TB-500 in 2026: What It Is, Where FDA Stands, and What the Evidence Shows

TB-500 is a synthetic 7-amino-acid peptide (Ac-LKKTETQ), the acetylated 17-23 fragment of the natural protein thymosin beta-4. [1][2] It is not the same molecule as full-length thymosin beta-4.

• TB-500 is not FDA-approved for any use.
• It was in FDA's Category 2 (substances that may present significant safety risks in compounding) from September 29, 2023 until April 22, 2026, per HHS. CDER recommended against listing it for compounding; FDA's advisory committee (PCAC) voted in favor. [2]
• FDA's page now lists TB-500 under "nominated but withdrawn." FDA says compounded TB-500 may pose a risk of immune reactions for certain routes, and that it has not identified any human exposure data. [8]
• HHS published a draft policy (proposed, not final) on Oct 30, 2026 naming TB-500 among six peptides that licensed pharmacies could compound under strict conditions. It is not binding, and comments are open. [2][3]
• Compounded drugs are not FDA-approved, which means FDA does not review their safety, effectiveness, or quality before they are marketed. [2]

• We found no published controlled human trials of TB-500 itself (PubMed search, Nov 4, 2026). Most TB-500 papers are lab, animal or doping-detection studies. [1][4]
• Trials exist for full-length thymosin beta-4, including a phase 1 safety study and a phase 2 eye-drop study. These test a different molecule, so they do not show TB-500 is safe or that it works. [5][6]
• A 2026 review of peptides in sports medicine found "rigorous human safety data are scarce" for unapproved peptides including TB-500, with "potential for serious harm." [7]
• HHS's justification says "the safety concerns CDER identified are inferential rather than observed." [2] Put simply, harm has not been documented, but it also has not been studied in people; FDA reports no human exposure data. [8]

• Much of the TB-500 sold online is labeled "research use only," which means it is not made for people.
• HHS describes "research-use-only peptides supplied by foreign manufacturers that pose a safety risk to US patients." [2]
• Under the draft policy, a pharmacy would need a lot-specific certificate of analysis from an FDA-registered manufacturer testing identity, purity, endotoxins, microbes and residual solvents. Online vials may come with none of this. [2]
• What to ask any prescriber: Is it from a state-licensed pharmacy? Is there a lot-specific COA from an FDA-registered maker? Is it single-ingredient?

No. TB-500 is prohibited by WADA: the 2026 Prohibited List names "Thymosin-β4 and its derivatives e.g. TB-500" under S2.3, prohibited at all times. [9]

1. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500. Drug Test Anal. 2012. PMID 22962027. https://pubmed.ncbi.nlm.nih.gov/22962027/
2. HHS policy PDF (Appendix A/B), read Nov 4, 2026. https://www.hhs.gov/sites/default/files/hhs-503a-peptide-enforcement-discretion-policy-appendix-b.pdf
3. 91 FR 68526, Oct 30, 2026, Docket HHS-OASH-2026-0496. https://www.federalregister.gov/documents/2026/10/30/2026-22192/
4. Doping control analysis of TB-500. J Chromatogr A. 2012. PMID 23084823.
5. Thymosin β4 in severe dry eye, phase 2 RCT. Cornea. 2015. PMID 25826322.
6. Phase I study of recombinant human thymosin β4. J Cell Mol Med. 2021. PMID 34346165.
7. Mendias CL, Awan TM. Sports Med. 2026;56(8):1921-1935. PMID 41966639.
8. FDA, Safety Risks Associated with Certain Bulk Drug Substances Nominated for Use in Compounding (content current as of 04/22/2026). https://www.fda.gov/drugs/human-drug-compounding/safety-risks-associated-certain-bulk-drug-substances-nominated-use-compounding
9. WADA, 2026 Prohibited List, S2.3. https://www.wada-ama.org/en/resources/world-anti-doping-code-and-international-standards/prohibited-list