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Also: TB 500 · thymosin beta-4 fragment (17-23) · Ac-LKKTETQ · LKKTETQ · Thymosin Beta-4, Fragment (LKKTETQ) · TB4 frag · TB-550 · TB-5000 · thymosin beta 4 500
Four separate measurements, never averaged. Independent evidence counts only studies with no sponsor attached. Methodology version v2.0-2026-10 · how we grade evidence
Not approved by any regulator, anywhere, for any indication. Claims — stated as claims — include accelerated healing of soft tissue, tendon and ligament injury, reduced scar tissue and adhesions, improved flexibility, muscle and wound repair and recovery from overtraining; it is often sold in a blend with BPC-157 as a "healing stack." It was originally marketed for performance in racehorses and greyhounds.
There are no human studies of TB-500. FDA, 15 May 2026, after searching the literature: "no articles were found in which TB-500 was administered to humans." No clinical studies, no human pharmacokinetics, no human pharmacodynamics, no case reports. The nomination to FDA contained no human clinical evidence, and the independent 2026 scoping review in the American Journal of Sports Medicine confirms separately that no human trials exist for TB-500 specifically. FDA also found no acute toxicity, repeat-dose toxicity, genotoxicity, reproductive, developmental or carcinogenicity studies in any species, and concluded that "potential safety risks associated with the use of these substances in humans are unknown."
TB-500 is not thymosin beta-4. FDA states it in terms — "thymosin beta-4 and TB-500 are not the same substance" — and specifically flags websites that use the two names interchangeably. Thymosin beta-4 is a 43-residue endogenous peptide with a real, registered, placebo-controlled trial record; TB-500 is a 7-residue N-acetylated fragment with none. FDA warns further that wound-healing properties reported for the non-acetylated fragment "cannot be directly extrapolated" to the N-acetylated molecule that is actually sold.
FDA concludes the substance is not physically and chemically well characterized: naming is inconsistent, a single CAS number is used for both forms, the nominated molecular formula matches neither, a representative public certificate of analysis covers only appearance, identity and peptide purity with no testing for assay, impurities, bacterial endotoxins or aggregates, and storage recommendations conflict between sources. In the grey-market survey TB-500 was the worst of fourteen peptides on every measure: 10.0% of samples failed identity testing, median purity was the lowest at 97.29%, and 22.5% and 7.5% of samples met compounded-product and manufactured-drug benchmarks respectively.
At a glance
The proposed mechanism is actin sequestration via the LKKTETQ actin-binding motif, promoting cell migration, angiogenesis and tissue remodelling. FDA notes that its own metabolism data undercut that rationale: studies in horses, rats and human-derived systems show TB-500 is broken down into smaller C-terminal-truncated peptides, and in cell culture the molecule itself did not significantly increase wound closure versus vehicle while one of its metabolites produced a small significant effect. FDA identified no human pharmacokinetic or pharmacodynamic studies by any route, so there is no human mechanistic evidence of any kind.
Dated determinations, each flagged for whether it is final and whether it covers safety only. A safety review is not an efficacy finding, and most reporting in this category conflates the two.
A recommendation to permit compounding of a substance that, on the agency’s own systematic search, has never been administered to a human in any published study. Non-binding: a proposed rule, a comment period and a final rule are all still required, and none had issued as of 13 October 2026. The readable record for the vote is trade coverage rather than an FDA minutes document.
FDA, verbatim: "no articles were found in which TB-500 was administered to humans." There is therefore no human safety data at any dose, route or duration. FDA also found no acute toxicity, repeat-dose toxicity, genotoxicity, reproductive, developmental or carcinogenicity studies in any species and no intramuscular pharmacokinetic studies, and concluded that "potential safety risks associated with the use of these substances in humans are unknown." It states in terms that "thymosin beta-4 and TB-500 are not the same substance," flags websites that use the names interchangeably, and warns that wound-healing properties reported for the non-acetylated fragment "cannot be directly extrapolated" to N-acetylated TB-500. FDA concludes both forms are "not physically and chemically well characterized": inconsistent naming, one CAS number used for both forms, a nominated molecular formula matching neither, a public certificate of analysis covering only appearance, identity and peptide purity with no testing for assay, impurities, bacterial endotoxins or aggregates, and conflicting storage recommendations. A FAERS search through 26 March 2025 returned no reports, and two Human Foods Complaint System cases involved blended TB-500 and BPC-157 with no safety assessment attached; FDA cautions these systems cannot support conclusions about safety. No outsourcing facility has ever reported compounding TB-500 products to the agency.
Removed with 11 other peptides in mid-April 2026 and now listed only in the "nominated but withdrawn" table on FDA’s page, content current as of 22 April 2026. The substance was never in Category 1, the only category covered by FDA’s interim enforcement-discretion policy, so nothing about its compounding status improved.
The 2026 list names TB-500 explicitly as a derivative of thymosin beta-4, so both the fragment and the parent are prohibited at all times.
One of the 19 substances added to Category 2 on 29 September 2023. FDA’s formal listing name is the fragment, not "TB-500", which it treats as a common name and not a USAN.
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The proposed mechanism for this ingredient has not been confirmed in humans. Marketing copy frequently states it as settled. It is not.
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