What is TB-500, and is there human evidence for it?

What is TB-500?

TB-500 is a synthetic fragment of thymosin beta-4, a 43-amino acid protein found in platelets and white blood cells. FDA lists it as "thymosin beta-4, fragment (LKKTETQ), also known as TB-500": seven amino acids of the parent protein. It is not an approved drug and is sold online as a research chemical.

TB-500 is a piece of a natural protein, made in a lab. The parent protein is thymosin beta-4 (often written Tβ4), a 43-amino acid peptide that is a major component of platelets, macrophages and other white blood cells, where it binds actin, the protein that gives cells their shape and lets them move [2]. Thymosin beta-4 is also present in body fluids such as tears [3].

FDA's own name for TB-500 makes the relationship plain: "thymosin beta-4, fragment (LKKTETQ), also known as TB-500" [1]. Each of those seven letters is one amino acid, so TB-500 is a seven-amino acid stretch of a 43-amino acid protein. Its appeal rests on the idea that this short stretch can do what the whole protein does. That idea is a hypothesis, and the rest of this article is about how far the evidence for it goes.

Many sellers describe TB-500 as "synthetic thymosin beta-4" and use the two names as if they were one. They are different molecules, and the difference decides which studies tell you anything about TB-500.

Is there human evidence for TB-500?

Not for TB-500 itself. FDA states that it has not identified any human exposure data for products containing the fragment. The human trials people cite tested full-length thymosin beta-4, mostly as eye drops or as an intravenous protein, and their results cannot be assumed to carry over to a seven-amino acid piece.

The short answer is no. On its list of compounding substances that may pose significant safety risks, FDA writes that it has not identified any human exposure data for drug products containing the thymosin beta-4 fragment, and that it lacks important information about whether it would cause harm if given to humans [1]. ClinicalTrials.gov does list one phase 1/2 safety study of TB-500 in adults with heart disease as recruiting, but the record's own summary calls it a fictional study written as an example of a ClinicalTrials.gov-style record, and it has posted no results [8].

What does exist is human research on the whole protein. Full-length thymosin beta-4 was developed as an eye drop, RGN-259, and tested in dry eye and in a corneal disease [2] [3]. Synthetic thymosin beta-4 was given intravenously to healthy volunteers in a safety study [4], and a recombinant (genetically engineered) version was tested in people who had just had a heart attack [5]. The next section summarizes what those studies found.

Why can those results not simply be applied to TB-500? Because a fragment is not the protein. It is a fraction of the size, it may be broken down differently, and it may or may not do what the full molecule does in a given tissue. No published human study has measured any of that for TB-500, so a claim that TB-500 heals tendons, muscles or injuries in people has no human trial behind it.

What the thymosin beta-4 trials found

Mixed results, mostly in the eye. The phase II dry-eye trial missed both primary endpoints. A phase III corneal trial healed 6 of 10 treated patients versus 1 of 8 on placebo, a difference that fell short of significance (p = 0.0656). A 96-patient heart attack trial found no overall difference in infarct size.

Dry eye. A phase II trial of 0.1% thymosin beta-4 eye drops in 72 people with dry eye found that neither primary endpoint, ocular discomfort or inferior corneal staining, differed significantly between treatment and control groups at the main visit [2]. Several differences appeared only in ad hoc analyses adjusted for baseline. Eye-related adverse events were uncommon: two subjects (5.6%) on thymosin beta-4, both mild, and five (13.9%) on placebo [2].

Neurotrophic keratopathy (a nerve-damage disease that stops the cornea from healing). In a phase III trial of the same eye drop, complete healing occurred after 4 weeks in 6 of the 10 treated patients and 1 of the 8 on placebo, with a p-value of 0.0656, a difference that fell short of statistical significance [3]. The trial was stopped early after 18 subjects had completed it, and no significant adverse effects were observed [3]. A promising signal, in a small trial, for an eye drop.

Safety in healthy volunteers. A randomized study gave synthetic thymosin beta-4 intravenously to four cohorts of 10 healthy subjects each, as a single dose and then daily for 14 days [4]. Adverse events were infrequent and mild or moderate, with no dose-limiting toxicities or serious adverse events [4].

Heart attack. A randomized, placebo-controlled trial of recombinant human thymosin beta-4 in 96 patients treated for a STEMI heart attack found that the overall difference in infarct (damaged heart muscle) area between the drug and placebo groups was not significant [5]. Infarct area was smaller in the subgroup that received its first dose within 8 hours after the artery was reopened, and the authors call for further randomized studies [5].

Read together: in these four studies thymosin beta-4 was well tolerated [2] [3] [4], the dry-eye and corneal trials missed significance on their primary endpoints [2] [3], and the heart attack trial found no overall difference [5]. None of them tested TB-500.

Is TB-500 safe?

Nobody knows, because it has not been studied in people. FDA warns that injected TB-500 may pose a risk of immunogenicity, an immune reaction against the peptide, because it can clump and carry peptide-related impurities. The safety record of full-length thymosin beta-4 in trials does not transfer to it.

Safety is the same story as effectiveness: there are no human data to judge it by. FDA's entry for TB-500 states the specific concern it does have. Compounded drugs containing the fragment may pose risk for immunogenicity for certain routes of administration, meaning the body may mount an immune response against them, because of the potential for aggregation (molecules clumping together) and peptide-related impurities [1]. FDA adds that it lacks important information about safety issues, including whether the substance would cause harm if given to humans [1].

The reassuring safety findings in the thymosin beta-4 trials above belong to those products: an eye drop and an intravenous protein, each made for a clinical trial [3] [4]. A research-labeled vial of TB-500 bought online is a different molecule, made to an unknown standard. Our guide to vetting research peptides covers how to read a certificate of analysis and what third-party testing adds.

Is TB-500 legal? Where FDA and HHS stand in 2026

TB-500 is not an approved drug. In July 2026 an FDA advisory panel voted, by a narrow margin, to recommend letting compounding pharmacies use it, a non-binding vote. On October 30, 2026 HHS published a draft policy not to take enforcement action against licensed pharmacies compounding certain peptides while rulemaking proceeds.

TB-500 has had an unusual few years in US regulation. FDA had listed it among the bulk substances that may pose significant safety risks in compounding; the agency's page now places it in a table of substances "nominated but withdrawn", still with the safety rationale quoted above [1].

In July 2026, a panel of advisers to FDA made recommendations on seven popular peptides, including TB-500, for possible use by compounding pharmacies. By narrow margins, the panel voted in favor of all but one, and NPR reported that if the agency follows it, clinicians could prescribe six peptide therapies: BPC-157, MOTS-c, KPV, TB-500, Epitalon and Semax [6]. The vote was a recommendation: FDA is not required to follow advisory committee recommendations, though it typically does [6].

Then, on October 30, 2026, HHS announced a draft enforcement discretion policy: while the list of substances allowed in 503A compounding is developed through rulemaking, HHS does not intend to take enforcement action against a state-licensed pharmacy, federal facility or licensed physician compounding with a peptide listed in the policy's Appendix A [7]. The notice itself says the policy is not binding on HHS or the public, and that compounded drugs are not FDA-approved [7]. Our explainer on the 2026 FDA peptide reclassification follows the whole process. None of it makes TB-500 an approved drug, and none of it produces the human evidence that is missing.