The biggest TB-500 risks come from a mismatch: the human trials were done on thymosin beta-4, a natural 43-amino-acid protein, while TB-500 is sold as a short synthetic fragment that has not been through those trials. The FDA says it has not identified any human exposure data for that fragment. Neither TB-500 nor thymosin beta-4 is an FDA-approved drug.
Thymosin beta-4 and TB-500 are not the same thing
Thymosin beta-4 is a protein your body makes. It binds actin, a building block of the cell skeleton, and has been studied for wound healing and heart repair. A synthetic copy of the full protein is 43 amino acids long.
TB-500 first appeared as a veterinary preparation. Hong Kong Jockey Club chemists who developed a doping test for racehorses describe its key ingredient as the seven-amino-acid segment LKKTETQ, with an artificial chemical cap (acetylation) added to one end. That segment is the region of thymosin beta-4 linked to actin binding and cell migration, but a fragment is a different molecule from the whole protein.
| Thymosin beta-4 | TB-500 | |
|---|---|---|
| What it is | Natural protein, 43 amino acids | Synthetic 7-amino-acid fragment, acetylated |
| Origin as a product | Developed by drug companies for eye, skin and heart conditions | Veterinary preparation, now sold online as a “research” peptide |
| Human trials | Several, in narrow settings (below) | FDA reports no human exposure data |
| FDA approval | None | None |
| WADA status | Banned at all times, class S2.3 | Banned at all times, named as a thymosin beta-4 derivative |
A search of the FDA’s Drugs@FDA database returns no approved product containing thymosin beta-4 or its fragment.
What the human thymosin beta-4 trials tested
Thymosin beta-4 has been tested in people, but only in specific forms, routes and conditions. Each was a formal study of a product made for that research, with participants monitored by investigators.
- Intravenous safety study, healthy volunteers. Four groups of 10 adults received synthetic thymosin beta-4 or placebo, first as a single infusion and then daily for 14 days. Adverse events were “infrequent, and mild or moderate”, with no serious adverse events.
- Recombinant thymosin beta-4 in China. A first-in-human phase 1 study gave single infusions to 54 healthy adults and repeated infusions for 10 days to another 30, and reported no serious adverse events. Three of the authors were employees of the sponsor.
- Eye drops for dry eye. A trial in nine people with severe dry eye reported improved symptoms and signs. A larger trial of 72 people missed both of its primary endpoints, though some secondary measures improved. Phase 3 dry-eye trials enrolling 601 and 700 people are listed as completed on ClinicalTrials.gov (ARISE-2 and ARISE-3).
- Topical gel for leg ulcers. A placebo-controlled study of 73 patients with venous ulcers at eight European sites found safety “comparable to placebo”.
- Epidermolysis bullosa. A phase 2 trial in this rare blistering skin disease was terminated, citing lack of patient availability and expiry of the study drug.
Why those trials do not cover a gray-market vial
It is tempting to read “well tolerated in human trials” and apply it to TB-500. That does not work, for four reasons.
- Different molecule. The trials used the full protein. TB-500 is a capped fragment. A 2024 study from the Korea Institute of Science and Technology’s doping control center stated plainly that “the biological effects of TB-500, however, have not been documented”.
- Different route. The trials used eye drops, a skin gel or a supervised infusion. A self-injected vial is none of those.
- Different manufacturing. Trial drug is produced for a regulated study. A vial labeled TB-500 has no such oversight, and may not contain what it says, at the strength it says.
- Short follow-up. The safety studies above lasted days to weeks. None tells you about repeated use over months.
The same 2024 study, done in cells and rats, found that TB-500 is broken down into smaller pieces in the body and that one of those breakdown products, rather than TB-500 itself, showed the wound-healing activity in cell tests. In other words, even the lab-level case for the fragment is unsettled.
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TB-500 risks: what is known and what is not
| Risk | Basis | What the evidence says |
|---|---|---|
| Immune reaction | FDA review of compounding nominations | The FDA says the fragment “may pose risk for immunogenicity” because of the potential for aggregation and peptide-related impurities. |
| Unknown harms in humans | No human data for the fragment | The FDA “lacks important information regarding any safety issues raised by this drug, including whether it would cause harm.” |
| Cancer-related questions | Laboratory and mouse research on thymosin beta-4 | Higher thymosin beta-4 levels are associated with tumor progression and metastasis, and knocking the gene out reduced metastasis in mice (Morita, 2018). This is an association in lab studies, not evidence that TB-500 causes cancer, and it has not been tested in people. |
| Infection | Injecting a non-sterile product | A risk of the injection itself. See peptide injection infections. |
| Wrong or contaminated contents | Unregulated supply | A purity certificate does not prove sterility. See what a COA can and cannot prove. |
How the FDA treats TB-500
TB-500 was nominated for use by compounding pharmacies and placed by the FDA in category 2, the group of bulk substances that may present significant safety risks. As of April 2026, the FDA’s category 2 page lists “thymosin beta-4, fragment (LKKTETQ), also known as TB-500” among substances previously in category 2 whose nominations were later withdrawn by the nominators. The FDA’s stated concerns remain on that page. A Federal Register notice of April 16, 2026 scheduled the agency’s Pharmacy Compounding Advisory Committee to discuss TB-500 for the 503A list, for wound healing, on July 23, 2026.
It is not on the 503A bulks list of substances pharmacies may compound from. Our guide to whether peptides are legal explains what these categories mean for a buyer.
TB-500 in sport
The World Anti-Doping Agency’s 2026 Prohibited List names “Thymosin-ß4 and its derivatives e.g. TB-500” in class S2.3, growth factors. Substances in S2 are banned in and out of competition and are non-Specified, which carries heavier default penalties.
In September 2025, USADA announced a four-year sanction for a triathlete who possessed and used BPC-157 and TB-500. USADA noted that use and possession of non-Specified Substances carries a default sanction of four years. Testing laboratories have published methods to detect the fragment and its breakdown products, first in racehorse urine and plasma.
See peptides and the WADA Prohibited List for how the classes work.
If you have injected TB-500 and develop a reaction, tell the treating clinician exactly what the vial was labeled as, and report it to FDA MedWatch, which accepts reports directly from patients and consumers.
Sources
- US Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks.
- US Food and Drug Administration. Drugs@FDA: FDA-Approved Drugs.
- US Food and Drug Administration. MedWatch: The FDA Safety Information and Adverse Event Reporting Program.
- Federal Register. Pharmacy Compounding Advisory Committee; Notice of Meeting (FR Doc 2026-07361), April 16, 2026.
- Electronic Code of Federal Regulations. 21 CFR 216.23.
- Ho EN, et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin β4, in equine urine and plasma by liquid chromatography-mass spectrometry. Journal of Chromatography A, 2012.
- Rahaman KA, et al. Simultaneous quantification of TB-500 and its metabolites in in-vitro experiments and rats. Journal of Chromatography B, 2024.
- Ruff D, et al. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers. Annals of the New York Academy of Sciences, 2010.
- Wang X, et al. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin β4 in healthy Chinese volunteers. Journal of Cellular and Molecular Medicine, 2021.
- Sosne G, Dunn SP, Kim C. Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial. Cornea, 2015.
- Sosne G, Ousler GW. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial. Clinical Ophthalmology, 2015.
- Guarnera G, et al. The effect of thymosin treatment of venous ulcers. Annals of the New York Academy of Sciences, 2010.
- Morita T, Hayashi K. Tumor Progression Is Mediated by Thymosin-β4 through a TGFβ/MRTF Signaling Axis. Molecular Cancer Research, 2018.
- ClinicalTrials.gov. NCT02974907 (ARISE-2), NCT03937882 (ARISE-3) and NCT00311766 (epidermolysis bullosa).
- World Anti-Doping Agency. The 2026 Prohibited List.
- US Anti-Doping Agency. Triathlon Athlete Anthony McCauley Accepts Sanction for Anti-Doping Rule Violations.