TB-500

A seven-amino-acid peptide (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH). The one-letter code LKKTETQ captures the amino acids but not the chemical group added to one end. The sequence matches positions 17–23 of mature thymosin β4, which is 43 amino acids long (UniProt P62328). TB-500 is therefore a fragment, not thymosin β4. It is sold as a research peptide, often mixed with BPC-157.

Sold as

Sold for injection under the skin or into muscle. There is no approved way to take this fragment.

Legal status in the EU

Not approved as a medicine in the EU. Named on the doping list under growth factors: "Thymosin-ß4 and its derivatives, e.g. TB-500" (Antidoping Sweden, 2023 doping list, section S2, point 3). Selling it to consumers in Sweden is not permitted.

Thymosin β4 binds and holds onto actin, one of the proteins cells use to build their internal scaffolding, and so stops that scaffolding assembling (UniProt P62328). The LKKTETQ fragment sits inside that protein, but whether the free seven-amino-acid piece reproduces the whole protein's biology in humans has not been shown.

Part of the bodyWhat happensHow it is knownKind of evidence

Tendons

This is what most people buy it for, and it is the only place TB-500 itself has been put into a living body and measured. Rats had their Achilles tendon cut through and stitched back together. Four weeks later, the repaired tendon took more force to pull apart than in the control group, which had the same operation and no peptide. Under the microscope the healing tissue was less disorganised. Nothing resembling this has been measured in a person.

32 male rats, eight per group, injected into the abdomen every day for four weeks after surgery. The authors call it an exploratory study and say the amounts and the length of treatment still need working out.

Only seen in animals

Source [08]

Skin, where a wound is healing

Small punch wounds in the skin of very old mice closed further, and laid down more of the fibre that gives skin its strength, than wounds given the carrier fluid alone. Two things about that study matter and are usually left out. The peptide used had no acetyl group on the end — a small chemical cap — and TB-500 does have one. The FDA's reviewers write that this cap changes how long a peptide lasts, how it folds and what it sticks to, so the two cannot be read across. It was also dabbed onto the wound, not injected.

26-month-old female mice, four 3 mm wounds each, treated on the day of wounding and again 48 hours later, assessed at seven days. Only one strength was ever tested, so the study cannot say how large the effect is or whether more would do more.

Only seen in animals

Source [09]Source [10]

The bloodstream, after an injection

It gets into the circulation and is then taken apart fast. In horses given a single injection under the skin, the level in blood peaked between one and two hours later. It could no longer be measured at all somewhere between six and ten hours. What the blood and urine carried instead were shorter pieces of the same peptide, with the amino acids chopped off one at a time from the tail end.

Thoroughbred geldings given one injection under the skin, and rats given one injection into the abdomen. The same breakdown pattern appeared when TB-500 was left sitting in human blood serum in a test tube for 22 hours. The timings above are the FDA's reading of the horse study; the paper's own abstract gives only the detection limits.

Only seen in animals

Source [10]Source [11]Source [12]

The surface of the eye

Mice had their corneas burned with a strong alkali. A gel that sets into a mesh at the injury site and slowly releases TB-500 sped up the regrowth of the eye's outer layer and calmed the inflammation. The authors say this is the first time TB-500 has been tried on an eye at all. The gel is a laboratory construction, not the powder sold for injection.

Mice with chemical burns to the cornea, plus human corneal cells grown in a dish. The published abstract gives neither the number of animals nor the length of follow-up.

Only seen in animals

Source [13]

The blood vessels that grow into a healing area

The seven-amino-acid stretch made human cells that line veins crawl across a dish, and made rings of chick artery sprout new vessels, about as strongly as the whole 43-amino-acid protein did. Versions missing any part of those seven did nothing. Growing new blood vessels and making cells move are also things a tumour does. Nobody has tested what this peptide does to a tumour: the FDA searched for animal cancer studies of TB-500 and found none.

Human vein-lining cells and chick artery rings in a dish. No animal, no person, and the peptide was in contact with the cells directly rather than having to survive a journey through a body.

Only seen in a dish, not in a body

Source [10]Source [14]Source [15]

There are no published controlled human studies of TB-500 itself. The human data usually cited is for RGN-259, an eye drop containing full-length thymosin β4 — a different molecule given a different way. In the phase 3 study SEER-1 (Sosne et al., Int J Mol Sci 2022), 18 people with neurotrophic keratopathy took part (10 active, 8 placebo), and the main outcome did not reach statistical significance (p = 0.0656). The study was stopped early because recruitment was slow: 46 participants were planned, 18 analysed.

Nothing has been timed in a person, because no person has been given it in a published study. The only timings that exist are in animals. In rats whose cut Achilles tendon was repaired, tendon strength was measured once, four weeks after surgery. In very old mice, skin wounds were assessed seven days after the peptide was applied. In horses, a single injection under the skin peaked in the blood between one and two hours later and was gone from the blood between six and ten hours later.

What was measuredWhat happenedIn whomHow it is known

Force needed to pull a repaired Achilles tendon apart

Higher in the rats given TB-500 than in the control group, which had the same operation and no peptide. The gap was large enough that chance is an unlikely explanation (p < 0.05). BPC-157, tested in the same experiment, also came out higher but not by enough to clear that mark.

32 male Sprague-Dawley rats, eight per group, measured four weeks after the tendon was cut and repaired

Only seen in animals

Source [08]

How damaged the healing tendon looked under a microscope

Two standard damage scores were used, and on both a lower number means healthier tissue. The TB-500 rats scored lower than the control group on both, by margins chance is an unlikely explanation for (p = 0.016 and p = 0.017). Adding BPC-157 on top of TB-500 gave no further improvement over TB-500 alone.

The same 32 rats, eight per group, tissue taken four weeks after surgery

Only seen in animals

Source [08]

Skin wound closure in very old mice

More of the wound had closed over and more collagen had been laid down at seven days than in wounds given the carrier fluid alone. The seven-amino-acid peptide did about as well as the whole protein. No percentage appears in the paper's abstract or in the FDA's summary of it, and the peptide used lacked the acetyl group TB-500 carries.

26-month-old female mice, four 3 mm punch wounds each, treated twice, assessed at day seven

Only seen in animals

Source [09]Source [10]

Closing a scratch drawn through a sheet of cells in a dish

TB-500 closed no more of the scratch than the plain liquid it was dissolved in. Ac-LKKTE — one of the shorter pieces TB-500 breaks down into — closed a small but real amount more. The FDA's reviewers draw the obvious conclusion: it is not known whether TB-500 does anything itself, or only through what it turns into.

Cultured connective-tissue cells, eight hours, one concentration only; no living animal involved. Neither the paper's abstract nor the FDA's summary names the species the cells came from.

Only seen in a dish, not in a body

Source [10]Source [12]

New blood vessels sprouting from a piece of artery

The seven-amino-acid stretch produced about the same amount of sprouting and cell movement as the full 43-amino-acid protein, while shortened versions missing part of it produced none. Adding loose actin — the protein this peptide grips — switched the effect off.

Human vein-lining cells and chick artery rings in a dish

Only seen in a dish, not in a body

Source [14]

There is no human safety data at all for injected TB-500. Regulating the cell's scaffolding and growing new blood vessels are general processes, which leaves theoretical effects on tumour growth unexamined. Product quality among peptides sold online is unregulated: an analysis of semaglutide products sold without prescription (Ashraf et al., J Med Internet Res 2024) measured purity of 7.7–14.37 per cent against a promised 99 per cent, and found bacterial toxin in every sample. That study tested semaglutide, not TB-500, but two of its three sellers were online shops marketing research peptides.

EffectWhat it feels likeHow often

An immune reaction to the injected peptide

Short peptides injected under the skin or into muscle can prompt the body to make antibodies against them. They can also clump together in the vial, which the FDA says makes such a reaction more likely. The FDA writes that the result ranges from antibodies that cause no symptoms at all to reactions it calls life-threatening. It also writes that it found no study in people of either clumping or antibody formation for TB-500.

Not measured. No published study has given TB-500 to a person, so there is no rate to quote.

Source [10]Source [16]

Whatever else is in the vial

The FDA read the certificate of analysis a supplier filed with the nomination. That is the sheet meant to say what is in a batch. It sets a ceiling on impurities without saying what any of them are. It carries no check on identity, no measure of how much peptide is present, and nothing on clumping, microbial quality or bacterial toxin. The way these short peptides are built leaves behind truncated and mis-assembled versions that closely resemble the real thing. Researchers once ordered vials of a different peptide from online sellers with no prescription. Three arrived, and their measured purity was between 7.7 and 14.37 per cent against the 99 per cent on the label. Bacterial toxin was present in all three.

Not measured for TB-500. Nobody has published an analysis of what the vials people actually buy contain.

Source [06]Source [10]

An effect on a cancer nobody knew was there

The two effects this peptide has shown in a dish are growing new blood vessels and making cells move. Whether that matters for a tumour is untested. The same FDA search found no repeat-dose toxicity study, no test for damage to DNA and no study in pregnancy either. The headings are in the document and every one of them is empty.

Not measured, in any species. The FDA searched for animal studies of cancer risk for TB-500 and reported that none exist.

Source [10]Source [14]

What people report

• Whether anything measured in a rat, a mouse or a horse also happens in a person — no published study has given TB-500 to a human being.
• Whether an injected dose survives long enough to reach an injury, given that in horses it was gone from the blood within ten hours and was being chopped shorter the whole time.
• Whether any healing seen in animals belongs to TB-500 or to one of the shorter pieces it breaks into — in a dish, TB-500 did nothing to a wound and one of those pieces did.
• What repeated courses do over months: no repeat-dose toxicity study of TB-500 exists in any species.
• Whether it causes or feeds cancer: no animal carcinogenicity study of TB-500 exists.
• Whether it damages DNA: no genetic toxicity study of TB-500 exists.
• What it does in pregnancy or to a developing animal: no developmental or reproductive study of TB-500 exists.
• Whether the body makes antibodies against it after injection, and what happens if it does.
• What is actually in a vial bought online, since the certificate a supplier filed carried no check on identity, no measure of how much peptide was present, and nothing on clumping, microbial quality or bacterial toxin.
• How many people are injecting it and what is happening to them — the US regulator's side-effect database held no reports for TB-500 when it was searched in March 2025.
• Whether it does anything for the injuries people actually buy it for — tendon, ligament, muscle, joint — in a human body.

Is TB-500 the same thing as thymosin β4?

No, and the FDA says so in writing. Thymosin β4 is a human protein 43 amino acids long; TB-500 is seven of them, positions 17 to 23, with an acetyl group stuck on one end. The FDA's evaluation notes that adding that group changes a peptide's charge, size and lifespan, so results from the bare seven-amino-acid sequence cannot simply be read across to TB-500. The human eye-drop trial people point to used the whole protein, not either fragment.

Source [02]Source [04]Source [10]

Has anyone ever been given TB-500 in a study?

Not in anything published. In its evaluation dated 15 May 2026 the FDA searched PubMed, Embase, ClinicalTrials.gov, DailyMed and its own databases. It stated that it found no article in which TB-500 was given to humans. It also found no measurement in a person of what the body does with it, by any route. One trial is registered and is not mentioned in that evaluation. It plans 80 adults with stable heart artery disease, run by a company called Hudson Biotech at Peking University Shenzhen Hospital in China. It was first posted on 23 March 2026 and is still recruiting. The registry entry names three dose levels without giving the amounts, and no results have been posted.

Source [10]Source [17]

Why is it nearly always sold mixed with BPC-157?

Because the two are marketed together as a repair pairing, not because anyone showed the combination works better. The FDA's evaluation cites seller pages promoting exactly that blend. The one experiment that gave both to the same animals found no added benefit from the combination over either peptide on its own. In that experiment it was TB-500, not BPC-157, whose effect on tendon strength was large enough that chance is an unlikely explanation.

Source [08]Source [10]

Would it show up on a doping test?

It is named on the World Anti-Doping Agency list by name — "Thymosin-ß4 and its derivatives e.g. TB-500" — under growth factors, banned at all times, in and out of competition. A laboratory method published in 2012 detects it in horse urine down to 0.01 nanograms per millilitre. Swedish public television had peptides bought online tested in May 2026 and found eight substances from that WADA list, TB-500 among them. In the same report a prosecutor says that possessing or using a substance on the Swedish Public Health Agency's separate list of doping agents risks a criminal charge.

Source [11]Source [18]Source [19]

Why do so many websites say TB-500 has 17 amino acids?

Because 17 is a position, not a length. The sequence sits at positions 17 to 23 inside thymosin β4, which makes it seven amino acids long, and the FDA writes it out in full as Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH. The same evaluation notes that TB-500 is a trade nickname rather than an official drug name, and that suppliers sell more than one substance under it. That alone is a safety problem, because a buyer cannot know which one is in the bottle.

Source [10]Source [20]

Can a pharmacy make it up legally?

Not in the United States. A compounding pharmacy nominated TB-500 for the list of substances pharmacies may use, then withdrew the nomination. The FDA finished the assessment anyway and proposed not adding it. Its reasons were the absence of any human data, the absence of animal toxicity studies, and the risk of an immune reaction to an injected peptide. The substance still appears on the FDA's published list of nominated substances that may present significant safety risks.

Source [10]Source [16]

Each letter represents one amino acid.

1. [01]2023 doping list, Antidoping Sweden — S2 point 3, growth factors: "Thymosin-ß4 and its derivatives, e.g. TB-500" (in Swedish) (2023)
2. [02]UniProt P62328 — Thymosin beta-4, Homo sapiens (sekvens, kedja 2–44, aktinbindande funktion)
3. [03]TB-500, PubChem synonymer — Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH
4. [04]0.1% RGN-259 (Thymosin ß4) Ophthalmic Solution ... Phase III Clinical Trial (SEER-1, n=18) (2022)
5. [05]ReGenTree Announces Efficacy Results of the Neurotrophic Keratopathy Phase 3 Trial with RGN-259 (SEER-1 stopped early, 18 of 46 planned)
6. [06]Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription (JMIR) (2024)
7. [07]SVT reporter poses as a customer – given buying advice despite the ban on injectable peptides (in Swedish) (2026)
8. [08]Biçer O 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 (2026)
9. [09]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 (2003)
10. [10]FDA Evaluation of TB-500-Related Bulk Drug Substances (TB-500 free base and TB-500 acetate), dated 15 May 2026, in the briefing document for the Pharmacy Compounding Advisory Committee meeting of 23–24 July 2026 (2026)
11. [11]Ho EN 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 (2012)
12. [12]Rahaman KA 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 2024;1235:124033 (2024)
13. [13]Lu P et al. Alkaline phosphatase-triggered spatiotemporal repair of corneal injury with TB500 peptide hydrogel. ACS Appl Mater Interfaces 2025;17(50):67503-67518 (2025)
14. [14]Philp D et al. The actin binding site on thymosin beta4 promotes angiogenesis. FASEB J 2003;17(14):2103-5 (2003)
15. [15]Sosne G et al. Biological activities of thymosin beta4 defined by active sites in short peptide sequences. FASEB J 2010;24(7):2144-51 (2010)
16. [16]FDA — Certain bulk drug substances for use in compounding that may present significant safety risks: "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" (page current as of 22 April 2026) (2026)
17. [17]NCT07487363 — TB-500 (thymosin beta 4 17-23 fragment) in adults with stable atherosclerotic cardiovascular disease, phase 1/2, 80 planned, recruiting, no results posted (2026)
18. [18]WADA International Standard Prohibited List 2026, section S2.3 growth factors and growth factor modulators, page 8: "Thymosin-ß4 and its derivatives e.g. TB-500" (2026)
19. [19]SVT's review: peptides sold online contain doping agents, buyers can be prosecuted (in Swedish, 26 May 2026) (2026)
20. [20]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):733-8 (2012)