What isTB-500FDA Status, Evidence and Risks (2026)

The short answer

TB-500's side effects in people are unknown. FDA found no studies in which TB-500 was given to humans ⁴. FDA's concerns center on immune reactions, impurities and a tumor-progression question raised at its 2026 panel ^4,5.

What TB-500 is

FDA describes TB-500 as "a seven amino acid synthetic fragment of thymosin β4" with an acetyl group on one end. Its sequence is N-acetyl-LKKTETQ ⁴. On FDA's lists it appears as thymosin beta-4 fragment (LKKTETQ) ¹.

Thymosin beta-4 is the full protein: 43 amino acids, and the most abundant of its family in the body ⁶. FDA notes the short LKKTET stretch is the part of thymosin beta-4 "thought to contribute to the wound-healing properties" of the full protein ⁴. That is why it was studied. It does not show the fragment works alone.

The nomination asked for TB-500 as a 3 mg/mL powder for injection under the skin or into muscle, for "wound healing." It did not say what kind of wound ⁴.

Where FDA stands

FDA's stated reasons for leaving it off: TB-500 is poorly characterized, how much it is compounded is unknown, and there is not enough human data on safety or effectiveness ⁴. Committee members who voted no called the data gap "particularly concerning" for a peptide meant to be injected ⁵.

The vote is advice. FDA is not bound by it, and adding a substance to the 503A list takes notice-and-comment rulemaking ⁵. As of FDA's list dated May 14, 2026, TB-500 is not on it ².

What the HHS policy does. HHS says it does not intend to take enforcement action against a state-licensed pharmacy, a federal facility or a licensed physician that compounds TB-500 acetate, given under the skin or into muscle, when every condition in the policy is met ¹². Conditions include one active ingredient per product, a bulk ingredient from an FDA-registered maker, reporting serious side effects to FDA, and a label that says "This is a compounded drug product. Not for resale" ¹³. The policy is interim and not binding. It lasts while the 503A list goes through rulemaking, and HHS can revisit it if a safety or quality problem shows up ¹². It approves nothing and puts nothing on the 503A list ¹². Read the full breakdown in HHS's Oct 28 peptide policy. The FDA Peptide Tracker logs each step. The gap between "off Category 2" and legal is covered in Are Peptides Legal?.

What the research shows

TB-500 itself: lab and metabolism work only. FDA found no human studies and no published animal wound studies of TB-500 ⁴. In one lab study, TB-500 "did not promote 'wound' closure" in cultured fibroblasts, a type of connective tissue cell ⁴. Studies in horses and rats show it reaches the bloodstream and breaks down into shorter pieces ⁴. A 2026 scoping review of thymosin beta-4 and TB-500 found just one study that tested TB-500 directly, and it was not in people ⁶.

Thymosin beta-4: small human trials, different molecule. The same review lists human trials of full-length thymosin beta-4, mostly as eye drops for dry eye and a cornea condition, plus venous leg ulcers and safety studies in healthy volunteers ⁶. The authors call the human evidence for muscle and joint repair "limited" ⁶. Results for thymosin beta-4 do not transfer to a 7-amino-acid fragment.

Side effects and FDA's concerns

There is no human data to list side effects from. FDA and its panel flagged these ^4,5:

• No safety studies. No toxicity studies in animals were submitted or found ⁴.
• Immune reactions. Injected peptides "may pose a significant risk for immunogenicity," FDA said, and clumping (aggregation) and impurities can raise that risk. No human data on this was provided ⁴.
• Poor characterization. FDA could not pin down its physical and chemical profile well ⁴.
• A tumor-progression question. FDA reviewers and some committee members raised concerns about TB-500's "potential relationship to tumor progression." FDA also noted no studies had been done on TB-500 itself ⁵. It was raised as a concern. No study has tested it in people.
• Banned in sport. WADA prohibits "Thymosin-β4 and its derivatives e.g. TB-500" under S2.3, growth factors ¹⁰.

Why it's sold online anyway

TB-500 is widely sold online with "research use only" labels ¹¹. That label is the seller saying the product is not for people. No prescriber reviews health history. No licensed pharmacy makes it for one patient or checks it for human use. The name on the vial is the seller's word, and FDA's impurity concern applies to whatever is inside.

Compare it with its shelf neighbors: BPC-157 and LL-37, or see BPC-157 vs TB-500.

Asked a lot.

The sources.

1. FDA. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks (content current as of Apr 22, 2026). Read it
2. FDA. Bulk Drug Substances Used in Compounding Under Section 503A of the FD&C Act (content current as of May 14, 2026). Read it
3. FDA. July 23 to 24, 2026 Meeting of the Pharmacy Compounding Advisory Committee. Read it
4. FDA. Presentation to the Pharmacy Compounding Advisory Committee, including TB-500 (Jul 23, 2026). Read it
5. Bass Berry. FDA Advisory Committee Recommends Six Peptides for Compounding (Jul 31, 2026). Read it
6. McGuire F, Hughes E, Maak T, Cushman DM. Thymosin Beta-4 and TB-500 in Tissue Healing, Regeneration, and Musculoskeletal Repair: A Scoping Review. Appl Sci. 2026;16(12):6202. Preprint: Read it
7. Frier Levitt. FDA Peptides "Do Not Compound" List Update (Apr 16, 2026). Read it
8. Holland & Knight. Peptides at a Crossroads: What Wellness Providers Need to Know (Oct 16, 2026). Read it
9. AP via ABC News. Trump administration advances plan for peptide drugs (Oct 15, 2026). Read it
10. World Anti-Doping Agency. The 2026 Prohibited List. Read it
11. AJMC. FDA Panel Backs 6 Peptides for Compounding (Jul 31, 2026). Read it
12. HHS. Enforcement Discretion Policy Regarding 503A Compounding Using Certain Peptide Bulk Drug Substances. Docket HHS-OASH-2026-0496, FR Doc 2026-22192 (filed for public inspection Oct 28, 2026; scheduled to publish in the Federal Register Oct 30, 2026). Read it
13. Hyman, Phelps and McNamara, FDA Law Blog. The PEPTIDE-L Wave Reaches HHS: Enforcement Discretion … Not the Finish Line (Oct 28, 2026). Read it