Promise Peptides mypromise.com Its clinicians now prescribe TB-500 within prescription-only care. After reading each request, a U.S.-licensed clinician decides. Review can end with a refusal. This care serves patients within the United States. The options follow state rules and the treatment selected. The compounded TB-500 offered in this care has no FDA approval. # TB-500 and thymosin beta-4: be specific. > TB-500 and thymosin beta-4: be specific — TB-500, thymosin beta-4 and Ac-SDKP are different substances. A close look at the fragment LKKTETQ, human studies and the FDA entry. **The particulars** The name has the sound of a prescription. The papers tell a messier story: a fragment, a full protein and results that will not sit under one label. ## The first surprise was a horse paper I expected healing to be the subject of the most useful fragment paper. Instead, one of the most useful papers was about [detecting TB-500 in horse blood and urine](https://pubmed.ncbi.nlm.nih.gov/23084823/). Its business was identification for doping control. It answered a chemical question. It identified the synthetic acetylated fragment Ac-LKKTETQ. That is a much smaller claim than “recovery,” and a much firmer place to begin. “Prescribed” is a demanding word. A prescription concerns a particular substance and a particular person; the word cannot settle whether a finding transfers between molecules. That is the snag in this name. The substance called TB-500 in the analytical literature is a short fragment of thymosin beta-4, while most of the repair research used the whole protein. I trust the horse paper to identify a molecule. The name earns no such trust from me when it carries a different molecule's results across that gap. The [2026 sports-medicine review](https://pubmed.ncbi.nlm.nih.gov/41966639/) describes encouraging animal findings alongside scarce rigorous human safety data and the possibility of serious harm. There are no completed controlled human trials establishing the TB-500 fragment's efficacy. ## What is TB-500, exactly? TB-500 is Ac-LKKTETQ, a synthetic chain of 7 amino acids corresponding to residues 17–23 of thymosin beta-4. The parent has 43 amino acids. In the [biochemical and structural work on thymosin beta-4](https://pubmed.ncbi.nlm.nih.gov/15329672/), the full protein binds loose actin, the material cells assemble into filaments when changing shape and moving. The size difference is substantial: the fragment's molecular weight is 889.02 daltons, while the full protein is approximately 4963 daltons. The [review of the parent protein's repair biology](https://pubmed.ncbi.nlm.nih.gov/22074294/) describes it in nearly all human cells and its release by platelets and macrophages around injury. TB-500 is a synthetic construct; the complete protein occurs naturally in the body. TB refers to thymosin beta-4; TB-500 is a research designation. Chemical identity still has to be established for the substance being discussed. Calling it the “actin-binding fragment” locates the piece. It does not establish that the piece performs every job of the whole. ![A strip of forty-three positions with the TB-500 region and the separate Ac-SDKP region marked](images/the-missing-pieces.svg) *A location map, not a model of the protein’s folded shape. TB-500 copies the actin-binding region; Ac-SDKP comes from the N-terminal end.* ## Ac-SDKP comes from the other end Ac-SDKP is another reason the names deserve care. It is an N-terminal cleavage product of full-length thymosin beta-4, with separate anti-fibrotic and blood-vessel activity discussed in the [parent-protein literature](https://pubmed.ncbi.nlm.nih.gov/22074294/). TB-500 contains a different region and cannot generate Ac-SDKP. An Ac-SDKP capsule question therefore concerns a different substance. These TB-500 studies do not establish a capsule amount or an oral treatment. I object to folding the two fragments into one attractive story: the chemistry has already separated them. ## The human participants received thymosin beta-4 The human work deserves its place. A [randomized intravenous safety study](https://pubmed.ncbi.nlm.nih.gov/20536472/) tested full-length synthetic thymosin beta-4 in healthy volunteers. The [ophthalmic research on corneal healing](https://doi.org/10.3390/ijms24010554) also used thymosin beta-4, in the formulation studied as RGN-259. Neither is a controlled human trial of the Ac-LKKTETQ fragment. For a ClinicalTrials.gov search, the distinction to preserve is TB-500 fragment LKKTETQ versus full-length thymosin beta-4. A trial record identifies a planned investigation; assessing efficacy requires its findings. I give the human papers credit for the substance and setting they actually tested; the interesting work becomes less interesting when its identity gets blurred. FDA names the substance explicitly as thymosin beta-4, fragment (LKKTETQ), also known as TB-500. Its [safety-risk entry](https://www.fda.gov/drugs/human-drug-compounding/certain-bulk-drug-substances-use-compounding-may-present-significant-safety-risks) records immune-response concerns and missing human safety information. On FDA's page current as of April 22, 2026, the nomination appears among withdrawn requests, rather than in Category 2. TB-500 remains unapproved as a drug. The [legal-status essay](/legal-status) follows the entry without supplying a decision FDA has not supplied. ## A recovery label gathers unlike things TB-500 and BPC-157 are chemically distinct. TB-500 carries a region from thymosin beta-4; BPC-157 is a separate gastric-derived peptide with 15 amino acids. The [sports-medicine review considers both among unapproved peptides](https://pubmed.ncbi.nlm.nih.gov/41966639/). A shared reputation for recovery does not establish either compound's human benefit, or a benefit from combining them. TB-500 is not a steroid. It is a peptide, and its sports prohibition is a separate question from its chemistry. The more interesting argument is over the word “repair.” In [one muscle study, extra regenerating fibers brought no extra strength](/research). That result troubles a sweeping recovery claim more than any slogan could. The [half-life question](/dosage) has no validated human-fragment number. The [community accounts](/effects) have no clinical denominator. Those are different problems, and they deserve different essays. --- A developer-console reading of the TB-500 record — the actin-binding fragment logged against its studies, with the full-length thymosin beta-4 caveat flagged in every diff, no clinic behind the terminal and nothing here prescribed or sold.