TB-500 and Thymosin Beta-4: Checking the Exact Substance

Products called TB-500 are often described as "thymosin beta-4". In most technical descriptions they are not the same molecule, and research on one does not transfer to the other. This profile explains the naming problem, then sorts the evidence by molecule, form and study type. We do not give dosing or usage guidance.

Two different molecules

Thymosin beta-4 is a naturally occurring protein of 43 amino acids. It is found in many cell types and in body fluids such as blood, saliva, tears and wound fluid, and it binds actin, a protein that shapes cells and helps them move ^{Source [1]}.

TB-500, as FDA describes it in 2026, is a synthetic fragment of only seven amino acids: positions 17 to 23 of thymosin beta-4 (the sequence LKKTETQ), with an acetyl group attached at one end ^{Source [1]}. Anti-doping chemists identified that acetylated fragment as the key ingredient of a veterinary preparation called TB-500 that had emerged in equine sports, where testing labs needed a way to detect its misuse ^{Source [2]}, and confirmed it by mass spectrometry in a product sold as TB-500 ^{Source [3]}.

FDA notes that many websites call TB-500 a synthetic version of thymosin beta-4 and use the two names interchangeably, and it states plainly that they are not the same substance ^{Source [1]}. TB-500 is also a common name rather than an official drug name. FDA has encountered different salts and derivatives sold under it, and found supplier sites that apply the free-base registry number to the acetate salt ^{Source [1]}. At FDA's July 2026 advisory committee meeting, according to ABC News, a senior FDA official asked how the agency could list TB-500 when neither he nor a prescriber knows which form a patient actually receives ^{Source [4]}.

What a document needs to say

A label or report that names only "TB-500" or "TB4" does not tell you which molecule is meant. To know what a study tested, or what a document describes, look for the full amino-acid sequence, whether the chain is acetylated, the salt form, and an identity result, usually from mass spectrometry, that matches that exact molecule. Our guide to purity, identity, and quantity explains why these are separate checks.

Evidence on the fragment

The animal study of the fragment that FDA discusses in its review was done in mice. Thymosin beta-4 applied to skin wounds sped healing in diabetic and aged mice, and the seven-amino-acid fragment LKKTETQ, without the acetyl group, promoted repair in aged mice about as well as the full protein ^{Source [5]}. FDA cautions that acetylation changes a peptide's charge, size and binding behavior, so results with the non-acetylated fragment cannot be assumed to apply to TB-500 ^{Source [1]}.

When researchers tested TB-500 itself in a 2024 cell study, it did not significantly speed the closure of scratch wounds in cultured fibroblasts, while one of its breakdown products did. The authors suggest that activity previously credited to TB-500 may come from that metabolite ^{Source [6]}.

FDA's review found no published study in which TB-500 was given to humans, and no clinical evidence for its nominated use in wound healing ^{Source [1]}. The only registry record we found that names TB-500 describes itself as a fictional example, and its sponsor’s other 2026 filings include a mock study, so we do not count it as a trial ^{Source [7] Source [8]}.

Evidence on full-length thymosin beta-4

The full protein has been tested in people, but only in specific pharmaceutical forms. That is a separate evidence base:

• Intravenous infusion. In a placebo-controlled phase 1 study in 40 healthy volunteers, adverse events were infrequent and mild or moderate after single doses and after 14 days of daily dosing, with no serious adverse events ^{Source [9]}. The study measured safety and blood levels; it was not designed to show any benefit.
• Topical gel. In a placebo-controlled phase 2 dose-ranging trial in 73 people with venous leg ulcers, safety was comparable to placebo, and the authors described the efficacy findings as suggestive rather than conclusive ^{Source [10]}.
• Eye drops. In a phase 2 trial in 72 people with dry eye, neither primary endpoint differed from placebo, although several secondary measures improved ^{Source [11]}.

These results concern particular formulations studied under trial conditions. They say nothing about TB-500, and nothing about other forms or routes of thymosin beta-4.

Regulatory status as of November 3, 2026

Neither TB-500 nor thymosin beta-4 is an FDA-approved drug, and FDA notes that TB-500 is not a component of any approved drug ^{Source [1]}. FDA's page on compounding substances that may present significant safety risks lists "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" among substances previously in Category 2 whose nominations were withdrawn, an entry that names the sequence without mentioning the acetyl group; FDA had identified no human exposure data for it ^{Source [12]}.

For its July 2026 Pharmacy Compounding Advisory Committee meeting, FDA evaluated TB-500 for wound healing and proposed not adding it to the 503A bulks list ^{Source [1]}. According to ABC News, the committee voted to recommend it anyway; the report did not give a vote count for TB-500. Such recommendations are non-binding, and adding a substance to the list requires FDA rulemaking ^{Source [4]}. HHS's October policy document says FDA's drug center recommended against listing either form of TB-500 and that the committee voted in favor of both ^{Source [13]}; FDA's meeting page listed no minutes or official vote results when we checked on November 3, 2026 ^{Source [14]}.

On October 30, 2026, HHS published notice of a draft interim policy on 503A compounding with certain peptides ^{Source [15]}. The policy's list names six, TB-500 among them, and identifies it as TB-500 acetate with the acetylated sequence Ac-LKKTETQ; it does not name full-length thymosin beta-4. Until a final rule decides whether TB-500 goes on the 503A list, HHS says it does not intend to take enforcement action against state-licensed pharmacies, federal facilities or licensed physicians that compound TB-500 acetate in subcutaneous or intramuscular injectable forms, as long as its conditions are met. They include a patient-specific prescription, a single active ingredient, bulk material from an FDA-registered manufacturer with a valid certificate of analysis, and reporting of serious adverse events to FDA ^{Source [13]}. HHS may revisit the policy if a safety signal or product quality problem arises, and the draft is not binding. Compounded drugs are not FDA-approved, and FDA does not review their safety, effectiveness or quality before they are sold ^{Source [15]}. The policy is enforcement discretion, not an approval and not a listing. We found no proposed rule on TB-500 in the Federal Register as of November 3, 2026.

In sport, the 2026 WADA Prohibited List names thymosin-β4 and its derivatives, with TB-500 given as an example, under S2.3 (growth factors and growth factor modulators), part of a class prohibited at all times ^{Source [16]}.

Research materials and consumer products belong to different categories. A listing, laboratory report, or research-use label does not establish suitability for personal use. The HHS policy concerns pharmacy compounding on a prescription, not products sold as research use only ^{Source [13]}.

Open questions

• Which molecule, and which salt form, is in a given product?
• Does the acetylated fragment have any effect in living animals or people, or only through its breakdown products?
• What are the risks of injectable products made from a peptide that FDA considers poorly characterized, including immune reactions?
• Do the results from specific thymosin beta-4 formulations apply to anything sold outside trials? Nothing published so far shows that they do.