Thymosin beta-4 is a 43-amino-acid peptide found in most human cells and in high concentration in platelets and wound fluid. It is a major intracellular actin-sequestering protein.

An important distinction that most discussion collapses: TB-500 as sold is generally a synthetic fragment, commonly the Ac-LKKTETQ sequence region, not full-length thymosin beta-4. The clinical work that exists, largely from RegeneRx in ophthalmology and dermal wound healing, used the full-length peptide. Citing that work in support of the fragment is a substitution that happens constantly and is rarely flagged.

The characterised intracellular function is sequestration of G-actin monomers, regulating actin polymerisation and therefore cell motility, and this is well established as cell biology.

The extracellular effects proposed for therapeutic use, promotion of angiogenesis, cell migration into wounds, modulation of inflammatory signalling, and reduction of fibrosis, are less well characterised, and the receptor or receptors mediating extracellular activity are not definitively identified. The heptapeptide region is the fragment usually credited with the actin-binding activity, which is the basis for using it in place of the whole molecule.

Early clinical, in narrow indications, using the full-length peptide.

Randomised trials in dry eye disease and in neurotrophic keratopathy have reported signals on ocular surface endpoints, with mixed results across trials and endpoints. Work in dermal wound healing, including venous stasis and diabetic ulcers, has produced early-phase results without a definitive programme.

For the musculoskeletal and recovery uses that dominate community discussion, there is no adequately powered human evidence for either the full peptide or the fragment. Preclinical cardiac work in animal infarct models is frequently cited and has not translated into completed human outcome trials.

Human safety data comes mostly from topical and ophthalmic formulations, where tolerability has generally been acceptable. That does not transfer to systemic administration.

Mechanistically the concerns are the same family as any pro-angiogenic and pro-migratory agent: effects on undiagnosed malignancy, and unknown consequences of sustained systemic exposure. No long-term systemic safety data exists.

Prohibited in sport under WADA's S2 category. Material sold as TB-500 is research-grade with unverified identity. And given the full-peptide/fragment confusion, "unverified identity" is not a theoretical concern here.

Not approved for human use in any major jurisdiction. Full-length thymosin beta-4 has held investigational status in ophthalmology programmes. TB-500 as sold is a research chemical, prohibited in sport.