TB-500 Fragment (17-23)

What exactly does “17-23 fragment” describe?

Thymosin beta-4 is a 43-amino-acid protein that was first isolated from calf thymus and later shown to be present in many cell types and released into blood, saliva, tears and wound fluid; it binds actin, a building block of the cytoskeleton. ^[1] In the FDA’s substance registration system the preferred name of the full protein is timbetasin, and the record contains the full sequence of the protein: positions 17–23 of the 43 amino acids, which begin with N-terminal acetylated serine, correspond to the leucine-lysine-lysine-threonine-glutamate-threonine-glutamine (LKKTETQ) sequence. ^[2]

The name “fragment (17-23)” describes these seven amino acids. According to the FDA, TB-500 free base is this sequence with an acetyl group added at the N-terminus (Ac-LKKTETQ); TB-500 acetate is the acetate salt of the same substance, and the FDA evaluates the two as separate bulk drug substances. ^[1] The first six amino acids of the sequence (LKKTET) are known as the actin-binding region of thymosin beta-4; that LKKTETQ could also bind actin and perform the actin-buffering function of the full protein is only a hypothesis. ^[1] The substance defined by the FDA and detected by an anti-doping laboratory in a product called “TB-500” is the seven-amino-acid acetylated LKKTETQ; the six-amino-acid LKKTET is the actin-binding core of this sequence. ^[3]

“Active sites” of the full protein

A 2010 review by researchers who have long studied thymosin beta-4 attributes different functions to different short sequences of the protein: the four-amino-acid Ac-SDKP at the N-terminus suppresses inflammation and fibrosis; the 15-amino-acid N-terminal region, which also contains Ac-SDKP, supports cell survival; and the LKKTETQ region in the center is associated with blood vessel formation, wound healing and cell migration. ^[5] Most of these functions were shown with the full protein or with non-acetylated sequences. The FDA stresses that acetylation can irreversibly change a peptide’s charge, its water-repelling property, its size and its binding properties, and that findings obtained with non-acetylated LKKTETQ therefore cannot be transferred directly to acetylated TB-500. ^[1]

What is actually in the products sold?

According to the FDA’s literature review, TB-500 appears to have first been synthesized from thymosin beta-4 in 2003. In 2012 an anti-doping laboratory in Belgium analyzed a product called “TB-500,” detected the N-terminal acetylated 17–23 piece of thymosin beta-4 (Ac-LKKTETQ) in it, synthesized the substance and proposed a method for detecting it in blood and urine. ^[3] According to the FDA, a veterinary preparation of TB-500 was marketed in 2011 as a performance enhancer in horse and greyhound racing. ^[1]

Name confusion is evident in today’s sales channels:

• Same name, different substances: The FDA writes that multiple salts and derivatives containing different active substances are sold under the common name TB-500, creating a risk that a substance different from the one the physician requested is used. On some supplier sites the CAS number given for the free base is used for the acetate salt. ^[1]
• Full protein or piece: The FDA states that TB-500 is often promoted online as “a synthetic version of thymosin beta-4” and that the two names are used interchangeably, but they are not the same substance. ^[1] In the list of unauthorized injectable peptides that Health Canada, Canada’s health authority, seized from a wellness center in April 2025, products labeled “17-23 fragment of thymosin-β4” and “Thymosin-β4 (TB4 or TB-500)” appear separately. ^[4]
• Missing quality documents: The publicly available certificate of analysis the FDA examined contained only appearance, identity and purity; quantity, impurities, bacterial endotoxin and aggregation had not been tested. The certificate attached to the submission for TB-500 acetate also lacked identity, quantity, aggregation and endotoxin results. ^[1]

For this reason, which substance, in what amount and at what purity is in a vial labeled “TB-500,” “TB-500 fragment” or “TB4” cannot be determined from the label.

What evidence exists for the fragment?

The FDA’s conclusion is clear: there is no preclinical study showing that the acetylated fragment supports wound healing in live animals, and none of the acute, repeat-dose, genetic damage, reproductive and carcinogenicity studies that would serve a safety assessment have been done. ^[1] The authors of the 2024 cell study suggest that the wound-healing effect attributed to TB-500 in the literature may come not from the parent substance but from the Ac-LKKTE breakdown product. ^[6]

What did human studies of full thymosin beta-4 find?

The substance that has been studied in humans is full-length thymosin beta-4, not the fragment, and the most advanced program is an eye drop with the code name RGN-259. These results cannot be transferred to the fragment used by injection.

• Neurotrophic keratopathy, phase 3 (SEER-1): In 18 patients with persistent corneal surface damage, complete healing at the end of 4 weeks was seen in 6 of 10 people in the RGN-259 group and 1 of 8 people in the placebo group (p=0.0656; not significant). Improvement in some secondary measures and in discomfort symptoms was reported, and no significant adverse effects were seen. Two of the authors are employees of the company that developed the drug and one is an employee of the parent company; the study was designed by company employees and the first author. ^[7]
• Dry eye, phase 3 (ARISE-3): According to the ClinicalTrials.gov results record, in the 700-person trial the change in inferior corneal staining at day 15 was −0.41 in the drug group (351 people) and −0.46 in the placebo group (348 people); the change in eye discomfort was −0.4 points in both groups. The record contains no statistical analysis. ^[8]
• Neurotrophic keratopathy, second phase 3 (SEER-2): According to the ClinicalTrials.gov record, the trial, which began in April 2023 with a target of 70 patients, appears as recruiting at the last update (December 23, 2025); the estimated primary completion date is April 2026, and no results have been published. ^[9]

Even in the most advanced human program of thymosin beta-4, a consistent benefit on primary outcomes could not be shown. According to the FDA, there is no TB-500 product authorized by the EMA in the EU; there is also no approved TB-500 product in Canada, Australia, the United Kingdom or several EU countries. ^[1]

Has the fragment been tested in humans?

No. The FDA states that it found no article, clinical trial or human exposure data in which TB-500 (free base or acetate) was given to humans, and that the possible safety risks in humans are therefore unknown. In the agency’s adverse event database FAERS, no report for TB-500 had appeared as of March 2025, while its food and supplement complaint system contained two records related to a “TB-500 and BPC-157 mixture,” without a safety assessment. ^[1]

Many uses are attributed to the substance online, such as tissue repair, tendon and muscle healing, muscle growth, endurance, heart health and hair growth; the FDA compiled these one by one from sales and clinic sites and found no study in which TB-500 was given to humans for any of them. ^[1]

What are the risks of TB-500 fragment?

• Immune response: According to the FDA, peptides given by injection can produce an immune response; outcomes range from antibodies that cause no symptoms to life-threatening reactions and cannot be predicted in advance. Aggregation and peptide-related impurities can increase this risk; no study has examined this for TB-500. ^[1]
• Unknown toxicity: There are no preclinical toxicity data at all. ^[1]
• Unauthorized injectable products: According to Health Canada, such products can cause infections, allergic reactions and drug interactions, may contain ingredients or contaminants not listed on the label, and may not have been manufactured or stored under safe conditions. ^[4]
• Mixture products: The two complaints in FDA records concern products mixed with BPC-157; when an adverse effect develops, it cannot be distinguished which substance it is due to. ^[1]

For general peptide risks: Peptide side effects

Is TB-500 fragment prohibited in sport?

In section S2.3, “growth factors and growth factor modulators,” the WADA 2026 Prohibited List names “thymosin-β4 and its derivatives, e.g., TB-500”; substances in class S2 are prohibited at all times, in and out of competition. ^[11] The list gives TB-500 by name as an example of thymosin-β4 derivatives. An anti-doping laboratory in Belgium has proposed an analytical method for detecting the substance in blood and urine. ^[3]

What is the status in the US?

On the FDA’s page dated April 22, 2026, the line “thymosin beta-4 fragment (LKKTETQ), also known as TB-500” appears in the table of substances that were previously in Category 2 (bulk drug substances that may present significant safety risks) and were withdrawn by their nominators. The FDA’s rationale on this line is the risk of immune response due to aggregation and peptide impurities with some routes of administration, and the absence of any human exposure data for products containing the substance. ^[10] In its report prepared for the July 2026 advisory committee, FDA staff recommended that TB-500 free base and acetate not be added to the list of substances pharmacies may use in compounding. ^[1] On July 23, 2026, contrary to the staff recommendation, the committee (PCAC) advised that the free base and acetate forms of TB-500 be added to the list; according to a law firm that summarized the meeting, the vote was 8 yes, 6 no, 1 abstention, and for the recommendation to take effect the FDA must carry out a rulemaking process with public comment. ^[15] Committee recommendations are not binding and do not mean a drug approval; details are in the TB-500 guide.

Detailed guideTB-500: evidence, risks and legal status→

What is the status of TB-500 fragment in Türkiye?

Status in Türkiye

License

No product containing TB-500, thymosin beta-4, timbetasin or their fragments appears on the TİTCK (Turkish Medicines and Medical Devices Agency) Licensed Human Medicinal Products List dated October 16, 2026 ^[12]

Law

Law No. 1262 on Pharmaceutical and Medicinal Preparations (İspençiyari ve Tıbbi Müstahzarlar Kanunu); Law No. 6197 on Pharmacists and Pharmacies (Eczacılar ve Eczaneler Hakkında Kanun) ^{[13] [14]}

Sport

WADA S2.3: always prohibited ^[11]

Article 3 of Law No. 1262 makes the placing of preparations on the market and the import of those produced abroad subject to Ministry permission. Article 19 provides imprisonment of one to five years for those who produce unlicensed preparations and for those who knowingly sell, offer for sale or have them sold, and treats the commercial import of a preparation produced abroad without a license as smuggling; it also provides that, where a product that is not a preparation is sold and promoted with a diagnosis or treatment claim over the internet, the access-blocking provision of Article 18 applies. Additional Article 7 provides that the Ministry may permit the import of unlicensed preparations for examination, experiment or personal treatment, provided that the amount does not exceed the quantity to be accepted by the Ministry. ^[13] Under Article 24 of Law No. 6197, medicines cannot be sold on the internet or any other electronic medium. ^[14] How products labeled “for research purposes” stand under these provisions is assessed in the specific case by the administration and the courts; this page is not legal advice.

For the general framework: Are peptides legal? and What does “research peptide” mean?

Evidence table: claim, population and limits

Limits of this guide

This page summarizes published evidence and official documents on TB-500 fragment (17-23) and full thymosin beta-4. It contains no dose, preparation, administration, course or product source; conditions in animal and cell studies are not transferred to humans. Because the fragment has never been studied in humans, no safe form of use has been defined. The regulatory status is changing rapidly; the information on this page is valid as of October 2026.