"TB-500" and "thymosin beta-4" are used interchangeably by many sellers and forums, and sometimes printed together on one label. They are two different molecules. One is a protein the body makes and that has been tested in people; the other is a short synthetic fragment of it that has not. This page sets out the chemistry, where the name TB-500 came from, which molecule each human study used, and what that means for anyone reading a vial label or a dosage figure.
Plain explainers of each molecule are on Peptide Lexicon's thymosin beta-4 and TB-500 entries. The adverse-event record — and its absence for TB-500 — is on our TB-500 side effects page. Nothing here is a recommendation; any decision about an unapproved substance belongs with a clinician.
The two molecules side by side
| Thymosin beta-4 | TB-500 | |
|---|---|---|
| What it is | Natural protein, found in most cells; binds actin | Synthetic acetylated fragment of that protein |
| Length | 43 amino acids (acetylated N-terminus) | 7 amino acids, Ac-LKKTETQ |
| Sequence | SDKPDMAEIEKFDKSK · LKKTETQ · EKNPLPSKETIEQEKQAGES (dots mark the shared stretch) | LKKTETQ (positions 17–23), N-acetylated |
| Molecular mass | ~4,964 Da (our calculation from UniProt P62328) | 889.0 (PubChem CID 62707662) |
| Molecules per mg, relative | 1 | ~5.6× |
| Human trials | Yes — eye drops, skin gel, IV infusion | None published (FDA, May 2026) |
| Approved anywhere | No | No |
| Sport | Prohibited (WADA S2.3) | Prohibited (WADA S2.3, named); NCAA banned |
The bold stretch is the only thing the two share. Thymosin beta-4's other 36 residues — including its first four, which the body cleaves off to make a separate signalling peptide, Ac-SDKP — are absent from TB-500.
The mass arithmetic. UniProt records the encoded chain as 44 residues and 5,053 Da; removing the initiator methionine (about 131 Da) and adding the acetyl group (42 Da) gives about 4,964 Da for the mature protein. Dividing by TB-500's 889 gives 5.58. So a milligram of TB-500 holds about 5.6 times as many molecules as a milligram of thymosin beta-4. If a figure in milligrams was worked out for one and applied to the other, the number of molecules is off by more than fivefold in one direction or the other.
Where the name TB-500 came from
TB-500 entered the literature not through a drug company but through doping control. In 2012, Ghent University's doping laboratory reported analysing a product sold as TB-500 and identifying in it the N-terminal acetylated 17–23 fragment of thymosin beta-4, Ac-LKKTETQ (Esposito et al., Drug Testing and Analysis, PMID 22962027). The same year, a group developing equine tests described TB-500 as "a synthetic version of an active region of thymosin β4" and built a method to detect it in horse urine and plasma (PMID 23084823).
So "TB-500" names a product category before it names a molecule. The doping lab's finding established what was in the product it tested. Whether every product sold as TB-500 today contains the heptapeptide, the full protein, or something else is a question only a laboratory analysis of that vial can answer; our page on how to read a COA explains what such a report should show.
What the human trials used — all of them thymosin beta-4
Every human study in the record gave the full-length protein, and none by the route TB-500 is sold for:
| Study | Molecule and route | People | Result |
|---|---|---|---|
| IV safety study, Ann N Y Acad Sci 2010 (PMID 20536472) | Thymosin beta-4, intravenous | Healthy volunteers | Single and multiple doses, placebo-controlled; a safety study, no efficacy outcome |
| Venous ulcers, 2007 and 2010 (PMID 17495250, 20536470) | Thymosin beta-4 gel on the skin | Patients with leg ulcers | Small European trial; reported on safety, tolerability and healing |
| Severe dry eye phase 2, Cornea 2015 (PMID 25826322) | RGN-259 (0.1% thymosin beta-4) eye drops, 6× daily for 28 days | 9 patients | Day 56: 35.1% less discomfort and 59.1% less corneal staining than vehicle (12 eyes vs 6) |
| ARISE-3, completed 2021 (NCT03937882) | RGN-259 eye drops | ~700 adults with dry eye | Primary endpoints at day 15: staining −0.41 vs −0.46 on placebo; discomfort −0.4 vs −0.4 |
| Phase I, J Cell Mol Med 2021 (PMID 34346165) | Recombinant human thymosin beta-4 | Healthy Chinese volunteers | First-in-human, randomised, double-blind single- and multiple-dose safety study |
The table contains the most important fact on this page twice over. First, the only sizeable efficacy trial posted primary results that did not separate from placebo: ARISE-3's means for both co-primary measures were practically the same on drug and placebo. The positive dry-eye result came from nine patients. Second, no row involves TB-500, and no row involves an injection under the skin for healing or recovery — the use TB-500 is marketed for.
FDA's reviewers made the same point in regulatory language in their May 2026 evaluation: they found no clinical study and no human exposure data for TB-500 by any route, and wrote that its pharmacology cannot be directly extrapolated from thymosin beta-4. The detail is on our TB-500 side effects page.
How the two are labelled for sale
Product listings use the names in three ways: "TB-500" alone, "thymosin beta-4" alone, and "TB-500 (Thymosin Beta-4)" as if they were synonyms. The third form describes two molecules with one label, and it matters for anyone reading a figure off a page — a milligram amount quoted for the full protein, applied to the fragment, delivers about 5.6 times as many molecules; the reverse delivers about one-sixth. The pricing consequence — one name buying two different molecules — is noted on our peptide cost page. Figures quoted for TB-500 are reported on our TB-500 dosage page, which identifies which molecule each source describes.
Sport testing treats them as one class
For athletes the distinction does not help. The 2026 WADA Prohibited List places "thymosin-β4 and its derivatives, e.g. TB-500" under S2.3, prohibited in and out of competition (WADA Prohibited List), and the NCAA names TB-500 in its 2026–27 banned classes. Methods to detect the fragment in urine and plasma were published in 2012. MuscleLedger covers testing from the athlete's side on its TB-500 page.