TB-500 legal status: a meeting is not approval
A safety warning, a withdrawn nomination and an advisory agenda describe different things. The dates do not add up to an approved medicine.
The dates do not write an approval
TB-500 is not FDA-approved for any human therapeutic indication or as a finished drug. I get impatient with accounts that let an administrative change do the work of a clinical or regulatory decision.
On September 29, 2023, FDA placed thymosin beta-4 fragment (LKKTETQ), also known as TB-500, in 503A Category 2. The FDA safety-risk page cites potential immunogenicity for certain routes and a lack of important safety information.
On FDA’s page current as of April 22, 2026, TB-500 is no longer in Category 2; it appears in the nominated-but-withdrawn table after the nomination was withdrawn. FDA still prints the safety concerns. The agency publishes no date on which it moved TB-500 out of Category 2, and has not explained the implications for routine pharmacy compounding.
The July 23–24, 2026 Pharmacy Compounding Advisory Committee meeting listed TB-500 (free base) and TB-500 acetate for consideration for the 503A Bulks List. FDA staff had proposed against adding them. The meeting took place, but FDA has not posted the advisers’ votes in the record used here. The committee discussion leaves FDA’s final decision unresolved in this record.
A withdrawn nomination does not fill the safety gap
FDA’s Category 2 designation identified substances that may present significant safety risks. It did not carry the enforcement discretion associated with Category 1. On January 7, 2025, FDA stopped placing new nominations into those categories. The 503A bulk-substance framework explains the categories; the analysis of the retired interim policies explains why a renewed nomination did not restore Category 2 enforcement discretion.
I take the withdrawn listing literally: it describes a nomination and leaves the peptide’s effects in a human body unaddressed. It supplies no new safety findings and no approval. The distinction between historical Category 2 placement and the later withdrawn listing matters even when the practical compounding implications remain unexplained.
The historical Category 2 designation and the withdrawn nomination describe different stages of the FDA record. Neither establishes drug approval or a favorable clinical safety result.
A prescription and an eligible ingredient are separate requirements
Section 503A generally concerns patient-specific compounding after a valid prescription; Section 503B concerns registered outsourcing facilities with a different framework. The FDA compounding questions and answers describes those routes, while the bulk-substance rules determine which ingredients can be used.
A prescription alone cannot make an ineligible bulk ingredient eligible. Nor does the way a consultation takes place change the ingredient requirements. FDA has not clarified routine TB-500 compounding access after the withdrawn nomination in the record described above.
I treat the access question as unresolved where the official record leaves it unresolved. The applicable ingredient rule still governs when a request, a meeting or a prescription exists. This is general regulatory information, not legal advice about a particular case.
Sport has its own prohibition
TB-500 and thymosin beta-4 are prohibited by the World Anti-Doping Agency in and out of competition for the relevant classes. The sports-medicine review of peptide therapies supplies the athletic context; the equine mass-spectrometry paper describes detection of TB-500 and its metabolites and leaves human recovery untested.
The sport ban is separate from FDA drug approval and pharmacy ingredient eligibility. Australia and New Zealand are among the jurisdictions that require a prescription. That jurisdictional fact does not establish U.S. approval or permission in drug-tested sport.
The thymosin beta-4 and FDA references bring these records together. They describe distinct rules, and none can silently stand in for another.