Peptides

TB-500 and thymosin beta-4: the evidence and the legal status, audited

By the US Health Digest editorial team · Published August 26, 2026 · Every claim linked to its primary source

TB-500 is not thymosin beta-4. It is a synthetic seven-amino-acid fragment of it — residues 17 to 23, with an acetyl group on the leading leucine — carved out of a 43-amino-acid protein the body makes on its own, per FDA's May 15, 2026 review. No FDA-approved drug contains it and there is no USP or NF monograph for it. FDA's reviewers reported that "no articles were found in which TB-500 was administered to humans," and concluded the statutory criteria "weigh against" adding it to the list of substances pharmacists may compound with. On July 23, 2026, FDA's advisory committee recommended it anyway, inside a single 8–6 vote with one abstention covering BPC-157, KPV and TB-500 together — one of six substances of seven nominated that the panel backed. That vote is advisory and non-binding, it is not an FDA approval, and it changed no law. The full-length protein does have a real trial record, and it is not an encouraging one. We publish no dosing, vendors or sourcing.

Three substances, one confusing word

Almost every question readers arrive with collapses three molecules into the word "thymosin." They are not interchangeable, and a result for one transfers to another only by assumption — the rule governing every peptide claim.

What FDA's own reviewers found

The briefing document FDA prepared for the July 23–24, 2026 Pharmacy Compounding Advisory Committee is the most thorough public audit of TB-500 that exists, and it is worth reading for what the agency could not find. On effectiveness: "The nomination did not include, and FDA did not find any information in the medical literature where, TB-500 was administered to patients to treat any disease or condition including its use in wound healing." On human safety: "FDA did not identify clinical studies in humans assessing pharmacokinetics or pharmacodynamics of TB-500 (free base) or TB-500 acetate via any ROA." On the published record: "We did not find case reports in the published medical literature on the use of TB-500 in humans."

On identity, FDA was blunter still. TB-500 "is not physically and chemically well characterized," because it is a common name rather than a formal one, and the agency "has encountered multiple salts, and derivatives, including different active moieties, sold commercially under the same common name" — naming that "represent[s] a safety risk for patients as they may be dosed with a different BDS than the physician ordered." The nomination itself demonstrated the problem: submitted by a compounding pharmacy, Wells Pharmacy Network, it nominated the free base while attaching a certificate of analysis for the acetate salt, and gave a molecular formula and an alternate CAS number matching neither. It was later withdrawn; FDA evaluated the substances on its own initiative.

The conclusion: "a balancing of the criteria weighs against TB-500 (free base) and TB-500 acetate being placed on that list… Accordingly, we propose not adding TB-500 (free base) or TB-500 acetate to the 503A Bulks List."

The vote, and what it did not do

The advisory committee heard that review and voted the other way. TB-500 was taken on day one, July 23, for the nominated use of wound healing, alongside BPC-157, KPV and MOTS-c. Pharmaceutical Executive reported the day-one result as 8–6 with one abstention in favour of BPC-157, KPV and TB-500 — a single tally covering all three, not a separate count for TB-500 — and 7–5 with two abstentions for MOTS-c. Across the two days the panel backed six of the seven nominated substances, rejecting only emideltide, as counsel following the meeting summarised it. We attribute the counts to that reporting: FDA's meeting page publishes the materials but no tally, and had still posted no minutes or transcript when we re-checked on August 26, 2026.

A committee recommendation is not a rule, and it is not an approval. FDA's meeting page says so plainly: "Advisory committees make non-binding recommendations to the FDA, which generally follows the recommendations but is not legally bound to do so." The 503A list is a regulation, 21 CFR 216.23, so adding a substance to it takes rulemaking, not a show of hands. Nothing about TB-500's legal status changed on July 23, and a Federal Register search on August 26, 2026 found no 503A bulk-substance rulemaking published since. Same pattern as BPC-157, carried in the same vote the same afternoon.

Counting the human evidence, with denominators

We ran the search ourselves. PubMed, searched August 26, 2026 for "TB-500," returns 26 records; restricted to clinical-trial or randomized-controlled-trial publication types, it returns zero. Reading all 26: most are anti-doping analytical chemistry — detection methods, extraction, in-vitro metabolism — the rest are narrative reviews, animal work, or false hits matching only the character string. None describes giving TB-500 to a person.

The nonclinical record is also thinner than the marketing implies, and FDA flagged why. The most-cited animal result, Philp 2003, applied the heptapeptide topically to punch wounds in diabetic and aged mice — but used the non-acetylated sequence. Because acetylation "irreversibly alters their charge, hydrophobicity, and size," FDA wrote, "the pharmacological profile of the non-acetylated heptapeptide LKKTETQ cannot be directly extrapolated to the N-acetylated heptapeptide TB-500." When the acetylated molecule that is actually sold was tested, in a 2024 in-vitro study, wound closure in scratched fibroblast cultures "was not significantly different" from vehicle at the concentration tested; one of its breakdown products, not the parent, produced a small effect. The newest primary study we found, published July 23, 2026, is still rats and Achilles tendons; the only record added since is a narrative review.

ClinicalTrials.gov lists one interventional record for the fragment, NCT07487363, a phase 1/2 cardiovascular study posted March 23, 2026. Its own brief summary describes it as "a fictional study… an example of a ClinicalTrials.gov-style record." We treat it as a registration artefact, not evidence — the same call we made on a similar record in our melanotan II audit.

The counterintuitive part: an empty safety database is not a safety record

FDA's surveillance search found almost nothing. Its Office of Surveillance and Epidemiology queried the FDA Adverse Event Reporting System for TB-500 through March 26, 2025 and "did not retrieve any reports"; a search of the human-foods complaint system from January 1, 2004 to March 10, 2025 surfaced two complaints about "blended TB-500 and BPC-157," neither containing a safety assessment.

That is not reassurance, and FDA attached the caveat itself: reporting is voluntary, "FDA does not receive all AE reports that may potentially occur with a product, especially for compounded products," and "considering these limitations, FDA cannot make definitive conclusions regarding the safety of TB-500." Few people file a federal adverse-event report after injecting something bought as a research chemical. Zero reports measures the reporting pathway, not the risk. FDA's affirmative concern is the one chemistry supports: peptides given by injection "may pose a significant risk for immunogenicity, potentially amplified by aggregation as well as potential peptide-related impurities," and no study exists showing TB-500 does not.

The full-length protein did run trials. Here is what they found.

This is the part the marketing skips. Searched again on August 26, 2026, ClinicalTrials.gov holds 19 records for thymosin beta-4 and its formulations — run mainly by RegeneRx Biopharmaceuticals, ReGenTree and Beijing Northland Biotech, in dry eye, neurotrophic keratopathy, pressure ulcers, venous stasis ulcers, epidermolysis bullosa and acute myocardial infarction. Eighteen of the nineteen used the full-length protein or an eye-drop formulation of it, and none of those tested TB-500. The nineteenth is the fragment record discussed below.

The largest completed trial, ARISE-2, randomized 601 participants with dry eye. Its posted results, filed January 5, 2022, give change from baseline at day 29 on both co-primary endpoints: ocular discomfort +0.07 on the drug versus −0.04 on placebo (0–5 scale), and corneal fluorescein staining +0.07 versus −0.01 (0–4 scale). Higher is worse, and no statistical analysis is posted; neither co-primary favoured the drug. A larger sibling, ARISE-3 (700 participants), completed October 7, 2021 and had still posted no results when we re-checked the record on August 26, 2026.

In neurotrophic keratopathy, a phase 3 trial terminated at 18 participants; the publication reports complete healing at four weeks in 6 of 10 treated versus 1 of 8 placebo, p=0.0656, which the authors called "a strong efficacy trend." Cochrane, reviewing the field on December 5, 2025, included that trial and rated it low-certainty: risk ratio 9.00, 95% confidence interval 0.57 to 141.88 — an interval that wide means the study answered nothing. Two registered injectable studies, a phase 1 intravenous trial in healthy volunteers and a phase 2 in acute myocardial infarction, were withdrawn with zero participants enrolled.

Two decades, real sponsors, real registrations, no US approval, and a phase 3 programme whose largest readout missed. That is the strongest evidence base any thymosin peptide has — and it belongs to a different molecule than the one being sold.

Sterility, labels and the name on the vial

FDA's enforcement database records five Class II recalls of compounded thymosin injections — Florida 2018, Indiana 2020 (one thymosin alpha, one thymosin beta-4), and two in Florida in 2025, one a BPC-157/thymosin beta-4 combination. Every one was for lack of sterility assurance. Note the labels: those products were sold as thymosin beta-4, while FDA reported that "no outsourcing facilities have reported compounding drug products containing TB-500" and that "no pharmacies were found that market compounded drug products containing TB-500" — which is a narrower finding than it sounds, and says nothing about the gray-market sellers below. The name on the vial and the molecule inside it are not reliably the same thing — which is why a vendor's certificate of analysis cannot settle the question.

Nor does a disclaimer change what a product legally is. In a March 31, 2026 warning letter concerning two other compounds, FDA wrote: "Despite statements on your product labeling marketing your products for 'Research Use Only,' and 'not intended for human consumption, medical use, or veterinary use,' evidence obtained from your website establishes that your products are intended to be drugs for human use."

FDA has named the fragment in enforcement correspondence more than once. A June 12, 2023 warning letter to Warrior Labz SARMS lists TB-500 among the unapproved new drugs the firm was marketing, quoting its own site copy back to it. And in a January 20, 2026 warning letter to GenoGenix LLC — the Boca Raton facility behind two of the five recalls above — FDA wrote that "Products such as Thymosin Beta-4 are unapproved new drugs under section 505 of the FDCA and also biological products under section 351 of the Public Health Service Act," and that its "products intended to be sterile are produced in an environment that may not provide adequate protection against the risk of contamination."

Banned in sport, and it started with horses

TB-500 entered the doping world before it entered the wellness one. FDA's review records that a veterinary preparation appeared in 2011 and "was marketed to boost performance in equine and greyhound sports," that the World Anti-Doping Agency funded a project in 2013 to establish detection limits for its metabolites, and that "TB-500 is on the list of prohibited substances under section S2.3 of the WADA." The US Anti-Doping Agency's advisory on the 2018 Prohibited List records the addition in the List's own words: "Thymosin-β4 and its derivatives, e.g. TB-500, were added as examples of prohibited growth factors" under S2.3. Anyone drug-tested for work or sport should treat that as disqualifying, and the size of the consumer peptide market means many buyers do not know it.

Questions for a clinician

• I have been offered or have used a peptide labelled "TB-500" or "thymosin beta-4." Which molecule is it, and is there any way to know?
• What FDA-approved options exist for the wound, tendon or injury I am actually trying to treat?
• I self-injected an unlabelled compound. Should I be tested for bloodborne infections, and monitored for injection-site or immune reactions?
• I am subject to drug testing at work or in sport. What does an S2 growth-factor listing mean for me?
• What would you want to see published before you would consider this reasonable?

Peptides are not one category with one answer — the GLP-1 drugs are peptides too, and they carry phase 3 trials and approved labels. TB-500 carries neither. We publish no doses, sources or preparation instructions, and nothing here should be read as telling anyone how to use it.

Sources

1. FDA. Briefing document, TB-500-Related Bulk Drug Substances (TB-500 free base and TB-500 acetate), Pharmacy Compounding Advisory Committee, July 23–24, 2026; memorandum dated May 15, 2026 — chemical identity, absence of human data, FAERS and complaint-system searches, immunogenicity assessment, and the conclusion that the criteria weigh against listing. Link.
2. FDA. July 23–24, 2026 Meeting of the Pharmacy Compounding Advisory Committee — agenda, per-substance briefing documents, and the statement that committee recommendations are non-binding. Link.
3. Jacobus N. FDA Panel Votes to Loosen Restrictions for Four Peptides. Pharmaceutical Executive, July 24, 2026 — day-1 vote counts. Link.
4. FDA. Certain Bulk Drug Substances for Use in Compounding May Present Significant Safety Risks (updated April 22, 2026) — "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" and "Thymosin-alpha 1 (Ta1)" both appear under "Bulk drug substances nominated but withdrawn," flagged as substances that "may pose risk for immunogenicity for certain routes of administration." Page content current as of April 22, 2026; re-checked August 26, 2026. Link.
5. FDA. Thymosin Alpha-1 (Ta1) Related Bulk Drug Substances, Pharmacy Compounding Advisory Committee, December 4, 2024 — 28-amino-acid sequence, and "Ta1 is not approved in the United States, Japan, or Europe (except Italy)." Link.
6. FDA. Warning letter to Gram Peptides, 721806, March 31, 2026 — "Research Use Only" labeling does not determine intended use. Link.
7. FDA. Warning letter to Warrior Labz SARMS, 655280, June 12, 2023 — TB-500 named among unapproved new drugs marketed by the firm, with its own website claims quoted. Link.
8. FDA. Warning letter to GenoGenix LLC, 718739, January 20, 2026 — thymosin beta-4 products are unapproved new drugs under section 505 and biological products under PHS Act section 351; insanitary conditions for products intended to be sterile. Link.
9. FDA drug enforcement (recall) records for thymosin products — five Class II recalls of compounded injections, all for lack of sterility assurance, retrieved via the openFDA API and re-verified August 26, 2026. Link.
10. US Anti-Doping Agency. 2018 Prohibited List: Summary of Major Changes — thymosin-β4 and its derivatives, e.g. TB-500, added as examples of prohibited growth factors under S2.3. Link.
11. Philp D, et al. Thymosin beta 4 and a synthetic peptide containing its actin-binding domain promote dermal wound repair in db/db diabetic mice and in aged mice. Wound Repair Regen. 2003;11(1):19–24. PMID 12581423.
12. Sosne G, et al. Biological activities of thymosin beta4 defined by active sites in short peptide sequences. FASEB J. 2010;24(7):2144–2151. PMID 20179146.
13. Rahaman KA, et al. Simultaneous quantification of TB-500 and its metabolites in in-vitro experiments and rats, and their screening by wound healing activities in vitro. J Chromatogr B. 2024;1235:124033. PMID 38382158.
14. Ho ENM, et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin β4, in equine urine and plasma. J Chromatogr A. 2012;1265:57–69. PMID 23084823.
15. Esposito S, et al. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Test Anal. 2012;4(9):733–738. PMID 22962027.
16. Biçer O, Adanir O, Güleryüz Y. Effects of BPC-157 and TB-500 on Achilles tendon healing in rats: a histopathological and biomechanical study. Jt Dis Relat Surg. 2026;37(3):822–837. PMID 42542926.
17. Mendias CL, Awan TM. Safety and Efficacy of Approved and Unapproved Peptide Therapies for Musculoskeletal Injuries and Athletic Performance. Sports Med. 2026;56(8):1921–1935 — treats Tβ4 and TB-500 as separate entries and notes rigorous human safety data are scarce. PMID 41966639.
18. Sosne G, et al. 0.1% RGN-259 (Thymosin ß4) Ophthalmic Solution Promotes Healing and Improves Comfort in Neurotrophic Keratopathy Patients in a Randomized, Placebo-Controlled, Double-Masked Phase III Clinical Trial. Int J Mol Sci. 2022;24(1):554. PMID 36613994.
19. Kruoch Z, et al. Medical and surgical interventions for neurotrophic keratopathy. Cochrane Database Syst Rev. 2025;12(12):CD015723 — RGN-259 trial, 18 participants, RR 9.00 (95% CI 0.57 to 141.88), low-certainty evidence. PMID 41347649.
20. ClinicalTrials.gov. ARISE-2, NCT02974907 (601 participants, results posted January 5, 2022). Link. ARISE-3, NCT03937882 (700 participants, completed October 7, 2021, no results posted). Link. Phase 3 neurotrophic keratopathy trial, NCT02600429 (terminated, 18 participants). Link. TB-500 fragment record, NCT07487363. Link.