Peptides & growth hormone

TB-500 (Thymosin Beta-4)

Updated · 23 sources · Editorial policy

TB-500 is a synthetic peptide sold online and by some clinics for injury recovery, tendon and muscle repair, and endurance. It is usually marketed as “synthetic thymosin beta-4,” but that label is wrong. FDA and doping-control laboratories identify TB-500 as a short 7-amino-acid fragment of thymosin beta-4 (Tβ4) with an acetyl group added: Ac-LKKTETQ. Thymosin beta-4 itself is a 43-amino-acid peptide found naturally in many types of human cells.

The single most important fact: no study has ever given TB-500 to humans. FDA’s 2026 review found no clinical trials, no human pharmacokinetic data and no animal toxicology studies for it. The human trials of “thymosin beta-4” tested the full-length natural molecule, a different substance, mostly as eye drops or skin treatments. The two largest of those trials missed their main goals. TB-500 is banned in sport at all times.

This page summarizes what FDA’s review, the animal studies and the thymosin beta-4 trials actually show. It is for education only and is not medical advice.

Property Detail
Class Synthetic 7-amino-acid peptide (Ac-LKKTETQ), a fragment of thymosin beta-4; investigational
Parent molecule Thymosin beta-4: a natural 43-amino-acid peptide (molecular weight 4,982)
Approval TB-500 is not part of any FDA-approved drug and is not approved in the EU, UK, Canada or Australia; full-length thymosin beta-4 is also still investigational
Human studies of TB-500 None identified by FDA (2026)
Half-life No human data for TB-500. In horses, plasma levels became unmeasurable 6–10 h after a 10 mg injection. Full-length thymosin beta-4 given IV to humans: 0.5–2.1 h
Mechanism Contains the actin-binding region (LKKTET) of thymosin beta-4; whether TB-500 itself is active is unproven
US compounding status Not on the 503A bulks list; FDA staff advised against adding it; an advisory committee voted 8–6 in favor in July 2026; no final FDA action
WADA Prohibited at all times (S2.3, growth factors); “Thymosin-ß4 and its derivatives e.g. TB-500” are named explicitly

What it is

Thymosin beta-4, the parent peptide

Thymosin beta-4 was first sequenced in 1981, from calf thymus. It has 43 amino acids, a molecular weight of 4,982, and an acetylated starting end (Low et al., PNAS 1981). It was first thought to be a thymus hormone. Later work showed it is present in many cell types and in blood, saliva, tears and wound fluid (FDA briefing document, 2026).

Its best-established role is inside cells. Thymosin beta-4 binds single actin molecules (G-actin) one-to-one and stops them from joining into filaments. Actin filaments form the cell’s internal skeleton, which cells use to move and change shape. In resting human platelets, at least half of all actin is unpolymerized, and most of it is held by thymosin beta-4 (Safer et al., J Biol Chem 1991).

TB-500, the fragment

A veterinary product called TB-500 appeared around 2011 and was marketed to boost performance in horse and greyhound racing (FDA summary of Ho et al. 2012). When doping laboratories analyzed it, the key ingredient was amino acids 17–23 of thymosin beta-4 with an acetyl group added at the front (Esposito et al., Drug Test Anal 2012; Ho et al., J Chromatogr A 2012).

Feature Thymosin beta-4 (Tβ4) TB-500
Length 43 amino acids 7 amino acids (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln)
Molecular weight 4,982 g/mol 889.0 g/mol (free base); 949.1 g/mol (acetate salt)
Origin Made naturally in the body Synthetic only
Tested in humans Yes: phase 1–3 trials (IV, topical on skin, eye drops) No
Safety data Phase 1–3 trial safety data None: no human studies and no animal toxicology studies identified by FDA

Two forms are sold, the free base and the acetate salt, and FDA treats them as separate substances. FDA also found that “TB-500” is not an official drug name, and that it has seen different salts and derivatives, including different active molecules, sold under the same name. A patient may therefore receive a different substance than the one prescribed (FDA briefing document, 2026).

“TB-500 is synthetic thymosin beta-4.” Many sellers use the two names interchangeably. FDA states plainly that thymosin beta-4 and TB-500 “are not the same substance.” Results from thymosin beta-4 trials cannot be assumed to apply to TB-500.

Mechanism. The TB-500 sequence contains LKKTET, the actin-binding region of thymosin beta-4. It has been suggested that the fragment might mimic the parent’s actin-binding effects, but this is a hypothesis. FDA also notes that adding an acetyl group changes a peptide’s charge, size and binding behavior. So results with the plain fragment (LKKTETQ) cannot be directly applied to acetylated TB-500 (FDA briefing document, 2026).

Pharmacokinetics

There are no human pharmacokinetic data for TB-500. What is known comes from horses, rats and lab enzyme systems (FDA briefing document, 2026):

Finding Source
After 10 mg under the skin in horses, plasma TB-500 peaked at 0.05–0.08 ng/mL at 1–2 h and was no longer measurable at 6–10 h Horses (Ho et al. 2012)
Broken down by trimming amino acids from the tail end, into Ac-LKKTET, Ac-LKKTE, Ac-LKKT, Ac-LKK and Ac-LK Horse liver, horse plasma and urine
Ac-LK was the main metabolite in rat urine at 0–6 h; Ac-LKK was still detectable up to 72 h Rats, intraperitoneal injection (Rahaman et al. 2024)
The same breakdown products form in human serum and in human liver and kidney preparations In vitro

The long-lasting metabolites matter for drug testing, not for effect. No study has shown how long TB-500 or its fragments act in the body.

For comparison, full-length thymosin beta-4 has been measured in people. In a phase 1 trial of a recombinant (genetically engineered) version given IV to healthy volunteers, peak levels came 3–15 minutes after dosing and the half-life was 0.5–2.08 h. Levels did not build up over 10 days of daily dosing (Wang et al., J Cell Mol Med 2021).

Regulatory status

  • Not an approved drug. TB-500 has no US drug monograph and is not a component of any FDA-approved drug. FDA found no approved TB-500 products in 10 other countries checked, and none authorized by the European Medicines Agency (FDA briefing document, 2026). Full-length thymosin beta-4 eye drops are still in phase 3 testing (ClinicalTrials.gov NCT05555589).
  • FDA safety listing. FDA’s list of bulk substances that may present significant safety risks includes “thymosin beta-4, fragment (LKKTETQ), also known as TB-500” among withdrawn nominations. It says FDA “has not identified any human exposure data” and lacks information on “whether it would cause harm if administered to humans” (FDA, updated April 22, 2026).
  • 2026 advisory committee review. A compounding pharmacy had nominated TB-500 as a 3 mg/mL injection for wound healing, then withdrew the nomination. FDA reviewed it on its own initiative (Federal Register, docket FDA-2025-N-6895). FDA staff recommended not adding either form to the 503A bulks list, citing poor chemical characterization, no evidence of effectiveness, no human safety data, and immunogenicity risk (FDA briefing document, 2026).
  • The committee disagreed. On July 23, 2026, the panel voted 8–6, with 1 abstention, to recommend TB-500. The vote is not binding. FDA would still have to accept it and complete notice-and-comment rulemaking before pharmacies could legally compound it. As of July 31, 2026, it had not been added (Steinzor, AJMC 2026).
  • Sport. The 2026 WADA Prohibited List names “Thymosin-ß4 and its derivatives e.g. TB-500” under S2.3, growth factors, prohibited in and out of competition (WADA 2026 Prohibited List).

What the research shows

TB-500 itself: lab and animal data only

The evidence for TB-500 as sold is thin and mixed:

  • Wound healing, cell culture. In a scratch-wound test on cultured fibroblasts (connective tissue cells), TB-500 at 50 µg/mL did not speed wound closure compared with control. One of its breakdown products, Ac-LKKTE, did have a small effect. The authors suggest earlier reports of wound healing may reflect this metabolite rather than TB-500 itself (Rahaman et al., J Chromatogr B 2024).
  • Skin wounds, in aged mice. The plain, non-acetylated fragment LKKTETQ, applied to skin wounds in 26-month-old mice, improved repair about as well as full thymosin beta-4 (Philp et al., Wound Repair Regen 2003). This was not TB-500, and FDA noted the study had no dose-response data.
  • Achilles tendon, in rats. In a 2026 study, 32 rats had their Achilles tendon cut and repaired, then received intraperitoneal TB-500 (60 µg/kg/day), BPC-157, both, or nothing for four weeks. The TB-500 group had a significantly higher load to failure and better tissue scores than controls. Combining it with BPC-157 added no benefit (Biçer et al., Jt Dis Relat Surg 2026). The authors called the study exploratory.

When FDA reviewed the literature, it found no in-vivo animal studies testing TB-500 for wound healing, and concluded that “nonclinical pharmacological evidence is currently lacking” (FDA briefing document, 2026). None of these results can be assumed to apply to people.

Full-length thymosin beta-4: human trials

Thymosin beta-4 itself has been tested in people, mainly by companies developing it as a drug, including ReGenTree (eye drops) and Beijing Northland Biotech (a recombinant IV form). These are the trials TB-500 marketing often points to, but they did not use TB-500:

Trial Design and n Product and dose Main result
Healthy volunteers (Ruff et al. 2010) Phase 1, randomized, placebo-controlled; 4 cohorts of 10 Synthetic Tβ4 IV, 42–1,260 mg once, then daily for 14 days No dose-limiting toxicity or serious adverse events
Healthy volunteers (Wang et al. 2021) Phase 1, randomized, double-blind; 84 subjects Recombinant Tβ4 IV, 0.05–25 µg/kg once or 0.5–5 µg/kg daily for 10 days Adverse events no more common than with placebo
Venous leg ulcers (Guarnera et al. 2010) Phase 2, double-blind, placebo-controlled, dose-escalation; 73 patients Topical Tβ4 Safety similar to placebo; authors say a 0.03% dose “may have the potential” to speed healing
Dry eye (Sosne & Ousler 2015) Phase 2, randomized, placebo-controlled; 72 subjects 0.1% eye drops (RGN-259) for 28 days Both primary endpoints missed; some secondary measures improved
Dry eye, ARISE-2 (NCT02974907) Phase 3, placebo-controlled; 601 patients 0.1% eye drops 4 times daily for 28 days Primary endpoints: discomfort change +0.07 vs −0.04; staining change +0.07 vs −0.01 (overlapping 95% CIs)
Dry eye, ARISE-3 (NCT03937882) Phase 3, placebo-controlled; 700 patients 0.1% eye drops 4 times daily for 14 days Primary endpoints: staining change −0.41 vs −0.46; discomfort change −0.4 vs −0.4
Neurotrophic keratopathy, SEER-1 (Sosne et al. 2022) Phase 3, randomized, double-masked; 18 patients 0.1% eye drops Complete healing at 4 weeks in 6 of 10 vs 1 of 8 on placebo (p = 0.0656, not significant)
Heart attack (STEMI) (Zhang et al. 2025) Randomized, double-blind, placebo-controlled; 96 patients Recombinant Tβ4 after artery reopening No significant difference in infarct size overall; a reduction only in the 43 patients dosed within 8 h

Neurotrophic keratopathy is a disease in which damaged corneal nerves lead to non-healing sores on the eye’s surface. The pattern across these trials is consistent: small studies and secondary or subgroup analyses look positive, while the largest trials did not show a clear benefit on their main endpoints.

Heart repair was first suggested by work in mice. After a coronary artery was tied off, thymosin beta-4 improved heart cell survival and heart function (Bock-Marquette et al., Nature 2004). The 2025 STEMI trial above is the first published randomized test of this idea in patients, and its overall result was negative.

Muscle growth, endurance and performance

No human study has tested TB-500 or thymosin beta-4 for muscle mass, strength, endurance or athletic performance. FDA lists muscle growth, endurance, stamina and performance among the claims made by online sellers, not among findings from research (FDA briefing document, 2026).

Side effects and risks

What has been reported in humans

Because TB-500 has never been studied in people, its side effects are unknown. FDA’s adverse event database (FAERS) had no TB-500 reports through March 2025. A separate FDA complaint database held two reports about “blended TB-500 and BPC-157” products, without safety details (FDA briefing document, 2026).

For full-length thymosin beta-4, the trials above reported mostly mild events. In the 700-patient ARISE-3 eye-drop trial, there were no serious adverse events in either group (ClinicalTrials.gov NCT03937882):

Event Tβ4 eye drops (n = 350) Placebo (n = 349)
Any non-serious adverse event 47 (13.4%) 31 (8.9%)
Pain at the instillation site 23 (6.6%) 16 (4.6%)
Reduced visual acuity 12 (3.4%) 7 (2.0%)
Serious adverse events 0 0

These safety data apply to eye drops and to short IV courses of the full peptide. They do not tell us about repeated injections of TB-500.

No toxicology data

FDA found no acute, repeat-dose, genetic toxicity, reproductive or carcinogenicity studies of TB-500 in any species. It concluded that “potential safety risks associated with the use of these substances in humans are unknown” (FDA briefing document, 2026).

Blood vessel growth and cancer: a theoretical concern

Thymosin beta-4 promotes cell migration and new blood vessel growth, which are also steps in tumor spread. In mice, melanoma cells engineered to make extra thymosin beta-4 formed larger tumors (21.7 mm vs 13.3 mm) and about four times more lung metastases (46.7 vs 10.9 nodules) than control cells (Cha et al., JNCI 2003). This shows the role of the body’s own thymosin beta-4 inside tumor cells. It does not show that injected TB-500 causes cancer. The authors of the phase 1 recombinant trial noted that thymosin beta-4 “has a potential for tumour promotion,” while other studies suggest anti-tumor effects (Wang et al., J Cell Mol Med 2021). No study has tested this question for TB-500.

Immunogenicity and product quality

FDA’s main safety concern is immunogenicity: the chance that the body makes antibodies against an injected peptide. Reactions can range from harmless antibodies to life-threatening ones. Neutralizing antibodies could, in principle, also block the body’s own matching peptide. FDA says this response “may be enhanced” when peptides are injected, that injection under the skin is generally more immunogenic than IM or IV dosing, and that the risk may be amplified by peptide clumping (aggregation) and synthesis impurities (FDA briefing document, 2026).

FDA’s review of quality documents found the following:

  • a public certificate of analysis for TB-500 free base tested only appearance, identity and purity;
  • the acetate certificate submitted with the nomination did not test identity, assay, aggregates or bacterial endotoxins (fever-causing bacterial toxins);
  • the nomination itself gave a molecular formula and an alternative CAS number that matched neither form.

TB-500 is sold online as 2–15 mg powders labeled for research use only (FDA briefing document, 2026; Steinzor, AJMC 2026).

Populations with no data

There are no TB-500 data in pregnancy, breastfeeding, children, people with a history of cancer, or people taking other drugs. Nothing is known about interactions.

Key takeaways

  • Not the same as thymosin beta-4. TB-500 is a synthetic 7-amino-acid fragment (Ac-LKKTETQ) of the 43-amino-acid natural peptide. FDA says the two are different substances.
  • No human data at all for TB-500. FDA found no clinical trials, no human pharmacokinetics and no case reports of its use in people.
  • The parent peptide’s trials were disappointing. Full-length thymosin beta-4 was well tolerated, but the two large phase 3 dry-eye trials (601 and 700 patients) showed no clear benefit on their main endpoints.
  • Animal and lab results conflict. TB-500 improved tendon repair in one rat study, but did not help wound closure in cell culture. Its effects may come from a breakdown product.
  • Safety is unknown. There are no toxicology or carcinogenicity studies. FDA’s main concerns are immunogenicity, impurities and inconsistent product identity.
  • Banned in sport; unsettled in US compounding. WADA prohibits it at all times. FDA staff opposed allowing it in compounding, an advisory panel voted narrowly in favor, and FDA has made no final decision.

Related: BPC-157, often sold in blends with TB-500. Thymosin alpha-1 is an unrelated thymus peptide with a similar name.

Sources

  1. Food and Drug Administration. FDA briefing document: Evaluation of TB-500-related bulk drug substances (TB-500 (free base) and TB-500 acetate) for inclusion on the 503A bulk drug substances list. Pharmacy Compounding Advisory Committee meeting, July 23–24, 2026 (memo dated May 15, 2026). FDA.
  2. Food and Drug Administration. Pharmacy Compounding Advisory Committee; notice of meeting; establishment of a public docket; request for comments—bulk drug substances nominated for inclusion on the section 503A bulk drug substances list. Docket FDA-2025-N-6895. Federal Register. April 16, 2026;91:20465 (FR Doc 2026-07361). Federal Register.
  3. Food and Drug Administration. Certain bulk drug substances for use in compounding that may present significant safety risks. Content current as of April 22, 2026. FDA.
  4. Steinzor P. FDA panel backs 6 peptides for compounding. AJMC. July 31, 2026. AJMC.
  5. World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List 2026 (in effect January 1, 2026). WADA.
  6. Low TL, Hu SK, Goldstein AL. Complete amino acid sequence of bovine thymosin beta 4: a thymic hormone that induces terminal deoxynucleotidyl transferase activity in thymocyte populations. Proc Natl Acad Sci U S A. 1981;78(2):1162–1166. PubMed.
  7. Safer D, Elzinga M, Nachmias VT. Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable. J Biol Chem. 1991;266(7):4029–4032. PubMed.
  8. Esposito S, Deventer K, Goeman J, 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. PubMed.
  9. Ho EN, Kwok WH, Lau MY, et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin β₄, in equine urine and plasma by liquid chromatography-mass spectrometry. J Chromatogr A. 2012;1265:57–69. PubMed.
  10. Rahaman KA, Muresan AR, Min H, et al. Simultaneous quantification of TB-500 and its metabolites in in-vitro experiments and rats by UHPLC-Q-Exactive orbitrap MS/MS and their screening by wound healing activities in-vitro. J Chromatogr B Analyt Technol Biomed Life Sci. 2024;1235:124033. PubMed.
  11. Philp D, Badamchian M, Scheremeta B, 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. PubMed.
  12. Biçer O, Adanir O, Güleryüz Y, et al. 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. PubMed.
  13. Bock-Marquette I, Saxena A, White MD, et al. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature. 2004;432(7016):466–472. PubMed.
  14. Ruff D, Crockford D, Girardi G, et al. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers. Ann N Y Acad Sci. 2010;1194:223–229. PubMed.
  15. Wang X, Liu L, Qi L, et al. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin β4 in healthy Chinese volunteers. J Cell Mol Med. 2021;25(17):8222–8228. PubMed; PMC.
  16. Zhang Y, Dong Q, Bian X, et al. Recombinant human thymosin beta 4 improves ischemic cardiac dysfunction in mice and patients with acute ST-segment elevation myocardial infarction after reperfusion. Cardiovasc Res. 2025;121(17):2747–2758. PubMed.
  17. Guarnera G, DeRosa A, Camerini R; 8 European sites. The effect of thymosin treatment of venous ulcers. Ann N Y Acad Sci. 2010;1194:207–212. PubMed.
  18. Sosne G, Ousler GW. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial conducted using the controlled adverse environment (CAE™) model. Clin Ophthalmol. 2015;9:877–884. PubMed.
  19. Sosne G, Kleinman HK, Springs C, 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. PubMed.
  20. ReGenTree LLC. Assessment of the safety and efficacy of RGN-259 ophthalmic solutions for dry eye syndrome: ARISE-2. ClinicalTrials.gov identifier NCT02974907 (results posted). ClinicalTrials.gov.
  21. ReGenTree LLC. Assessment of the safety and efficacy of RGN-259 ophthalmic solutions for dry eye syndrome: ARISE-3. ClinicalTrials.gov identifier NCT03937882 (results posted). ClinicalTrials.gov.
  22. ReGenTree LLC. Assessment of the safety and efficacy of 0.1% RGN-259 ophthalmic solution for the treatment of NK: SEER-2. ClinicalTrials.gov identifier NCT05555589. ClinicalTrials.gov.
  23. Cha HJ, Jeong MJ, Kleinman HK. Role of thymosin beta4 in tumor metastasis and angiogenesis. J Natl Cancer Inst. 2003;95(22):1674–1680. PubMed.