Is thymosin beta-4 FDA-approved, and what does the research show?

What is thymosin beta-4?

Thymosin beta-4 is a small peptide of 43 amino acids that the body makes itself ⁶, ². It is present in most cell types and in blood, saliva, tears and wound fluid ⁶. After an injury, platelets, macrophages and many other cells release it ¹. Researchers call it a regenerative peptide because it takes part in the repair of injured tissue ¹.

The name TB-500 belongs to a different molecule ⁶. TB-500 is a synthetic fragment of seven of the protein's 43 amino acids, positions 17 to 23, with an acetyl group, a small chemical cap, at one end ⁶. FDA writes that "thymosin beta-4 and TB-500 are not the same substance," although sellers often use the two names interchangeably ⁶.

Is thymosin beta-4 FDA-approved?

Thymosin beta-4 is not FDA-approved: Drugs@FDA lists no thymosin beta-4 or TB-500 product as of November 9, 2026 ³. FDA's reviewers wrote in May 2026 that TB-500 has no United States Pharmacopeia monograph, the official quality standard, and is not a component of any FDA-approved drug ⁶. The full-length peptide is an investigational drug, tested in people as an eye drop and a gel ⁷, ⁸, ².

TB-500 is one of the six peptides in Appendix A of HHS's enforcement-discretion policy, published October 30, 2026 ⁴, ⁵. The policy concerns compounding, a licensed pharmacy or physician making a medicine for one patient on a prescription, under section 503A of federal law ⁵. It is a draft, interim enforcement-discretion policy, not an approval ⁴. Enforcement discretion means HHS says it does not intend to act ⁵.

The policy describes itself as "allowing limited compounding of the substances addressed in this guidance under appropriate safeguards" to give patients "a more controlled alternative" ⁵. In HHS's words, it "does not intend to take enforcement action against a State-licensed pharmacy, Federal facility, or licensed physician compounding a drug product using a bulk drug substance in Appendix A," when all ten of its conditions are met ⁵.

For TB-500 the policy covers TB-500 acetate only, a salt form of the fragment, "for subcutaneous or intramuscular injectable dosage forms" ⁵. The ingredient must come with a certificate of analysis, a supplier's test report, that meets the policy's conditions ⁵. The notice adds that the policy "does not establish any rights for any person and is not binding on HHS or the public" ⁴. Appendix A names TB-500, not the full-length peptide ⁵.

From September 29, 2023, until April 22, 2026, TB-500 sat in Category 2 of FDA's list of substances nominated for compounding ⁵. Category 2 is FDA's heading for "Bulk Drug Substances that Raise Significant Safety Concerns" ¹⁰. HHS records that "neither the placement nor the removal changed its status under section 503A" ⁵. Neither name appears in any category of the list as updated May 14, 2026 ¹⁰.

FDA's reviewers proposed not adding TB-500 to the compounding list, and FDA's advisory committee on compounding voted in favor of both forms in July 2026; the policy followed ⁶, ⁵. In 2023 FDA told a seller of "TB-500" vials that its products were unapproved new drugs ¹¹. TB-500 itself has its own guide.

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How does thymosin beta-4 work?

The mechanism is proposed in a review published online in November 2011 ¹. Thymosin beta-4 binds to actin, a protein of the cell's internal skeleton, and promotes cell migration ¹. That includes the movement of stem and progenitor cells, the body's repair cells, which form new blood vessels and regenerate tissue ¹. It also reduces the number of myofibroblasts, cells involved in scarring, which the review links to less scar formation and fibrosis, the build-up of scar tissue ¹.

The seven amino acids copied in TB-500 are the stretch its nominator described as the protein's actin-binding site ⁶. Whether the fragment acts like the whole protein has not been shown in people ⁶, ¹². FDA found one study in cells in which TB-500 "did not induce wound healing" in scratched cultures of fibroblasts, the cells that build connective tissue ⁶. One of its breakdown products appeared to ⁶.

What does the research show?

The most studied form is RGN-259, a preservative-free eye drop containing thymosin beta-4 ⁷. Its largest trial is ARISE-3, a phase 3 trial, the late stage, usually in hundreds of patients ⁷. It was randomized, so chance assigned 699 adults with a reported history of dry eye to RGN-259 or a placebo drop without it ⁷. It was double-masked, so neither they nor the investigators knew which ⁷. Drops went in four times a day for 14 days, and the results were posted in August 2026 ⁷.

On both primary measures, the outcomes chosen in advance to judge the trial, the groups were close ⁷. Inferior corneal staining, a dye score of damage to the lower cornea, was measured on day 15 across a controlled dry-air challenge ⁷. It fell by 0.41 points on RGN-259 and by 0.46 on placebo ⁷. Ocular discomfort fell by 0.4 points in each group ⁷.

ARISE-2, a phase 3 trial of 601 adults treated for 28 days, posted no advantage for RGN-259 over placebo ¹³. Its two primary measures were ocular discomfort and fluorescein staining, a corneal dye score on the same scale measured in a subgroup ¹³.

Two phase 2 trials, the smaller mid-stage trials of effect in patients, came before them ¹⁴, ¹⁵. In 9 patients with severe dry eye at two United States sites, RGN-259 was compared with its vehicle, the same drop without the active ingredient ¹⁴. At day 56 the RGN-259 group had 35.1 percent less ocular discomfort and 59.1 percent less corneal staining than the vehicle group ¹⁴. In 72 adults with moderate to severe dry eye, neither primary endpoint differed from placebo, while some secondary measures did ¹⁵.

Studied in people: a randomized, double-masked, placebo-controlled phase 3 trial of 18 patients with stage 2 or 3 neurotrophic keratopathy ⁸, ¹⁶. Complete healing at four weeks occurred in 6 of 10 patients on RGN-259 and 1 of 8 on placebo ⁸. The difference fell short of the usual threshold for statistical significance (p = 0.0656), and the authors call it "a strong efficacy trend" ⁸. The sponsor ended that trial early, recording a "business decision," and a second trial of 70 patients completed in July 2026 with no results posted ¹⁶, ¹⁷. A Cochrane review, an independent systematic summary of trials, rated this in December 2025 as low-certainty evidence that RGN-259 may not increase the share of patients whose cornea heals ¹⁸.

Studied in people: two phase 2, randomized, placebo-controlled trials of a thymosin beta-4 gel; the venous-ulcer trial's design was described in a 2007 paper while it was still enrolling ¹⁹, ², ⁹. One enrolled 73 patients with venous ulcers in Italy and Poland (72 in the registry's safety count), the other 72 patients with pressure ulcers ², ⁹. The venous-ulcer authors wrote that their findings "suggest" the gel at one of its three strengths, 0.03 percent, "may have the potential to accelerate wound healing" ². They add that complete healing within three months came in about 25 percent of patients, a figure the abstract gives without naming the arm ². A 2012 review paper says the gel accelerated healing "by almost a month in those patients that did heal" across both trials ²⁰.

Studied in people for safety only: a phase 1 trial, the first stage in people, gave intravenous thymosin beta-4 or placebo to 40 healthy volunteers ²¹. They received single doses and then daily doses for 14 days ²¹. A phase 1 trial in China of a recombinant form, made in engineered cells, gave 54 and then 30 healthy volunteers single doses and then daily doses for 10 days ²². Beyond safety, two phase 2 trials in acute myocardial infarction, a heart attack, of 62 and 90 participants, measured change in the area of heart muscle damaged ²³, ²⁴. Both are listed as completed in China with no results posted ²³, ²⁴.

What has not been studied is the use people most often ask about. FDA's reviewers found no article "in which TB-500 was administered to humans," and a 2026 orthopedic review found thymosin beta-4's musculoskeletal evidence "limited to in vitro and rodent studies" ⁶, ¹². No registered trial of either molecule for such an injury was found on November 9, 2026.

What are the side effects?

ARISE-3's safety population of 699, which counts one person twice, recorded no serious adverse event ⁷. A non-serious adverse event above the record's 1 percent reporting threshold was recorded in 47 of 350 on RGN-259 and 31 of 349 on placebo ⁷. The most common was pain where the drop landed, in 23 and 16 ⁷. In ARISE-2, 3 of 299 on RGN-259 and 2 of 302 on placebo had a serious adverse event ¹³. In the 18-patient keratopathy trial one patient on RGN-259 had a serious adverse event, recorded as depression, and the authors report "no significant adverse effects" ⁸, ¹⁶.

The pressure-ulcer trial enrolled inpatients and outpatients with an ulcer through the full thickness of the skin ⁹. A serious adverse event was recorded in 13 of the 53 patients at risk on the gel and 5 of the 18 on placebo ⁹. In the venous-ulcer trial the counts were 3 of 55 and 1 of 17 ²⁵. The venous-ulcer authors judged the safety profile "acceptable and comparable to placebo" ². In healthy volunteers given it intravenously, adverse events were "infrequent, and mild or moderate in intensity," with no serious events in either phase 1 trial ²¹, ²².

FDA's own safety words are for the fragment, TB-500. Compounded drugs containing it "may pose risk for immunogenicity for certain routes of administration due to the potential for aggregation as well as peptide-related impurities" ²⁶. Immunogenicity is an immune reaction against the drug itself. FDA "has not identified any human exposure data" and "lacks important information regarding any safety issues raised by this drug, including whether it would cause harm if administered to humans" ²⁶. Its search of the adverse-event reporting system FAERS through March 26, 2025, retrieved no reports for TB-500 ⁶.

In sport

WADA, the World Anti-Doping Agency, names "Thymosin-β4 and its derivatives e.g. TB-500" in section S2.3 of its 2026 Prohibited List, growth factors and growth factor modulators ²⁷. Class S2 is prohibited at all times, in and out of competition ²⁷. FDA's reviewers record the same listing and a WADA project of 2013 to add TB-500's breakdown products to peptide screening ⁶.

What JumpstartMD offers

JumpstartMD does not prescribe thymosin beta-4. Our peptide library covers it so you have the facts on its FDA status and the evidence, current as of November 9, 2026.

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Compounded medications are not FDA-approved, and FDA does not review their safety, effectiveness, or quality before they are marketed. A licensed clinician decides after an evaluation whether a peptide is appropriate for you, and not every patient will qualify.