PCAC Briefing · Thursday, July 23, 2026
TB-500 before the compounding committee
FDA proposes not adding either TB-500 substance to the 503A Bulks List, citing a lack of physicochemical data, a lack of historical use in compounding, and no information on use in humans — plus a specific immunogenicity concern about the proposed injectable route.
Published July 16, 2026 · By the Grey Peptides Editorial Board · ✓ Sourced from FDA's briefing document
| Substances | TB-500 (free base) and TB-500 acetate |
| Also known as | Thymosin β4 fragment, Tβ4 (17-23) |
| Length | 7 amino acids (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH) |
| Use FDA reviewed | Wound healing |
| Forms proposed | Subcutaneous / intramuscular injection, 3 mg/mL, supplied as lyophilised powder |
| Nomination | Nomination withdrawn; FDA evaluated at its own discretion |
| FDA staff position | Proposes not adding either form to the 503A Bulks List |
What FDA reviewed it for
Nominated for wound healing.
This is not a detail to skip past. FDA evaluates a bulk drug substance in the context of the use proposed for it — so the question in front of the committee is not "is TB-500 useful?" but "does the evidence support compounding TB-500 for wound healing?" A substance can have an interesting research literature in a completely different area and still fail this test, because that literature was never the thing under review.
FDA's specific objections
FDA weighs four criteria: whether the substance is well characterised physically and chemically, whether it has been used historically in compounding, what evidence exists on effectiveness, and what safety issues its use in compounding raises. Here is where TB-500 landed on each.
FDA says TB-500 (free base) is not physically and chemically well characterised, citing naming that does not follow INN, USAN or IUPAC standards and missing critical characterisation data.
Unlike most of the seven, FDA found a lack of historical use in compounding for TB-500 specifically.
There was a lack of any evidence of effectiveness for either form.
FDA describes the information on human use as non-existent, and adds that peptides given by the proposed injectable routes may pose a significant risk for immunogenicity.
What is specific to TB-500
Six of the seven objections above could be written about almost any of the compounds on the July agenda. These next points could not — they are what makes this file different from the other six.
TB-500 is on the World Anti-Doping Agency's prohibited list under section S2.3. WADA approved a project in 2013 to determine detection limits for TB-500 metabolites and build them into peptide screening methods. Anyone subject to anti-doping testing should treat any change in compounding status as irrelevant to their situation — the two regimes are unrelated.
TB-500 is a short fragment of thymosin β4, not thymosin β4 itself. Full-length thymosin β4 has been studied in FDA-registered trials; that record does not transfer to the fragment, and FDA's evaluation does not treat it as if it does.
FDA describes websites selling TB-500 in 2 mg to 15 mg vials stating the products are for research use only, and at least one providing reconstitution and administration instructions. It also cites published analysis of online forum discussion in which TB-500 was among the most frequently mentioned substances, trending upward after 2010.
The evidence picture
Grey Peptides grades TB-500's evidence base as low. Human data is effectively absent. The frequent conflation of TB-500 with full-length thymosin β4 is the single most common factual error made about this compound.
Two readings of the same file are worth separating. FDA's conclusion is about a regulatory question — whether the record supports letting pharmacists compound this substance for this use. That is a higher bar than "there is some interesting science here," and a lower bar than "this substance is harmful." Almost everything FDA said about TB-500 is a statement about missing information, not a finding of harm. Reporting that as "FDA says TB-500 is dangerous" is wrong. So is reporting it as "FDA found no safety problems."
Where this sits in our encyclopedia
Our entry on this compound covers the underlying pharmacology, the published literature, and the status picture independent of this meeting. The two are separate reads: the encyclopedia entry is about the compound, and this page is about the regulatory file.
Mechanism, evidence grade, half-life, approval status, and sources. PCAC July 2026 — the full picture
All seven compounds, what the vote does and doesn't change, and what to watch. Peptide Regulatory Status Tracker
Category 1/2/3, 503A vs 503B, and where every peptide sits.
The other six compounds
BPC-157 · KPV · MOTS-c · Emideltide (DSIP) · Semax · Epitalon
FDA — July 23–24, 2026 Meeting of the Pharmacy Compounding Advisory Committee ↗
FDA briefing documents released June 29, 2026. Federal Register notice 91 FR 20465, published April 16, 2026 (FR Doc. 2026-07361), Docket No. FDA-2025-N-6895. FDA Briefing Document for TB-500 (free base) and TB-500 acetate, released June 29, 2026.