Glutathione

01 · What it is

Glutathione is the one compound in this family that every reader already has, in every cell, in millimolar concentration. The entire question is whether any route of administration raises it where it matters — and forty years of human research has answered that question most convincingly by giving something else.

Glutathione is a tripeptide, but it is not a signalling peptide. It is γ-L-glutamyl-L-cysteinyl-glycine, and the “γ” carries the important structural fact: the bond between glutamate and cysteine is formed through the glutamate side chain rather than its α-carboxyl group. That isopeptide linkage is not recognised by ordinary peptidases, which is how a three-residue molecule survives inside cells at concentrations of 1–10 mM. Its function is chemical rather than informational — it is the cell’s principal thiol antioxidant and the substrate of glutathione peroxidases and glutathione S-transferases, the enzymes that neutralise peroxides and conjugate electrophiles for excretion.

Nothing about that is in dispute. What is in dispute is everything downstream of it.

The bioavailability question was tested directly, and the answer is not what the market assumes. A randomised controlled trial of oral glutathione supplementation on body stores of glutathione in healthy adults set out from the observation that oral GSH had been shown bioavailable in animal models while its efficacy in humans had not been established ⁴. That is the correct framing, and it was still the framing in 2015.

The intravenous route, which is the most commercialised, is the least supported. A 2016 review of glutathione as a skin-whitening agent states that although intravenous glutathione injections are popular, there is no evidence to prove their efficacy ⁵. A 2025 review argues that intravenous glutathione is being mismatched with ozone therapy as though the two were interchangeable, and that reported symptom relief from high-dose intravenous antioxidants may reflect transient effects rather than durable ones ¹³.

The best result in the whole file comes from not giving glutathione. GlyNAC — glycine plus N-acetylcysteine, the two rate-limiting inputs to glutathione synthesis — was given to older adults in a placebo-controlled randomised clinical trial and reported improvement in glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, physical function and a set of measures the authors describe as ageing hallmarks ⁹. Twenty-four older adults and twelve young adults.

That is the shape of this evidence base: a molecule everyone has, a delivery problem nobody has solved, and a precursor strategy that works better than the molecule.

And there is one finding that should temper the whole category. A review of glutathione and immune function concluded that lymphocytes from healthy human subjects have, on average, an optimal glutathione level, and that there is no indication immunological functions such as resistance to infection or vaccine response can be enhanced in healthy people by raising it ¹.

02 · Evidence at a glance

Evidence grade

Mixed Evidence — large human randomised literature; results depend on population, route and formulation

Structure

γ-L-glutamyl-L-cysteinyl-glycine; the γ-linkage resists ordinary peptidases

Functional class

An antioxidant cofactor and enzyme substrate, not a signalling peptide

Endogenous concentration

Millimolar, intracellular, in every cell

Randomised or placebo-controlled human records

~1,084

Oral bioavailability in humans

Contested; tested directly in a randomised trial ⁴

Intravenous efficacy

No evidence supporting it for the most common commercial use ^5,13

Best-supported strategy

Precursor supply — glycine plus N-acetylcysteine ⁹

Skin-lightening evidence

Five RCTs and one open-arm study of oral glutathione, reviewed systematically ¹²; IV unsupported ⁵

Disease trials

Cystic fibrosis ⁶, autism ¹¹, acne ¹⁴

Finding against use in healthy people

Lymphocyte glutathione already optimal in healthy subjects; no indication immune function can be enhanced ¹

Approval status

Not an approved drug for any age-related or cosmetic indication

03 · Mechanism of action

What it actually does

Glutathione exists in a reduced form (GSH) and an oxidised disulfide dimer (GSSG), and the ratio between them is one of the most-used measures of a cell’s redox state. Three roles follow.

It is the substrate of glutathione peroxidases, which reduce hydrogen peroxide and lipid peroxides to water and alcohols, oxidising GSH to GSSG in the process. It is the substrate of glutathione S-transferases, which conjugate it to electrophilic compounds — including many drug metabolites — to make them water-soluble and excretable; this is the principal route by which the liver disposes of paracetamol’s toxic metabolite. And it maintains protein thiols in the reduced state, which is required for the function of a large number of enzymes.

None of this is a hypothesis. It is textbook biochemistry, and it is the reason glutathione depletion is genuinely harmful. It does not follow that supplementation in a person with normal glutathione is beneficial, and the distinction between correcting a deficiency and exceeding a normal value is the central issue in this file.

Synthesis, and the rate-limiting step

Glutathione is made inside cells in two ATP-dependent steps. Glutamate cysteine ligase joins glutamate to cysteine — this is the rate-limiting step, and cysteine availability is usually what limits it — then glutathione synthetase adds glycine.

That biochemistry explains why the precursor approach outperforms the molecule. Supplying cysteine (as N-acetylcysteine, which is stable where free cysteine is not) and glycine gives the cell what it needs to make glutathione at the point where it is needed ⁹. Related work has examined whether methionine supplementation can increase protein-derived cysteine and upregulate cysteine synthesis to improve glutathione synthesis, in children being treated for oedematous malnutrition ³, and whether N-acetylcysteine regenerates native mercaptoalbumin in subjects with redox imbalance after oral and intravenous administration ⁸.

The delivery problem

Administered glutathione has to reach the inside of cells to do any of the things described above, and several obstacles stand between an oral or intravenous dose and that destination.

Orally, the tripeptide encounters γ-glutamyl transpeptidase on the intestinal brush border, an enzyme whose function is to break the γ-linkage and recover the constituent amino acids. Whether intact glutathione crosses the gut in meaningful quantity is precisely what the randomised bioavailability trial was designed to establish ⁴.

Intravenously, the dose bypasses the gut but reaches plasma, where glutathione concentration is normally around a thousandfold lower than inside cells. Moving it from plasma into cells against that gradient requires transport that has not been demonstrated to occur at useful rates in humans. The 2025 critique of intravenous antioxidant practice makes essentially this point ¹³.

This is the same problem NAD+ has, for the same reason, with the same commercial response: infusions sold on a mechanism whose delivery step is unestablished.

04 · Key research findings

The precursor result, which is the strongest thing here. A placebo-controlled randomised clinical trial supplemented glycine and N-acetylcysteine in older adults, comparing 24 older adults against 12 young adults, and reported improvement in glutathione deficiency, oxidative stress, mitochondrial dysfunction, insulin resistance, physical function and measures the authors group as ageing hallmarks ⁹.

A randomised trial, in older adults, with a mechanistic rationale that follows directly from the rate-limiting step, reporting improvement across several domains. It is also small, single-centre, and tests two amino acid derivatives rather than glutathione.

Oral bioavailability, tested. A randomised controlled trial examined the long-term effectiveness of oral glutathione supplementation on body stores of glutathione in healthy adults, opening from the position that human efficacy had not been established despite animal bioavailability data ⁴.

That this trial had to be run in 2015, decades into commercial availability, is itself the finding.

Skin lightening — the largest commercial use, and the evidence for it. A 2016 review found that three randomised controlled trials supported a skin-lightening effect, and stated that although intravenous glutathione injections are popular there is no evidence to prove their efficacy ⁵. A 2025 systematic review identified five randomised controlled trials and one open-arm clinical study of oral glutathione, at doses of 250 mg once daily, 250 mg twice daily and 500 mg once daily, and reported greater improvement when glutathione was combined with microneedling than alone ¹². A separate randomised, double-blinded, benchmark- and placebo-controlled trial tested oral L-cystine combined with reduced L-glutathione on human skin pigmentation ⁷.

The oral formulation has randomised evidence in this indication. The intravenous formulation, which is what is most often sold and injected, does not — and the review that says so is a dermatology review, not a sceptic’s op-ed ⁵.

Disease trials, with mixed and mostly modest results. The GROW study was a prospective, multicentre, randomised, placebo-controlled, double-blind phase 2 trial of oral glutathione and growth in pancreatic-insufficient children with cystic fibrosis aged 2 to 10 ⁶; the portion of its abstract available for this guide reports no significant differences in secondary outcomes, and the primary result is flagged for verification rather than stated here. A double-blind placebo-controlled randomised crossover pilot tested glutathione alone and glutathione with vitamin C and N-acetylcysteine in 24 autistic children aged 5 and over with disruptive behaviour ¹¹. A randomised controlled trial tested 500 mg oral glutathione on serum nitric oxide and IL-1α concentrations and clinical improvement in mild to moderate acne vulgaris ¹⁴.

Three randomised trials, three unrelated conditions, none producing a result that has changed practice.

Vascular and redox endpoints. Combined citrulline and glutathione supplementation improved endothelial function and blood pressure reactivity in 44 healthy postmenopausal women randomised over four weeks to citrulline alone, citrulline plus glutathione, or placebo ¹⁰. A secondary analysis of a randomised crossover trial found creatine plus β-hydroxy-β-methylbutyrate supplementation associated with attenuation of the placebo-related increase in oxidised glutathione in older adults, with functional test changes examined only as exploratory correlates ¹⁵.

Both are combination studies in which glutathione is one component, and the second is explicit that its functional analyses were exploratory.

Parkinson’s disease. A review of antioxidants and supplements in Parkinson’s disease identified one study of glutathione alongside larger literatures on tocopherol and coenzyme Q10 ².

One study, two decades ago, in a disease where glutathione depletion in the substantia nigra is a well-documented pathological finding. The gap between a compelling rationale and a trial programme is as wide here as anywhere in this project.

The finding that argues against use in healthy people. A review of glutathione and immune function concluded that lymphocytes from healthy human subjects have, on average, an optimal glutathione level, with no indication that immunological functions such as resistance to infection or response to vaccination could be enhanced in healthy subjects ¹.

Published in 2000, and not superseded by anything in this file. A molecule that is already at its optimum in healthy tissue is not a molecule with headroom.

05 · Evidence overview

06 · Safety profile

Endogenous status changes what safety means here. Glutathione is present in every human cell at millimolar concentration and turned over continuously. It is not a xenobiotic, and the standard toxicological framework applies awkwardly. Oral doses in the studies cited are in the range of hundreds of milligrams daily ^12,14, against a body pool measured in grams.

Human data. Randomised trials have administered oral glutathione to healthy adults ⁴, to children with cystic fibrosis ⁶, to autistic children ¹¹, to people with acne ¹⁴ and to postmenopausal women in combination ¹⁰, without reports in the reviewed abstracts of serious harm attributable to the intervention.

What is genuinely unknown. Whether intravenous administration reaches the intracellular compartment at all, which is the question on which every claim for the intravenous route depends and which no study in this file answers ¹³. Whether raising glutathione above normal in a healthy person does anything, given that lymphocyte concentrations are already optimal in healthy subjects ¹. Whether sustained antioxidant supplementation blunts the adaptive responses that depend on transient oxidative signalling — a general concern for the antioxidant category that the exercise and mitochondrial literature raises and that these trials were not designed to detect. The oncology question, which is the most serious one: glutathione conjugation is a principal mechanism by which tumour cells resist alkylating chemotherapy, and elevated tumour glutathione is an established resistance mechanism, so systemic supplementation during cancer treatment has a theoretical capacity to work against it — a concern no trial in this file addresses. And the composition and sterility of intravenous preparations compounded outside a pharmaceutical supply chain, on which nothing is published.

07 · US regulatory status

Current as of 6 September 2026. Glutathione is not approved as a drug in the United States for any age-related, cosmetic or performance indication and is not a controlled substance.

Oral glutathione is sold as a dietary supplement ingredient, a category requiring safety but not demonstrated efficacy. Intravenous glutathione administered in clinics is a compounded injectable preparation of a substance with no approved injectable form in the United States, and the dermatology literature is explicit that its efficacy for the most common such use is unproven ⁵.

The compound has a legitimate clinical presence in a different setting: glutathione depletion is central to paracetamol toxicity, where the treatment is N-acetylcysteine — a precursor, given to restore synthesis, rather than glutathione itself. That is the clearest illustration in medicine of the strategy this field keeps rediscovering.

Under the World Anti-Doping Code, glutathione does not appear as a named prohibited substance in the classes reviewed for this guide, though intravenous infusions above defined volume limits are restricted by the prohibited-methods provisions on intravenous infusion. Competitors should consult the current Prohibited List directly rather than rely on secondary summaries, including this one.

08 · Limitations of the evidence

1. The intravenous route has the most commerce and the least evidence. A dermatology review states plainly that there is no evidence proving the efficacy of intravenous glutathione injections for skin lightening ⁵, and a 2025 critique argues the intravenous antioxidant field is conflating modalities and mistaking transient effects for durable ones ¹³.
2. Oral bioavailability was still an open question in a 2015 randomised trial ⁴, long after the product was widely sold. The γ-linkage that protects glutathione inside cells is the substrate of an intestinal enzyme whose job is to cleave it.
3. The best-evidenced intervention is not glutathione. GlyNAC supplies the precursors and reported improvements across several domains in a randomised trial ⁹. Citing that result as support for glutathione products is the same category error this project has flagged for TB-500, CJC-1295 No-DAC and NAD+.
4. Healthy people may have no headroom. Lymphocyte glutathione in healthy subjects is on average already optimal, with no indication that immune function can be enhanced by raising it ¹. Most commercial use is by people who are not deficient.
5. The GlyNAC trial is small and single-centre. Twenty-four older adults against twelve young adults ⁹, with a broad panel of outcomes. Broad outcome panels in small trials generate positive findings by construction, and replication at scale has not been reported here.
6. The disease trials are scattered and mostly modest. Cystic fibrosis ⁶, autism ¹¹ and acne ¹⁴ are three unrelated conditions; none of these results has changed practice, and for the cystic fibrosis trial this guide could not verify the primary outcome from the available abstract.
7. Several trials tested combinations, not glutathione alone. Citrulline plus glutathione ¹⁰, L-cystine plus glutathione ⁷, glutathione plus vitamin C plus N-acetylcysteine ¹¹, creatine plus HMB with glutathione as a readout ¹⁵. Attributing a combination’s effect to one component requires an arm those studies did not always include.
8. The oncology interaction is unexamined and is the most serious theoretical risk. Glutathione conjugation is an established mechanism of tumour resistance to alkylating agents. No trial here addresses supplementation during cancer treatment in either direction.
9. Antioxidant supplementation has a poor record at outcome level across medicine, and this file contains no long-duration outcome trial of glutathione in any indication.
10. Nothing is known about compounded intravenous material. No published analysis has examined the composition, concentration or sterility of intravenous glutathione preparations administered outside a pharmaceutical supply chain.

09 · References

1. Dröge W, Breitkreutz R. Glutathione and immune function. Proc Nutr Soc. 2000 Nov;59(4):595–600.

   PMID 11115795 ↗

2. Weber CA, Ernst ME. Antioxidants, supplements, and Parkinson’s disease. Ann Pharmacother. 2006 May;40(5):935–938.

   PMID 16622156 ↗

3. Green CO, Badaloo AV, Hsu JW, Taylor-Bryan C, Reid M, Forrester T, Jahoor F. Effects of randomized supplementation of methionine or alanine on cysteine and glutathione production during the early phase of treatment of children with edematous malnutrition. Am J Clin Nutr. 2014 May;99(5):1052–1058.

   PMID 24598154 ↗

4. 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. Eur J Nutr. 2015 Mar;54(2):251–263.

   PMID 24791752 ↗

5. Sonthalia S, Daulatabad D, Sarkar R. Glutathione as a skin whitening agent: facts, myths, evidence and controversies. Indian J Dermatol Venereol Leprol. 2016 May–Jun;82(3):262–272.

   PMID 27088927 ↗

6. Bozic M, Goss CH, Tirouvanziam RM, Baines A, Kloster M, Antoine L, Borowitz D, Schwarzenberg SJ; GROW study group. Oral glutathione and growth in cystic fibrosis: a multicenter, randomized, placebo-controlled, double-blind trial. J Pediatr Gastroenterol Nutr. 2020 Dec;71(6):771–777.

   PMID 32960827 ↗

7. Duperray J, Sergheraert R, Chalothorn K, Tachalerdmanee P, Perin F. 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. J Cosmet Dermatol. 2022 Feb;21(2):802–813.

   PMID 33834608 ↗

8. Altomare AA, Brioschi M, Eligini S, et al. N-acetylcysteine regenerates in vivo mercaptoalbumin. Antioxidants (Basel). 2022 Sep 6;11(9):1758.

   PMID 36139832 ↗

9. Kumar P, Liu C, Suliburk J, Hsu JW, Muthupillai R, Jahoor F, Minard CG, Taffet GE, Sekhar RV. 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. J Gerontol A Biol Sci Med Sci. 2023 Jan 26;78(1):75–89.

   PMID 35975308 ↗

10. Figueroa A, Maharaj A, Kang Y, Dillon KN, Martinez MA, Morita M, Nogimura D, Fischer SM. Combined citrulline and glutathione supplementation improves endothelial function and blood pressure reactivity in postmenopausal women. Nutrients. 2023 Mar 23;15(7):1557.

    PMID 37049398 ↗

11. Williams PG, Sears L, Watson WH, Gunaratnam B, Feygin Y, Wright SP, Sullivan JE. Glutathione, vitamin C, and cysteine use in autistic children with disruptive behavior: a double-blind, placebo-controlled crossover pilot study. J Dev Behav Pediatr. 2025 Jan–Feb;46(1):e17–e24.

    PMID 39960783 ↗

12. Sarkar R, Yadav V, Yadav T, P J, Mandal I. Glutathione as a skin-lightening agent and in melasma: a systematic review. Int J Dermatol. 2025 Jun;64(6):992–1004.

    PMID 39444151 ↗

13. Chirumbolo S, Franzini M, Ricevuti G, Valdenassi L. Intravenous glutathione should not be mismatched with ozone as an antioxidant therapy. Biomed Pharmacother. 2025 Jul;188:118197.

    PMID 40408806 ↗

14. Mawu FO, Anwar AI, Djawad K, Bukhari A, Kapantow MG, Christopher PM. Effectiveness of oral glutathione in reducing nitric oxide and IL-1α concentrations for clinical improvement in mild to moderate acne vulgaris: a randomized controlled trial. Acta Dermatovenerol Alp Pannonica Adriat. 2025 Sep;34(3):95–99.

    PMID 41014073 ↗

15. Ramos-Hernández R, Mielgo-Ayuso J, Fernández-Lázaro D, Abia A, Pérez-López JF, Saiz-Rodríguez M, Busto N. Creatine plus β-hydroxy-β-methylbutyrate supplementation is associated with preserved glutathione redox-balance and redox-function associations in older adults: a secondary analysis of a randomized crossover trial. Biogerontology. 2026 Feb 19;27(2):58.

    PMID 41712056 ↗