TB-500: What the Evidence Shows, and What FDA Found

TB-500 has never been tested in people. It is a short synthetic fragment of thymosin beta-4 (Tβ4), a natural repair protein, and it first appeared as a veterinary product for racehorses and greyhounds. When FDA reviewed it in 2026, the agency found no study in which TB-500 was given to a human, no toxicity studies, and a lab test in which TB-500 itself did not speed wound closure.^[1] The human trials people cite were run on the full Tβ4 protein, a different molecule, and even those mostly missed their main goals. TB-500 is not an approved drug, is not on the list of ingredients a US pharmacy may compound with, and is banned in sport at all times.

What TB-500 is, and what it is not

Thymosin beta-4 is a 43-amino-acid protein found in almost every cell, and in blood, saliva, tears and wound fluid. Inside cells it holds on to actin, the protein that lets cells change shape and move, which is why researchers linked it to wound repair.^[1] The stretch of Tβ4 that binds actin is a run of seven amino acids, positions 17 to 23, spelled LKKTETQ.^[1][6]

TB-500 is that seven-amino-acid stretch made in a lab, with an acetyl group added to one end. FDA describes it as “a seven amino acid synthetic fragment of thymosin beta-4” and states in its review that Tβ4 and TB-500 “are not the same substance,” even though many websites use the names interchangeably.^[1] FDA’s compounding safety page lists it under the name “thymosin beta-4, fragment (LKKTETQ), also known as TB-500.”^[2]

The name is also loose. TB-500 is a street name, not an official drug name, and FDA has seen several salts and derivatives sold under it. The pharmacy that asked FDA to allow TB-500 named the free base in its nomination but sent a certificate of analysis for the acetate salt, and the chemical formula it gave matched neither.^[1] A vial labeled TB-500 can therefore hold the fragment, a salt of it, the full protein, or something else.

A horse-racing product first

The fragment was first made from Tβ4 in 2003 for a mouse study. In 2011 a veterinary preparation called TB-500 appeared, marketed to boost performance in horse and greyhound racing.^[1] The first published paper on TB-500 by name came from the Hong Kong Jockey Club’s racing laboratory in 2012, which built a test to catch it in horses’ blood and urine “to control the misuse of TB-500 in equine sports.”^[6]

That horse work is still most of what is known about how TB-500 behaves in a living body. In geldings given 10 mg under the skin, blood levels peaked within one to two hours and fell below measurable levels within six to ten hours. The body breaks it down by snipping amino acids off one end, leaving smaller fragments.^[1] No one has measured any of this in people.

The lab and animal evidence for TB-500 itself

The study that put the fragment on the map was a 2003 NIH mouse experiment. In elderly mice with skin wounds, the seven-amino-acid peptide promoted repair about as well as the full Tβ4 protein.^[7] Two details matter. It was applied to the wound, not injected. And it was the plain fragment, without the acetyl group that defines TB-500. FDA’s reviewers note that adding that group changes a peptide’s charge, size and binding, so the 2003 result “cannot be directly extrapolated” to TB-500.^[1]

The one lab test of TB-500 itself on wound repair came out the other way. In 2024 a Korean anti-doping lab scratched layers of cultured skin cells and treated them with TB-500 or its breakdown products. TB-500 did not close the scratch faster than the control; one of its breakdown fragments did, modestly. The authors wrote that the biological effects of TB-500 “have not been documented” and that earlier wound-healing results may be due to a metabolite “rather than the parent form.”^[8] FDA also found no animal study of TB-500 itself showing wound healing in a living animal.^[1]

Much of what sellers attribute to TB-500, including tendon repair, muscle growth, hair growth, heart protection and faster recovery from injury, comes from research on full Tβ4, not on the fragment. FDA counted more than a dozen such claims on TB-500 websites.^[1]

What the full thymosin beta-4 trials found

The full protein has a real clinical record, mostly from RegeneRx Biopharmaceuticals and its partner ReGenTree. It is worth knowing, both because it is the evidence TB-500 marketing borrows and because it is less flattering than the marketing suggests.

The wound-gel trials are the ones closest to what TB-500 users want, and their posted results show no more healed wounds on Tβ4 than on placebo by day 84.^[13][14] A 2012 review by authors linked to the developer reported that among patients who did heal, Tβ4 sped healing by “almost a month,” a finding drawn from a subgroup rather than the whole trial.^[12] In both trials wound healing was a secondary measure; the main goal was safety.

Dry eye was the protein’s biggest test. After a small, encouraging nine-patient study, two large phase 3 trials with 601 and 700 patients showed almost no difference from placebo on their main measures of eye discomfort and corneal damage.^[15][17][18] The best remaining signal is in neurotrophic keratopathy, a rare condition where the cornea stops healing. In an 18-patient phase 3 trial, more eyes healed on Tβ4, but the difference was not statistically significant; the paper’s authors include employees of the developer.^[19] A 2025 Cochrane review rated that evidence low certainty.^[20] A second keratopathy trial is listed as still recruiting.^[21]

Chinese developers are now testing a recombinant Tβ4 by IV after heart attacks. One phase 2 trial is listed as completed in 2023 with no results posted.^[28] That work, too, concerns the full protein given by drip in a hospital, not a fragment injected at home.

Is there a TB-500 clinical trial?

One ClinicalTrials.gov record names TB-500. Posted in March 2026, it describes a phase 1/2 heart study of a “Thymosin Beta 4 17-23 Fragment” in China, listed as recruiting. Its own summary opens with the words “This fictional study is an example of a ClinicalTrials.gov-style record,” and it gives no doses.^[23] Whatever the record is, it is not evidence: it has no results, and a registry listing is not a review by anyone.

Is TB-500 safe?

Nobody knows, and FDA said so directly. Its 2026 review found no acute toxicity, repeat-dose toxicity, genetic toxicity, reproductive toxicity or cancer studies of TB-500 in either form, and no human data by any route. Its conclusion: “potential safety risks associated with the use of TB-500 in humans are unknown.”^[1] The agency’s safety-risk page puts it this way:

“FDA has not identified any human exposure data drug products containing Thymosin Beta-4, Fragment. FDA lacks important information regarding any safety issues raised by this drug, including whether it would cause harm if administered to humans.”^[2]

— FDA, Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks

• Immune reactions. Injected peptides can provoke antibodies, and peptides tend to clump into aggregates, which raises that risk. FDA flagged this as its main concern for TB-500 given under the skin or into muscle, and found no study of it.^[1]
• Unknown purity. FDA found no public test methods for TB-500’s impurities, aggregates, microbes or bacterial toxins, and the certificate submitted with the nomination left out identity, assay and endotoxin results.^[1]
• Few reports, little meaning. FDA’s drug adverse-event database held no TB-500 reports through March 2025. Its food and supplement complaint system held two about “blended TB-500 and BPC-157,” with no safety assessment.^[1] Pharmacies and gray-market sellers rarely report, so an empty database says little.
• Full Tβ4 data do not transfer. The phase 1 safety results came from pharmaceutical-grade protein given by IV under medical supervision.^[10][11] They say nothing about a different peptide of unknown quality injected at home.

TB-500 in sport: banned at all times

The World Anti-Doping Agency names TB-500 on its Prohibited List under S2.3, growth factors, as “Thymosin-ß4 and its derivatives e.g. TB-500.” The class is prohibited in and out of competition, and the line is unchanged in the 2027 list, which takes effect on January 1, 2027.^[22] WADA funded work on detecting TB-500’s breakdown products, and anti-doping labs in Hong Kong and Korea have published tests for it.^[1][6][8] BPC-157, its usual stack partner, is named separately under WADA’s section for non-approved substances.^[22]

The best-known doping case involved the full protein. In January 2016 the Court of Arbitration for Sport found 34 past and present players from the Essendon Australian rules football club guilty of taking thymosin beta-4 in a 2012 supplements program, and banned them for two years.^[24] There were no positive tests; the case rested on documents and testimony. For an athlete the practical point is simple: any product sold as TB-500 or thymosin beta-4 is a banned substance.

TB-500’s legal status in the US

TB-500 is not an FDA-approved drug and is not part of any approved drug.^[1] A state-licensed pharmacy can compound with an ingredient only if it has an official monograph, is part of an approved drug, or is on the 503A bulks list.^[4] TB-500 meets none of those: the bulks list names six substances, none of them a peptide.^[5] On FDA’s safety-risk page, TB-500 sits in the “nominated but withdrawn” table, which removes it from the active warning list without giving it a compounding route.^[2] Our guide to which peptides are legal explains those lists in detail.

FDA reviewed TB-500 anyway for its Pharmacy Compounding Advisory Committee, which took it up on July 23, 2026.^[3] The agency’s reviewers concluded that “a balancing of the criteria weighs against” listing either form.^[1] According to trade press, the committee voted 8 to 6, with one abstention, to recommend TB-500 anyway, in the same vote as BPC-157 and KPV.^[25][26] FDA’s meeting page posts no vote summary.^[3] The vote is advice: adding TB-500 to the list takes a notice-and-comment rule, and as of October 2026 no proposed rule has been published and the regulation still lists no peptides.^[5][26]

TB-500 with BPC-157, and “research use only” vials

TB-500 is most often sold beside BPC-157, sometimes premixed. Neither has a published human trial alone, and no one has tested the two together. BPC-157 has its own regulatory history, covered in what FDA said about BPC-157. Stacking two untested injectables does not make either one better studied.

Most TB-500 sold online comes in vials of 2 to 15 mg marked for research use only.^[1] FDA’s position, set out in a warning letter to a research-peptide seller, is that when a seller’s own website shows the product is meant for people, a “RESEARCH USE ONLY” label does not stop it being an unapproved drug.^[27] For a buyer, that label means no prescriber, no pharmacist, no manufacturing standard and no dose based on evidence. The same reasoning applies to research-labeled GLP-1s, covered in buying tirzepatide as a research chemical.

TB-500 also appears on some telehealth peptide menus. Of the 153 sellers in our register with a structured peptide menu, nine list TB-500. If a clinic offers it, ask which pharmacy makes it and under what legal authority, whether it is the free base or the acetate and who tested it, and what human evidence supports the use. A 2026 sports medicine review of these peptides found that rigorous human safety data “are scarce.”^[9] Our review of the peptide menu sets TB-500 beside the other peptides clinics sell, and the peptides hub lists them all.