TB-500 frag

Origin and structure

Thymosin beta-4 is a 43-amino-acid protein found in almost every human cell, where it keeps a reserve of actin, the building block of the cell's internal scaffolding. TB-500 frag is seven of those amino acids, Leu-Lys-Lys-Thr-Glu-Thr-Gln (LKKTETQ, positions 17 to 23), with an acetyl group on the first leucine. Its formula is C38H68N10O14, its molar mass 889 g/mol and its CAS number 885340-08-9.

The bare sequence was first tested on its own in 2003 by a group at the US National Institutes of Health, which found that it helped skin wounds heal in aged mice about as well as the full protein. An acetylated version turned up around 2011 as a veterinary product called TB-500, sold to boost performance in racehorses and greyhounds. In 2012 the anti-doping laboratory at Ghent University identified Ac-LKKTETQ in a TB-500 product, and the racing laboratory of the Hong Kong Jockey Club published the first method to detect it in horses.

How it is thought to work

The first six amino acids of the fragment, LKKTET, are the motif with which thymosin beta-4 holds single actin molecules. The working idea is that the fragment keeps some of the parent protein's effects on cell movement, blood-vessel growth and wound repair, which earlier cell and mouse studies traced to this region. Whether actin binding is what actually drives those effects has not been established.

The acetyl cap matters. Most of the positive findings were made with plain LKKTETQ, and the FDA points out that acetylation changes a peptide's charge, size and binding, so results for one form cannot simply be carried over to the other. In the body the fragment is cut down from its tail end into shorter acetylated pieces. In horses it peaked in the blood one to two hours after an injection under the skin and could no longer be measured after 6 to 10 hours; in rats the piece Ac-LKK could be detected in urine for up to 72 hours.

Is TB-500 frag approved?

No. The EMA has authorised no medicine containing it and it is not a registered medicine in the Netherlands, so it may not be sold or advertised as a medicine. It is available only as research material.

In the United States it is not FDA-approved and is not part of any approved drug; the debate there is about pharmacy compounding. On 29 September 2023 the FDA put 'thymosin beta-4 fragment (LKKTETQ)' in Category 2 of its 503A bulk-substances list, the group flagged for significant safety risks, and by April 2026 the nomination behind it had been withdrawn. The FDA then reviewed it on its own initiative for wound healing, and its staff advised against allowing it, citing an unclear identity, the complete absence of human data and a possible risk of immune reactions. On 23 July 2026 the Pharmacy Compounding Advisory Committee nevertheless voted 8 to 6, with one abstention, in favour. That vote is advice: no final decision had been published when this page was checked in October 2026.

In sport it is prohibited at all times. The WADA Prohibited List covers it under S2.3, growth factors and growth factor modulators, which names thymosin beta-4 and its derivatives with TB-500 as the example.

What the research shows

The literature on the fragment itself is small, and much of it comes from doping laboratories that wanted to detect it rather than find out whether it works. The FDA's 2026 review found no study of any kind in which it was given to people, no case reports, and no toxicity studies of any kind: acute, repeated-dose, genotoxicity, reproduction or cancer. The studies that do exist:

• Mice, 2003 (Philp et al.): plain LKKTETQ on skin wounds in 26-month-old mice improved wound closure and collagen at day 7, comparable to full thymosin beta-4. One model, no dose comparison, and not the acetylated form.
• Cells and rats, 2024 (Rahaman et al., Korea Institute of Science and Technology): in a fibroblast scratch test the acetylated fragment did not speed wound closure, while its breakdown product Ac-LKKTE did. The authors suggest earlier wound-healing results may be due to that metabolite.
• Horses, 2012 (Ho et al.): the first blood and urine test for the fragment and its metabolites, after single injections in horses. A detection study, not an efficacy study.
• Rats, 2026 (Biçer et al.): in 32 rats with a repaired Achilles tendon, the group given 'TB-500' had stronger tendons and better tissue scores at four weeks than controls, and adding BPC-157 gave no extra benefit. The paper describes TB-500 only as synthetic thymosin beta-4, so which molecule was used is unclear.
• Humans: none. The FDA searched PubMed, Embase and ClinicalTrials.gov, among others, and found no study in which the fragment was given to people.

TB-500, thymosin beta-4 and the fragment

TB-500 is a trade name, not an official drug name, and it is used for more than one molecule. The FDA reports that it has found several salts and derivatives, including different active substances, sold under that one name, and a 2023 laboratory analysis of products sold online as TB500 and TB1000 found that their contents did not consistently match their descriptions.

Our TB-500 article deals with the product sold under that name as thymosin beta-4 acetate, the full 43-amino-acid protein; this article covers the seven-amino-acid fragment. For the evidence the difference is large. Full-length thymosin beta-4 has been tested in people, for example as eye drops in a small phase III trial in 18 patients with a corneal disorder (Sosne et al., 2022), while the fragment has not been tested at all. Results for one cannot be read across to the other, and only an identity test that measures the sequence or the molecular mass shows which of the two a vial contains.

Frequently asked questions

What is the difference between TB-500 and thymosin beta-4?

Thymosin beta-4 is a natural protein of 43 amino acids. TB-500 in the strict sense used by doping laboratories and the FDA is a synthetic seven-amino-acid piece of it, positions 17 to 23, with an acetyl cap. Sellers often use the names interchangeably, so a label alone does not show which of the two a product contains.

Has TB-500 frag been tested in humans?

No. The FDA's 2026 review found no clinical study, no case report and no other human exposure data. The human trials sometimes quoted for TB-500 used full-length thymosin beta-4, a different molecule.

Is the TB-500 fragment legal in the Netherlands?

It is not a registered medicine in the Netherlands or anywhere in the EU, so it may not be sold or advertised as a medicine. It is offered only as research material, not for human use, and in sport it is prohibited at all times.

Is TB-500 banned in sport?

Yes, at all times, in and out of competition. WADA lists thymosin beta-4 and its derivatives under S2.3 and names TB-500 as an example; the Dopingautoriteit publishes the same list in Dutch. Doping laboratories published methods to detect the fragment and its breakdown products in 2012.

Did the FDA approve TB-500 in 2026?

No. In July 2026 an FDA advisory committee voted 8 to 6 to let pharmacies compound it, against the advice of FDA staff. That is a recommendation about compounding, not a drug approval, and the FDA had not acted on it by October 2026.

Storage and handling

Store according to product-label instructions and batch handling notes. Lyophilized research peptides are commonly kept dry, protected from light, and handled with clean laboratory technique.

References

Checked against these sources on 28 October 2026.

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