Sold as though it were thymosin beta-4. It is a fragment of it, and the distinction has consequences.
what it is
Thymosin β4 is a naturally occurring protein involved in actin binding and cell migration, with a real research history including human trials for corneal and dermal wound healing. TB-500 is a synthetic fragment of that molecule. Vendors and forums use the names interchangeably; they are not the same compound and the clinical work belongs to the full-length protein.
what the evidence shows
For TB-500 specifically: preclinical work only. No human efficacy trials. The human data that exists — and there is some — was generated with full-length thymosin β4 in ophthalmic and dermatological indications, and none of it has produced an approval.
Sold as though it were thymosin beta-4.
what stays uncertain
Whether the fragment reproduces the parent protein's activity at all, which is not a given. Systemic administration of a molecule involved in cell migration and angiogenesis raises questions about tissue you did not intend to affect, and those questions have not been studied in people.
the practical note
When you see the human thymosin β4 literature cited on a TB-500 product page, that is a different molecule doing the persuading.