TB-500 and Thymosin Beta-4, Tested in Skin and Eyes but Not Yet in Tendons

When a patient asks me about TB-500 for a torn hamstring or a shoulder that won't settle down, I start with a question of my own, which molecule do you mean? The name covers two different peptides, and nearly all of the research quoted in its favor was done on only one of them. The human trials, meanwhile, involved leg ulcers and damaged corneas, and no one has run one in the injuries that send people looking for it.

What thymosin beta-4 does in a cell

Thymosin beta-4 is a small protein of 43 amino acids [1], abundant inside cells and in body fluids, and platelets carry large amounts of it to the site of a wound [2]. Inside the cell its main job is to bind actin, the structural protein a cell assembles into filaments whenever it needs to change shape or crawl. By holding a reserve of unassembled actin, thymosin beta-4 lets a cell rebuild its internal skeleton quickly, which a skin cell or a vessel-lining cell has to do before it can migrate into a wound and close it.

TB-500 reached the public through the stables. In 2012, chemists at the Hong Kong Jockey Club's racing laboratory reported that the key ingredient of a veterinary preparation sold under that name wasn't the full protein. It was a seven-amino-acid fragment, written LKKTETQ, with an acetyl group attached to one end, and they built a test to catch it in the urine and plasma of racehorses [3]. That fragment wasn't an arbitrary choice, since it reproduces amino acids 17 to 23 of thymosin beta-4, the stretch that binds actin. When Philp and colleagues at the National Institutes of Health tested the same seven amino acids in 2003, the fragment healed skin wounds in aged mice about as well as the parent molecule did [4].

Today a vial labeled TB-500 may hold the full 43-amino-acid protein or the short fragment, depending on the supplier. Nearly everything I describe below was done with full-length thymosin beta-4, so applying it to a fragment product requires one more assumption. Neither form is FDA-approved, and no approved drug is built on thymosin beta-4.

What it does in injured animals

Thymosin beta-4 closes skin wounds faster in rodents. Malinda and colleagues, in a 1999 study in the Journal of Investigative Dermatology, made full-thickness skin wounds in rats and gave the protein topically or by injection into the abdomen. Re-epithelialization, the regrowth of skin across the wound, ran 42 percent ahead of saline controls at four days and as much as 61 percent ahead at seven days, and the treated wounds held more collagen and more new blood vessels. In a separate assay the protein made keratinocytes, the main cells of the outer skin, migrate two to three times faster [5]. The protein also promoted corneal repair in rats [4].

The heart experiments drew the most attention. In 2004 in Nature, Bock-Marquette and colleagues tied off a coronary artery in mice to produce a heart attack and found that thymosin beta-4 switched on a survival enzyme called Akt in the heart, kept more heart muscle cells alive early after the injury, and improved cardiac function afterward [6].

For the injuries patients bring to me, the animal evidence is thinner. Xu and colleagues, in a 2013 study in Regulatory Peptides, cut the medial collateral ligament of the rat knee and placed fibrin sealant carrying 1 microgram of thymosin beta-4 into the gap. At four weeks the treated ligaments showed evenly spaced collagen bundles, thicker collagen fibrils, and better mechanical strength than the controls [7]. That result is encouraging, but it's one rat study with the protein placed surgically inside the injury, which isn't how anyone uses TB-500.

The human trials that exist

Thymosin beta-4 has gone through controlled human trials, which sets it apart from BPC-157 and most other peptides patients ask me about.

In the phase 1 safety trial, Ruff and colleagues gave four groups of 10 healthy volunteers a single intravenous dose of 42, 140, 420, or 1,260 mg or a placebo, then repeated the same dose daily for 14 days. Adverse events were infrequent and mild or moderate, and the investigators recorded no serious adverse events and no dose-limiting toxicity [8]. This means healthy people tolerated the protein for two weeks, which says nothing about benefit or about longer use.

The skin trials applied the protein topically. In a double-blind, placebo-controlled phase 2 trial at eight European sites, Guarnera and colleagues randomized 73 patients with venous stasis ulcers, the chronic leg wounds caused by failing veins. Safety matched placebo, and the authors concluded that the 0.03 percent dose "may have the potential" to speed healing, with complete closure within three months in about a quarter of patients, mostly those with smaller wounds [1]. Phase 2 trials in pressure ulcers and in epidermolysis bullosa, an inherited blistering disease, reach PubMed only as summaries in review articles written by researchers involved in developing the drug, which report faster repair [2]. I can't find either one published as a full trial report, and I can't find a phase 3 trial for any skin condition.

The eye trials are the most advanced. RGN-259, an eye drop containing 0.1 percent thymosin beta-4, improved both discomfort and corneal staining compared with vehicle drops in a randomized phase 2 trial of nine patients with severe dry eye [9]. In a phase 3 trial of 18 patients with neurotrophic keratopathy, a corneal defect that won't heal because the cornea has lost its nerve supply, the defect closed completely within four weeks in 6 of 10 treated patients and 1 of 8 on placebo, a difference that fell short of statistical significance at p = 0.0656 [10]. These are small trials with a favorable direction, and neither settles the question.

What nobody has tested

No human trial has tested thymosin beta-4 or TB-500 for a tendon, ligament, or muscle injury. Mayfield and colleagues, reviewing injectable peptides in the American Journal of Sports Medicine in 2026, reached the same conclusion, that both forms promote repair in preclinical models while human orthopaedic data are lacking [11]. Every human trial above put the protein directly on a wound or an eye, or infused it into a vein. None injected it under the skin and measured whether a distant tendon healed faster.

I think the extrapolation from skin to tendon is reasonable biology and unproven medicine. Skin has a rich blood supply and closes a wound within weeks, while a tendon has few cells and few vessels and remodels over months, as I described in our post on fluoroquinolone tendon damage. A protein that helps cells migrate could plausibly help there too, though plausibility has misled medicine often enough that I hold this one loosely.

Who shouldn't use it

Cell migration and new vessel growth serve a healing wound, and they serve a tumor as well. Cha and colleagues, in a 2003 study in the Journal of the National Cancer Institute, engineered mouse melanoma cells to overproduce thymosin beta-4 and injected them into mice. The tumors grew larger (21.7 mm against 13.3 mm), seeded more lung metastases (a mean of 46.7 nodules against 10.9), and contained 4.4 times as many blood vessels [12]. Forcing a cancer cell to make the protein differs from injecting it into a healthy person, and no human study has linked treatment to cancer. Still, I wouldn't give it to anyone with an active or recently treated cancer, and I'd avoid it in pregnancy, where we know nothing.

Competitive athletes should stay away from it, since thymosin beta-4 and TB-500 are banned substances in sport [11] and racing laboratories can detect the fragment at hundredths of a nanogram per milliliter [3].

How we use it

We prescribe compounded TB-500 as an experimental adjunct, for a defined soft tissue injury and a limited course, to patients who understand that the human evidence comes from ulcers and corneas. For a chronic wound, the measures that carry the evidence are compression for venous ulcers, pressure relief, debridement, infection control, and good blood sugar control. For a muscle or tendon, the measure that carries the evidence is a progressive loading program, supported by enough protein and enough sleep, and I'd want those in place before adding a peptide whose benefit in that setting nobody has measured.

Ask a provider about TB-500

References

1. Guarnera G, DeRosa A, Camerini R, et al. The effect of thymosin treatment of venous ulcers. Ann N Y Acad Sci. 2010. PMID: 20536470
2. Kleinman HK, Sosne G. Thymosin β4 Promotes Dermal Healing. Vitam Horm. 2016. PMID: 27450738
3. Ho EN, Kwok WH, Lau MY, 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. J Chromatogr A. 2012. PMID: 23084823
4. Philp D, Badamchian M, Scheremeta B, et al. Thymosin beta 4 and a synthetic peptide containing its actin-binding domain promote dermal wound repair in db/db diabetic mice and in aged mice. Wound Repair Regen. 2003. PMID: 12581423
5. Malinda KM, Sidhu GS, Mani H, et al. Thymosin beta4 accelerates wound healing. J Invest Dermatol. 1999. PMID: 10469335
6. Bock-Marquette I, Saxena A, White MD, et al. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature. 2004. PMID: 15565145
7. Xu B, Yang M, Li Z, et al. Thymosin β4 enhances the healing of medial collateral ligament injury in rat. Regul Pept. 2013. PMID: 23523891
8. Ruff D, Crockford D, Girardi G, et al. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers. Ann N Y Acad Sci. 2010. PMID: 20536472
9. 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. PMID: 25826322
10. Sosne G, Kleinman HK, Springs C, et al. 0.1% RGN-259 (Thymosin ß4) Ophthalmic Solution Promotes Healing and Improves Comfort in Neurotrophic Keratopathy Patients in a Randomized, Placebo-Controlled, Double-Masked Phase III Clinical Trial. Int J Mol Sci. 2022. PMID: 36613994
11. Mayfield CK, Bolia IK, Feingold CL, et al. Injectable Peptide Therapy: A Primer for Orthopaedic and Sports Medicine Physicians. Am J Sports Med. 2026. PMID: 41476424
12. Cha HJ, Jeong MJ, Kleinman HK. Role of thymosin beta4 in tumor metastasis and angiogenesis. J Natl Cancer Inst. 2003. PMID: 14625258