Thymosin beta-4: evidence, FDA status and research

It is sold as an injection to heal tendon, muscle and joint injuries. No one has tested that use in people; the small human trials are on eye drops, the heart and safety.

Sold for

Thymosin beta-4 is sold as a regenerative injection for tendon, muscle and joint injuries and promoted as the full-length version of TB-500.

Studies say

Small randomized trials show safety and signals of benefit in eye and heart disease, but the injected use for sports injuries has never been tested in people, which is why it grades B for the studied indications and no better.

Why grade B. Thymosin beta-4 has two phase 1 safety trials in healthy volunteers (40 and 84 subjects), an 18-patient phase 3 eye-drop trial, a 96-patient randomized trial after heart attack, and a venous ulcer study. All are small and most tested topical or intravenous full-length protein for specific diseases, not the injected wound-healing use it is sold for. Animal work in rats and mice supports wound and cardiac effects. Grade B for the clinical indications under study; the injury-recovery use sold online has no trial. ⁴⁷¹⁰¹¹¹²

FDA. Thymosin beta-4 is not approved by the FDA for any use. Injectable products sold online, often labeled TB-500 (which the FDA reviewed as a seven-amino-acid fragment), are research chemicals; the FDA found no human exposure data for TB-500, and the agency has not acted on its compounding advisory committee's July 2026 vote to recommend adding TB-500 to the 503A bulks list. RGN-259 eye drops are in a phase 3 trial for neurotrophic keratopathy, and a recombinant intravenous form has been tested in heart attack patients.

Written from 14 cited sources. Last refreshed 2026-10-27. Not physician reviewed.

What this means for you

What people take it for: Healing injuries to tendons, muscles and joints
Can a US doctor prescribe it?: No. It is not approved and is sold as a research chemical.
Properly tested in people?: Partly: small trials of up to 96 people, none on the injury use it is sold for.

Evidence checklist

Randomised controlled trial Small randomized trials in eye, heartyes
Human study over 100 people Largest trial 96; phase 1 had 84yes
More than one research group Trials from several groups and sponsorsyes
Clinical outcomes, not surrogate markers Eye-drop missed primary endpoint; heart pendingno
Human safety data beyond case reports Volunteer trials only; no data on injection useno

Drawn from the cited studies below. A dash means the sources give no information.

What it is

Thymosin beta-4 is a protein with 43 amino acids. The TMSB4X gene on the X chromosome codes for it. It is one of the most abundant proteins inside human cells. This is especially true of blood platelets and white cells. Scientists first purified it from calf thymus extract in the 1960s. That gave it a misleading name. It is not a thymus hormone. ⁵¹⁰

RegeneRx is developing the protein in two forms. One is RGN-259 eye drops for dry eye and neurotrophic keratopathy. The other is an intravenous drug for heart attack. A Chinese recombinant version (NL005) has finished phase 1 and a randomized trial in heart attack patients. None of these is approved. The gray-market injectable sold to consumers is unrelated to these programs. ⁴⁷¹¹

How it works

Thymosin beta-4 binds single actin molecules and holds them in reserve. This lets cells quickly build and tear down the inner scaffolding they need to move. Platelets release it at a wound. There it draws in skin cells and blood-vessel cells. It promotes new vessel growth and collagen deposits. It also dampens inflammation. In heart muscle, it activates the ErbB2 survival pathway. This reduces cell death after a blocked artery is reopened. The actin-binding region is amino acids 17 to 23. That fragment is sold as TB-500. ⁶⁷¹²

What human research shows

Two randomized phase 1 trials tested healthy volunteers. One gave 40 Americans a single dose and a 14-day course into a vein. The other gave 84 Chinese volunteers a single dose and a 10-day course. Both found only mild to moderate side effects. They found no dose-limiting toxicity and no drug buildup. ¹⁰¹¹

The phase 3 trial of RGN-259 eye drops in neurotrophic keratopathy randomized 18 patients. After four weeks, 6 of 10 treated patients had fully healed. So had 1 of 8 on placebo. That missed statistical significance on the main measure. But discomfort and dryness scores improved significantly. A larger phase 3 trial is registered. ⁴⁹

A 2025 randomized, placebo-controlled trial enrolled 96 patients. All had an ST-elevation heart attack treated with stenting. Overall, recombinant thymosin beta-4 did not significantly shrink the infarct size at 90 days. But it did shrink it in the 43 patients who got the first dose within eight hours of the procedure. The authors called for larger trials. A European venous ulcer study was designed for 72 patients. The published report covers only design and early safety. ²⁷

Study	Model	Finding
RGN-259 neurotrophic keratopathy phase 3	Humans, n=18, randomized placebo-controlled double-masked	Complete healing in 6 of 10 treated versus 1 of 8 placebo eyes at 4 weeks, not statistically significant; comfort scores improved. ⁴
Recombinant thymosin beta-4 after STEMI (2025)	Humans, n=96, randomized placebo-controlled, plus mouse ischemia-reperfusion models	No overall difference in infarct size at 90 days; smaller infarcts in the subgroup dosed within 8 hours of stenting. ⁷
US phase 1 (Ruff 2010)	Humans, n=40 healthy volunteers, randomized placebo-controlled, intravenous single and 14-day dosing	No dose-limiting toxicity or serious adverse events across the dose range. ¹⁰
Chinese phase 1 (2021)	Humans, n=84 healthy volunteers, randomized placebo-controlled	Mild to moderate adverse events only, no accumulation after 10 days of dosing. ¹¹
Rat wound model (Malinda 1999)	Rats, full-thickness skin wounds	Topical or injected thymosin beta-4 increased re-epithelialization by 42 percent at day 4 and up to 61 percent at day 7 versus saline. ¹²

Safety

The full-length protein was well tolerated in the phase 1 trials and the eye-drop and heart trials. Side effects were similar to placebo. Those data cover use in a vein and on the eye under trial conditions. The courses lasted days to weeks. ⁴⁷¹⁰¹¹

No one knows what repeated injection under the skin does. That is how people actually use gray-market product, for injuries. The FDA's compounding assessment says it found no human exposure data for that use. Thymosin beta-4 promotes blood-vessel growth. No one has studied its effect on existing tumors in people. It is on the WADA prohibited list. ⁶⁸

Questions for your clinician

Has the injected form I am considering been tested in people?
Which injury treatments have real trial support for my condition?
What do we know about growth-promoting risks of long-term use?

Questions people ask

How does thymosin beta-4 differ from TB-500?

Thymosin beta-4 is the full protein with 43 amino acids. TB-500 is a lab-made copy of seven amino acids from the middle of it (positions 17 to 23). This is the segment that binds actin. Human trials exist only for the full protein. ⁵⁶

Has thymosin beta-4 won FDA clearance?

No. It is in clinical trials as eye drops and as an intravenous drug for heart attack. None has reached approval. Injectable product sold online is not approved. It has no human safety data. ³⁸

Does thymosin beta-4 heal wounds?

In rats, yes. A 1999 study found faster skin closure and more new blood vessels. In people, the evidence is an 18-patient trial on the cornea that showed a healing trend. There is also a venous ulcer study that has published its protocol but not final results. ²⁴¹²

What adverse events turned up in trials?

In trials of the full-length protein in a vein or on the eye, side effects were mild and similar to placebo. No one has studied long-term injection for injuries. The FDA notes there are no human exposure data for the compounded product. ⁸¹⁰¹¹

Is thymosin beta-4 banned in sport?

Yes. The World Anti-Doping Agency and horse-racing authorities prohibit it. Labs have published methods to detect both the protein and the TB-500 fragment. ⁶

Citations

Goldstein 2012, Expert Opin Biol Ther: Thymosin beta4, a multifunctional regenerative peptide doi.org
Guarnera 2007, Ann N Y Acad Sci: Thymosin beta-4 and venous ulcers, European randomized study doi.org
ClinicalTrials.gov NCT03937882: RGN-259 phase 3 in dry eye clinicaltrials.gov
Sosne 2022, Int J Mol Sci: RGN-259 in neurotrophic keratopathy, phase 3 pubmed.ncbi.nlm.nih.gov
2026, Peptides: Thymosin beta 4, an emerging therapeutic candidate for kidney diseases pubmed.ncbi.nlm.nih.gov
Ho 2012, J Chromatogr A: Doping control analysis of TB-500 in equine urine and plasma pubmed.ncbi.nlm.nih.gov
2025, Cardiovasc Res: Recombinant human thymosin beta 4 in mice and STEMI patients after reperfusion pubmed.ncbi.nlm.nih.gov
FDA: Certain bulk drug substances for use in compounding may present significant safety risks fda.gov
ClinicalTrials.gov NCT05555589: RGN-259 phase 3 in neurotrophic keratopathy clinicaltrials.gov
Ruff 2010, Ann N Y Acad Sci: Randomized placebo-controlled single and multiple dose study of intravenous thymosin beta4 in healthy volunteers pubmed.ncbi.nlm.nih.gov
2021, J Cell Mol Med: First-in-human phase 1 study of recombinant thymosin beta4 in healthy Chinese volunteers pubmed.ncbi.nlm.nih.gov
Malinda 1999, J Invest Dermatol: Thymosin beta4 accelerates wound healing pubmed.ncbi.nlm.nih.gov
STAT 2026: FDA panel to discuss broader access to peptides via compounding statnews.com
STAT 2026: FDA peptide compounding panel backs epitalon, rejects emideltide statnews.com