Glutathione is a tripeptide of glutamate, cysteine and glycine, made by essentially every cell in the body. It is the dominant intracellular antioxidant and a required cofactor for the enzymes that neutralise peroxides and conjugate toxins for excretion. None of that is disputed, and it is why the compound appears in serious clinical research across liver disease, neurodegeneration and chemotherapy toxicity.

It is in this catalogue for a different reason. Glutathione is sold at very large scale as an intravenous "skin brightening" and "detox" drip, and as an oral supplement making the same claim. That market has almost nothing to do with the biology above, and the gap between the two is the entry.

Two mechanisms are proposed and they are not equally supported.

The antioxidant role is established biochemistry: glutathione donates a reducing equivalent, is oxidised to GSSG, and is recycled by glutathione reductase. It is a cofactor rather than a drug in this role.

The skin-lightening claim rests on a separate proposal, that glutathione shifts melanin synthesis away from dark eumelanin toward lighter phaeomelanin, partly by interfering with tyrosinase. That effect is observable in cell culture. Whether it happens in human skin at concentrations achievable by swallowing or infusing the peptide is the open question, and it is the one the marketing skips.

The bioavailability problem sits underneath everything. Glutathione is a peptide, and an oral dose meets the same digestive enzymes any peptide meets.

Thin, and thinnest exactly where the market is largest.

For skin lightening, a randomised double-blind placebo-controlled trial of oral glutathione reported a change in melanin index over four weeks. It is small, short, and in one population. Systematic reviews since have found the overall body of work limited, heterogeneous and at meaningful risk of bias, and have declined to conclude that the effect is established.

For whether oral dosing raises body stores at all, a randomised trial addressed the question directly rather than assuming it, which is more than most of this literature does.

For the intravenous route, which is the most commonly sold and most expensive form, there is no adequately powered randomised evidence for a cosmetic outcome. That is the sharpest fact in this entry: the version people pay the most for is the version with the least behind it.

As an endogenous molecule taken orally, glutathione is generally well tolerated, and that tolerability is routinely used to imply the injectable route is equally benign. It does not.

Intravenous administration is a different exposure with different risks: infusion reactions, and the infection and contamination risks that belong to any injected product prepared outside pharmaceutical manufacturing. Several national regulators have issued public warnings specifically about intravenous glutathione marketed for skin lightening, on the grounds that it is unapproved for that use and that the products involved are frequently unlicensed. Reports of serious harm, including Stevens-Johnson syndrome and kidney injury, exist in that context.

There is also a category error worth naming. Skin lightening is not a medical outcome, and a product sold for it is being used cosmetically at a dose and route nobody has established.

Approved in some jurisdictions as an injectable for specific clinical indications, and widely available as an oral supplement under food rules rather than drug rules, which means its cosmetic claims are not assessed the way a medicine's would be.

Not approved anywhere for skin lightening. Regulators including the Philippine FDA and others have issued explicit advisories against intravenous glutathione sold for that purpose. Its legal status therefore differs sharply depending on the route and the claim, and a single sentence about whether glutathione is "legal" is meaningless without both.