenous glutathione was well tolerated, but these were short and small studies (from 7 to 54 people). The main limitations: contradictory results for the oral form; no head-to-head comparison of the ordinary and liposomal forms; liposomal data from a single open-label study funded by the manufacturer; no efficacy of the intravenous form in two placebo-controlled trials. Training is a separate issue. Ristow and colleagues (2009) showed in 39 young men that large doses of vitamins C and E abolished the beneficial effect of 4 weeks of exercise on insulin sensitivity and blocked the rise of the muscles' own antioxidant enzymes (including glutathione peroxidase). Hence the popular warning not to take antioxidants around training. Note: that study concerned vitamins, not glutathione — we found no equivalent study with GSH, so carrying this conclusion over to glutathione is an extrapolation. We deliberately give no methods of use and no doses.
The wider context
Glutathione is an exception in the encyclopaedia: it is not a signalling peptide but a “working” molecule of metabolism, and its biology resembles vitamins more than hormones. With another tripeptide — KPV — it shares only its size: three amino acids. For a reagent such as GSH, which oxidises easily, the quality and freshness of the batch are crucial — we explain how to read the analytical document in our piece on how to read a CoA certificate.
Summary
Glutathione is the main antioxidant of cells, a tripeptide made in the body from glutamate, cysteine and glycine. A single oral dose does not raise its blood level, and a 4-week trial showed no change — but a 6-month trial in 54 people showed a 30–35% rise in body stores. The liposomal form has only an open-label pilot study in 12 people behind it, and the intravenous form showed no efficacy either in Parkinson's disease or in skin lightening. The best-documented route to higher GSH is the precursors (glycine + NAC). Skin lightening is moderate and short-lived. The state of the evidence: moderate for the biochemistry itself, weak and contradictory for supplementation.
Sources
- Richie JP Jr, Nichenametla S, Neidig W, Calcagnotto A, Haley JS, Schell TD, Muscat JE. Randomized controlled trial of oral glutathione supplementation on body stores of glutathione. European Journal of Nutrition. 2015;54(2):251–263. PMID: 24791752. DOI: 10.1007/s00394-014-0706-z. pubmed.ncbi.nlm.nih.gov/24791752
- Allen J, Bradley RD. Effects of oral glutathione supplementation on systemic oxidative stress biomarkers in human volunteers. Journal of Alternative and Complementary Medicine. 2011;17(9):827–833. PMID: 21875351. DOI: 10.1089/acm.2010.0716. pubmed.ncbi.nlm.nih.gov/21875351
- Kumar P, Liu C, Suliburk J, et al. Supplementing Glycine and N-Acetylcysteine (GlyNAC) in Older Adults Improves Glutathione Deficiency, Oxidative Stress, Mitochondrial Dysfunction, Inflammation, Physical Function, and Aging Hallmarks: A Randomized Clinical Trial. Journals of Gerontology Series A: Biological Sciences and Medical Sciences. 2023;78(1):75–89. PMID: 35975308. DOI: 10.1093/gerona/glac135. pubmed.ncbi.nlm.nih.gov/35975308
- Hauser RA, Lyons KE, McClain T, Carter S, Perlmutter D. Randomized, double-blind, pilot evaluation of intravenous glutathione in Parkinson's disease. Movement Disorders. 2009;24(7):979–983. PMID: 19230029. DOI: 10.1002/mds.22401. pubmed.ncbi.nlm.nih.gov/19230029
- Sarkar R, Yadav V, Yadav T, P J, Mandal I. Glutathione as a skin-lightening agent and in melasma: a systematic review. International Journal of Dermatology. 2025;64(6):992–1004. PMID: 39444151. DOI: 10.1111/ijd.17535. pubmed.ncbi.nlm.nih.gov/39444151
- Witschi A, Reddy S, Stofer B, Lauterburg BH. The systemic availability of oral glutathione. European Journal of Clinical Pharmacology. 1992;43(6):667–669. PMID: 1362956. DOI: 10.1007/BF02284971. pubmed.ncbi.nlm.nih.gov/1362956
- Sinha R, Sinha I, Calcagnotto A, et al. Oral supplementation with liposomal glutathione elevates body stores of glutathione and markers of immune function. European Journal of Clinical Nutrition. 2018;72(1):105–111. PMID: 28853742. DOI: 10.1038/ejcn.2017.132. pubmed.ncbi.nlm.nih.gov/28853742
- Ristow M, Zarse K, Oberbach A, et al. Antioxidants prevent health-promoting effects of physical exercise in humans. Proceedings of the National Academy of Sciences of the United States of America. 2009;106(21):8665–8670. PMID: 19433800. DOI: 10.1073/pnas.0903485106. pubmed.ncbi.nlm.nih.gov/19433800
For in-vitro laboratory research only. It is not a human medicine and is not for treatment.