TB-500: what it is, how it differs from thymosin beta-4, and the risks

TB-500 is a tiny synthetic fragment of a natural human protein, and as far as FDA could find, it has never been studied in a person. It came out of the veterinary and doping world, and it is sold today as a healing peptide on the strength of research into a much larger molecule. It is not FDA-approved. Before anything else, understand that TB-500 and thymosin beta-4 are different things. Most of what you will read about it blurs them. It is usually paired with BPC-157, a peptide with its own thin evidence.

What is TB-500?

TB-500 is a seven-amino-acid peptide with an acetyl group added to one end. Its sequence is Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, or LKKTETQ, which matches positions 17 to 23 of thymosin beta-4. FDA’s July 2026 review describes it as “understood today to be” that fragment, and the hedge is deliberate. TB-500 is a common name, not an official drug name, and FDA has “encountered multiple salts, and derivatives, including different active moieties, sold commercially under the same common name.”

Its history explains a lot. FDA traces the fragment to a 2003 laboratory study, and a veterinary preparation appeared in 2011, marketed to boost performance in horse and greyhound racing. A 2012 anti-doping paper developed a test for it in racehorses. FDA found discussion of it on doping forums rising after 2010. At the time of its review, FDA could not find a pharmacy that marketed compounded TB-500, or any published study of a compounded product.

TB-500 vs thymosin beta-4

Thymosin beta-4 has real human trial data, and TB-500 does not. Results from one do not carry over to the other.

FDA states plainly that “thymosin beta-4 and TB-500 are not the same substance,” although many websites use the names interchangeably. Even a tiny chemical change matters: FDA notes that acetylation alters a peptide’s charge, size and binding, so results for the unacetylated fragment cannot be assumed to apply to TB-500 either.

How is TB-500 supposed to work?

The theory is borrowed from its parent protein. Thymosin beta-4 binds actin, a building block of the cell’s internal skeleton, and it has been linked to cell migration, new blood vessel growth and wound repair. The LKKTETQ stretch contains the actin-binding region, so the idea is that a small piece might do the same work. FDA says it remains to be determined whether actin binding explains the fragment’s reported effects, and it is unknown whether TB-500 needs to be broken down into something else to act at all.

What are the benefits of TB-500?

No benefit of TB-500 has been shown in people. FDA found that sellers market it for a remarkable range of things. Set against the evidence, the list looks like this.

FDA concluded that nonclinical evidence “is currently lacking” to support TB-500 for wound healing, and that there is “a lack of evidence to evaluate the effectiveness” in people. We cannot tell you what results to expect or how long they would take, because nobody has measured them.

The human trials people cite

The trials quoted for TB-500 tested full-length thymosin beta-4, mostly as eye drops (a product called RGN-259) and topical gels. Some used intravenous infusion. None used TB-500.

A 2025 Cochrane review rated the RGN-259 evidence in neurotrophic keratopathy as low certainty, and a 2025 laboratory paper notes that FDA approval of the eye drops is “still pending.” No thymosin beta-4 product is approved in the United States.

One registry entry deserves a warning. NCT07487363 lists a phase 1/2 study of TB-500 in heart disease, but the record’s own summary calls it “this fictional study” and an “example” record. We do not count it, and you should be wary of anyone who cites it as a human trial.

Is TB-500 safe?

Nobody knows, and FDA was unusually direct: “potential safety risks associated with the use of TB-500 in humans are unknown.”

• No toxicity studies. FDA found no acute, repeat-dose, genetic, reproductive or cancer studies of TB-500.
• No human exposure data. FDA found no clinical studies and no published case reports.
• Adverse event reports. FDA’s drug database held none through March 2025, and its food and supplement database held two reports about a TB-500 and BPC-157 blend with no safety assessment. Reporting is voluntary, so silence means little.
• Immune reactions. As an injected peptide, FDA says, TB-500 “may pose a significant risk for immunogenicity,” meaning the body could mount an immune response against it. Clumping (aggregation) and peptide impurities could make that worse.
• A reaction to a blend. One FDA report describes a woman who developed skin and gum darkening on a BPC-157 and TB-500 research product, repeating when she used it again. FDA could not say which peptide was responsible. The BPC-157 side effects guide gives the details.

The theoretical cancer question

A published concern exists, and it is indirect. In a 2003 mouse study in the Journal of the National Cancer Institute, melanoma cells engineered to make extra thymosin beta-4 formed more lung metastases and more tumor blood vessels. That experiment changed the tumor cells themselves. Nobody gave a drug, it did not involve TB-500, and it does not show that TB-500 causes cancer. But marketing TB-500 for “promoting angiogenesis” sits uneasily beside it, and with no carcinogenicity study there is no data to settle the question.

Quality problems

A 2023 laboratory analysis of products sold online as TB500 and TB1000 found their content was “not systematically consistent” with their descriptions. FDA found TB-500 is not well characterized, with public data missing on impurities, aggregates, microbial quality and endotoxin. Products labeled for research use only are not legal to use on people, and FDA warning letters have treated that label as no defense when the marketing points to human use, under the rule in 21 CFR 312.160.

Is there a TB-500 dose?

No human dose has been established, and no study supports any amount. FDA found that websites sell vials in several sizes for reconstitution, but vial size is packaging, and it should not be read as a dose. The doses in the rat and horse studies do not translate to people, for the reasons on the BPC-157 dosage page. Here the case is even weaker, because there is no toxicology at all to work from. We do not list online protocols.

FDA status and the October 2026 HHS policy

TB-500 has never had a New Drug Application.

As of November 2, 2026, HHS says it does not intend to take enforcement action against a state-licensed pharmacy, federal facility or licensed physician compounding TB-500 acetate by injection under the skin or into muscle, when all ten conditions are met, until a final rule decides whether it goes on the 503A bulks list. Those conditions include a single active ingredient, an FDA-registered and CGMP-compliant manufacturer, a valid certificate of analysis, a patient-specific prescription and reporting of serious adverse events. A BPC-157 and TB-500 blend falls outside the policy.

The limits matter as much as the policy. TB-500 stays unapproved and off the bulks list, no rule has been proposed, and FDA has not reviewed any compounded product. The notice says it is “not binding on HHS or the public,” and HHS may revisit it if a safety signal or quality problem appears. It covers 503A compounding only, and states can be stricter.

HHS and FDA staff disagree about the science. HHS’s appendix calls FDA’s safety concerns “inferential rather than observed,” and it leans on the record of the parent protein and on rapid breakdown of the fragment in human serum. FDA’s reviewers wrote that they were “particularly concerned about the absence of human data.” Both describe the same fact: nobody has measured what TB-500 does in people. The policy is a decision about enforcement, and it adds nothing to the evidence.

How does TB-500 compare with BPC-157?

Neither is proven, but TB-500 has less behind it. BPC-157 has five small human studies, and TB-500 has none. BPC-157 has a published toxicology paper, and TB-500 has none. In the one rat study testing both, TB-500 improved tendon strength over controls while BPC-157 did not reach significance, and the combination added nothing. That is one small rat experiment, and it should not be stretched further. The BPC-157 and TB-500 guide covers that study, and the Wolverine stack page covers the pairing as it is marketed.

Is TB-500 banned in sport?

Yes. The 2026 WADA Prohibited List puts “Thymosin-β4 and its derivatives e.g. TB-500” in class S2.3, growth factors and growth factor modulators, prohibited at all times. The anti-doping interest is why reliable tests for it exist.

Questions to ask a prescriber

If a clinician brings up TB-500, it is reasonable to ask:

• What is it meant to do for me, given that FDA found no human data?
• Is the product TB-500 acetate as a single ingredient, compounded for me by a state-licensed pharmacy?
• Does our state permit this?
• Can the pharmacy name the original manufacturer and show the certificate of analysis?
• What approved or better-studied options should we try instead?
• Who do I call if I react, and how will it be reported to FDA?

Pregnancy, cancer history, autoimmune disease and competitive sport are the situations where these unknowns weigh most. There is no label to say so, which is the point: with no toxicity data, no one can tell you who is at risk.