TB-500: Almost Every Number You Have Seen Belongs to a Different Molecule

01What it is

Thymosin beta-4 is a naturally occurring peptide, 43 amino acids long, present in most human cells. It has a genuine research literature.

What vendors sell as TB-500 is not that. It is a synthetic fragment corresponding to residues 17 through 23 of thymosin beta-4, sequence LKKTETQ, with an acetyl group attached to the front end. A doping control laboratory synthesised it and characterised it by high resolution and triple quadrupole mass spectrometry to establish what was actually circulating in the market. (PMID 22962027)

Seven amino acids out of 43.

What is sold as TB-500 is C38H68N10O14, average molecular weight 889.0, monoisotopic mass 888.49. (PubChem CID 62707662, CAS 885340-08-9) Full length thymosin beta-4 is C212H350N56O78S, average molecular weight 4,963. (PubChem CID 45382195)

If a vendor tells you TB-500 is thymosin beta-4 and their own report lists a molecular weight near 889, the report is describing the fragment regardless of what the marketing says. One thing on a certificate that looks wrong and is not: net peptide content below 100 percent is expected, because the powder is usually supplied as a salt carrying some water, so the vial weighs more per mole than the peptide itself does.

The FDA, the American drug regulator, says so outright. In a footnote listing vendor websites, it notes that several of them use the two terms interchangeably, then states plainly that thymosin beta-4 and TB-500 are not the same substance.

No receptor has been established for either one. Beta-thymosins are described in the primary literature as acting outside the cell by a mechanism that is not known. (PMID 21106936) The 2026 scoping review describes actin sequestration as the primary molecular function, with everything else downstream, rather than naming a receptor. Actin is the scaffolding that holds a cell's shape, and sequestration means mopping it up. For the fragment specifically there is no binding data of any kind.

02What the trials found

Thymosin beta-4 has been given to people twice, both into a vein.

One randomised placebo controlled study gave healthy volunteers intravenous thymosin beta-4 at 42, 140, 420 and 1,260 mg, first as a single dose and then daily for 14 days, reporting no dose limiting toxicities or serious adverse events. (PMID 20536472)

A later first in human study of a recombinant version dosed intravenously at 0.05 to 25 mcg/kg as a single dose, and 0.5 to 5 mcg/kg daily for 10 days. (PMID 34346165)

For the fragment being sold as TB-500, the FDA's literature search found no article describing administration to a human, and no case reports of human use. There is no published human dose, no half life, no peak blood level, and no figure for how much of an injection reaches the bloodstream, by any route. Nothing to inherit and nothing to scale from.

There is one listing that looks like it changes this, and it does not. A ClinicalTrials.gov record, NCT07487363, describes a Phase 1/2 randomized double blind placebo controlled dose escalation study of TB-500 in stable cardiovascular disease. 80 participants. Industry sponsor. Status: recruiting.

Its own summary opens with the sentence "This fictional study is an example of a ClinicalTrials.gov-style record."

It is not alone, and the wider picture belongs to Morgan McSweeney, who documented it on 31 July 2026. (Dr Noc) The number will look like a citation everywhere it appears except at the source.

Every rat dose in circulation was generated with full length thymosin beta-4. One study administered 2, 12 and 18 mg/kg and estimated 3.75 mg/kg, and its own abstract describes the test article as a 5K actin binding peptide, meaning the roughly 5 kilodalton parent. (PMID 25060418) Another used 6 mg/kg intraperitoneally in a rat embolic stroke model, with the first dose 24 hours after stroke induction and four more every three days. (PMID 20627173) Intraperitoneal means into the abdominal cavity.

Topical thymosin beta-4 at 5 mcg in 5 microlitres of buffer, twice daily in mice, sped up corneal healing and reduced inflammatory signalling. (PMID 11950239)

All parent molecule.

Work using the fragment itself exists, and it is thin. A rat metabolism study, discussed below. (PMID 38382158) A 2025 peptide hydrogel in a corneal alkali burn model, which its authors describe as the first ocular application of TB500, though earlier rabbit corneal work with the unacetylated fragment exists. (PMID 41359360) A doping control paper in which horses received a single dose containing 10 mg of N-acetylated LKKTETQ, which is the fragment rather than the parent. (PMID 23084823) And a 2003 study administering the unacetylated fragment to mice.

A 2026 scoping review screened 1,772 records and included 80 studies. Musculoskeletal work was sparse, and direct TB-500 evidence appeared in a single included study. (DOI 10.3390/app16126202)

One study. In the tissue category the compound is almost entirely sold for.

03The finding that complicates everything above

When researchers tracked what happens to TB-500 in rats, it came apart fast. Ac-LK was the dominant metabolite in the first six hours. Ac-LKK persisted and was still detectable at 72 hours.

Then they took the metabolites they had identified and screened them for wound healing activity in fibroblasts, the cells that lay down new connective tissue. The only one showing significant activity against control was Ac-LKKTE, a five residue breakdown product. Not the seven residue peptide that was administered. The authors' own conclusion is that the wound healing effects attributed to TB-500 may be due to that metabolite rather than to the compound itself. (PMID 38382158)

If that holds, then what people are injecting is closer to a shipping container than to cargo.

Plenty of real drugs work that way. But every dosing schedule built around how long TB-500 persists in the body is then timing the wrong molecule.

04How it compares

TB-500 is sold beside three other repair peptides.

Read the last column. No row there is evidence for any other row.

05Where the dose came from

The convention that circulates is roughly 2 mg twice weekly for 4 to 6 weeks, then 2 mg once weekly as maintenance. One source publishing it states directly that practical cycle planning for TB-500 is community derived, not validated by any published human trial of the fragment, and that no FDA approved dosing label exists.

Two partial explanations, and neither closes the case.

The first is the vial. TB-500 is most commonly sold in 10 mg vials. The repeated figures are simple fractions of that unit. 2 mg is one fifth. 2.5 mg is one quarter, and also half of a 5 mg vial. 5 mg is half, and also a whole 5 mg vial.

Intermediate figures like 3 mg and 4 mg are not clean fractions. And 2 mg twice weekly, the single most repeated figure of all, produces 4 mg per week, which is neither a whole vial nor a tidy fraction of one. The packaging explains some of the round numbers. It does not explain the schedule.

The second is a horse label. A veterinary equine product sold as TB-500 instructs: administer subcutaneously, one vial each week for six consecutive weeks as a loading dose, then one vial per month thereafter, with dosing after intense work at seven day intervals.

Four structural features of the human convention are already fully assembled there. The subcutaneous route, meaning under the skin. The load then maintain architecture. A six week loading block. A fixed whole vial amount rather than a weight based dose.

I could not find a published account of anyone transposing the veterinary schedule into human use. What I can say is that the human schedule has a closer antecedent in a horse product label than in any study of any kind.

06What could go wrong

Start with the vial.

An independent analysis of TB500 and TB1000 products bought over the internet found their contents were not systematically consistent with their descriptions. (PMID 36482504)

The veterinary side is worse. UC Davis's Kenneth L. Maddy Laboratory analysed a number of substances labelled TB-500, reported through the Racing Medication and Testing Consortium. Many contained no thymosin beta-4 or any peptide derived from it. Most did not contain any proteins, peptides or amino acids at all. Exactly one sample actually contained N-acetylated LKKTETQ.

A test that finds no amino acids in a peptide product has not found a low quality peptide. It has found something that was never a peptide.

The mislabelling starts at the source. The equine product label declares its active ingredient as "Thymosin Beta 4 10mg," while the compound generically sold under the TB-500 name is the acetylated heptapeptide. Two different molecules, named interchangeably, on the label itself.

Then the cancer question. Researchers used a virus to force melanoma cells that were already malignant to overproduce full length thymosin beta-4. Mean metastatic lung nodules rose from 10.9 to 46.7, with tumour vessel number up more than fourfold. (PMID 14625258) That is genetic overexpression of the parent protein inside an existing tumour. It is not somebody injecting a heptapeptide. Writing that TB-500 causes cancer would be unsupported, and so would writing that the concern has been debunked, because no carcinogenicity study of TB-500 exists in any species. The question has not gone unanswered. It has gone unasked.

And if you are tested in sport, this one is not ambiguous. TB-500 is named on the WADA Prohibited List by that exact trade name. The listed wording is "Thymosin-β4 and its derivatives e.g. TB-500," which appears under S2.3, Growth Factors and Growth Factor Modulators. (2026 Prohibited List) It is prohibited at all times, in competition and out. It is a non-Specified Substance, because the whole S2 class is. The default sanction is four years rather than two. And it has been listed since the 2018 List, effective 1 January 2018.

07What nobody knows

Whether the fragment does anything in humans. I found no published report of it being given to a person.

What either molecule binds to. No receptor for the parent, no binding data at all for the fragment.

Any toxicology, in any species. The category is empty, and empty is different from reassuring.

08My take

This is a compound with a naming problem sitting on top of an evidence vacuum. The dosing schedule traces more cleanly to a veterinary label than to any study.

What bothers me most is not the uncertainty. It is that the uncertainty is completely invisible in how the compound is sold. "TB-500, also known as thymosin beta-4" is not a simplification. It is the whole error, printed on the label.

If you have run TB-500, I would like to know whether you had a certificate of analysis for your specific batch, and what molecule it actually named. Given what the Maddy Laboratory found, that answer is more interesting than anything about the dose.

Frequently asked

Is TB-500 the same thing as thymosin beta-4?+

No. Thymosin beta-4 is a naturally occurring peptide 43 amino acids long. What vendors sell as TB-500 is a synthetic fragment of it, residues 17 through 23, sequence LKKTETQ, with an acetyl group on the front end. The FDA states plainly that the two are not the same substance, and nearly all research circulating under the TB-500 name was run on the larger molecule.

Has anyone actually given TB-500 to a person?+

The FDA searched the published literature for any article in which TB-500 was administered to a human and found none. A 2026 scoping review screened 1,772 records, included 80 studies, and found direct TB-500 evidence in exactly one of them. The two human studies people cite used full length thymosin beta-4, the other molecule, and the one ClinicalTrials.gov record for TB-500 calls itself a fictional example.

Is TB-500 safe?+

There is no published basis for answering that in either direction. The FDA searched for acute toxicity, repeat dose toxicity, genotoxicity, developmental and reproductive toxicity, and carcinogenicity studies of TB-500, and found none of them, in any species. Separately, UC Davis testing of veterinary product labelled TB-500 found most samples contained no proteins, peptides or amino acids at all.

Where does the 2 mg twice a week schedule come from?+

Not from a trial. The convention that circulates is roughly 2 mg twice weekly for 4 to 6 weeks, then 2 mg once weekly, and the sources publishing it concede it is community derived rather than validated by any published human trial of the fragment. A veterinary equine product label already carries the same shape, down to a six week loading block, and I could not find a published account of anyone transposing that schedule into human use.

Does TB-500 cause cancer?+

I could find no carcinogenicity study of TB-500 in any species, so the claim that it causes cancer is unsupported, and so is the claim that the concern has been debunked. Researchers used a virus to force melanoma cells that were already malignant to overproduce full length thymosin beta-4, and mean metastatic lung nodules rose from 10.9 to 46.7, with tumour vessel number up more than fourfold. That is overexpression forced inside an existing tumour, not somebody injecting the seven residue fragment.

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This content is for educational and informational purposes only and is not medical advice. Peptides discussed are research compounds and may not be approved for human use. Nothing here should be used to diagnose, treat, cure, or prevent any disease. Always consult a qualified healthcare professional before starting any peptide, supplement, or protocol. Individual responses vary. Do not self-administer compounds without proper medical supervision.