TB-500 Side Effects: What Is and Isn't Known About Serious Risks

What is on record in humans?

FDA's search of its food and supplement complaint system, covering January 1, 2004 to March 10, 2025, returned two cases about a blended TB-500 and BPC-157 product, with no safety assessment attached [1].

A separate case is described in FDA's review of BPC-157. A 40-year-old woman injected a 'research purposes only' product containing 5 mg BPC-157 and 5 mg TB-500 under the skin twice daily and within a week developed widespread skin hyperpigmentation and darkening of her gums, which came back when she used the product again two weeks later [2]. Because the vial held two peptides, FDA said it cannot tell whether either one caused the reaction [2].

Neither finding establishes a causal link to TB-500, and neither shows which serious reactions the peptide can or cannot cause.

The concern FDA does state: immune reactions to the injection

FDA's stated concern for injectable TB-500 is immunogenicity, an immune reaction to the peptide, which may be amplified by peptide aggregation and impurities [1]. FDA's safety-risk page for compounded bulk substances raises the same concern for injectable peptides [7].

How often that would happen with TB-500 is unknown. FDA found no study of TB-500 immunogenicity or aggregation, and no nonclinical toxicity studies of the peptide [1].

What the laboratory and animal work shows

FDA found no in-vivo animal study of TB-500 for wound healing [1]. In an in-vitro scratch-wound test, TB-500 at 50 microgram/mL did not induce wound closure in fibroblast cultures, while a shorter metabolite, N-acetylated LKKTE, did show activity [1].

Animal studies indicate that TB-500 given subcutaneously or intraperitoneally reaches the bloodstream and is broken down into smaller peptides; no animal studies by intramuscular injection were found [1]. FDA found no human pharmacokinetic data by any route [1].

Safety results from full-length thymosin beta-4 do not apply to TB-500

TB-500 is a synthetic, N-acetylated 7-amino-acid fragment (residues 17 to 23) of the 43-amino-acid protein thymosin beta-4, with a molecular weight of about 889 g/mol [1]. It is not the full protein, so trial results for the full protein describe a different molecule.

• A first-in-human study of recombinant human thymosin beta-4 assigned 54 healthy volunteers to single intravenous doses from 0.05 to 25 microgram/kg or placebo, and enrolled 30 volunteers in a 10-day multiple-dose part at 0.5, 2 or 5 microgram/kg. Adverse events were mild to moderate, with no dose-limiting toxicity and no serious adverse events [8].
• A phase 2 trial randomized 72 adults with moderate to severe dry eye to 0.1% thymosin beta-4 drops or placebo for 28 days. Neither primary endpoint differed significantly from placebo, and no adverse events were reported [9].
• A randomized, placebo-controlled trial in 96 people with ST-elevation myocardial infarction after angioplasty found no significant overall difference in infarct area at 90 days; a smaller infarct area appeared in the subgroup of 43 who received the first dose within 8 hours [10].

None of these results describes an injected fragment of the protein.

Regulatory status

TB-500 is not an FDA-approved drug. FDA's July 2026 review found no USP or National Formulary monograph for it, no FDA-approved drug containing it, and no approved product in Canada, Australia, the UK or the European countries it checked [1]. It sits in the 'nominated but withdrawn' table on FDA's page about bulk substances that may present significant safety risks; removal from category 2 was not a finding of safety [7].

FDA staff proposed in July 2026 that neither TB-500 free base nor TB-500 acetate be added to the pharmacy bulks list, citing poor characterization, unclear historical use, no evidence of effectiveness for the wound-healing use FDA evaluated, and immunogenicity risk for injection [1]. The Pharmacy Compounding Advisory Committee voted in favor of adding it on July 23, 2026, against staff's recommendation; that vote is advice only and not binding [11][12]. HHS's October 2026 draft policy names TB-500 among six peptides and says HHS does not intend to take enforcement action against state-licensed pharmacies and licensed physicians compounding with TB-500 acetate for subcutaneous or intramuscular injection if conditions are met; the notice states that it is not binding on HHS or the public and creates no rights [12][13]. Compounded drugs are not FDA-approved, and FDA does not review their safety, effectiveness or quality before marketing [12].