TB-500 legal status: read the actual entry

Withdrawn is the word on FDA’s page

FDA identifies the substance as Thymosin beta-4, fragment (LKKTETQ), also known as TB-500. It previously placed that substance in 503A Category 2, the group for bulk substances that may present significant safety risks. Its rationale included potential immunogenicity for certain administration routes and missing important safety information.

On FDA's page current as of April 22, 2026, TB-500 appears under “Bulk drug substances nominated but withdrawn.” It is no longer in Category 2. FDA publishes no date for that change and gives no instruction there explaining how a 503A pharmacy is to treat the withdrawal. I cannot squeeze a policy date out of an undated entry.

TB-500 is not FDA-approved and has no approved human therapeutic indication. Removing a nomination from one category does not confer drug approval. The 503A bulk-substance framework and the safety-risk entry concern compounding; approval of a finished medicine is a separate matter. This is general regulatory information, not medical or legal advice.

The July agenda did not decide the question

The published July 23–24, 2026 PCAC meeting agenda individually named TB-500 (free base) and TB-500 acetate as substances being considered for inclusion on the 503A Bulks List. The meeting was a discussion in an evaluation process. An agenda is neither a listing decision nor a reclassification.

FDA staff had proposed against adding TB-500. As of September 2026, FDA had published no record of the committee's votes. A final listing decision comes through FDA rulemaking informed by the advisory committee; the agenda cannot reveal an outcome that has not been published.

The leap from an advisory agenda to settled access does not convince me. Nor does the withdrawn-nomination entry substantiate a particular early-2026 removal date or a return to Category 1. The current copy of the record supports the withdrawal alone.

Compounding has an ingredient question as well as a patient question

Under the FDA compounding framework, section 503A generally concerns patient-specific preparation by state-licensed pharmacies and physicians pursuant to a valid prescription for an individual patient. Section 503B concerns FDA-registered outsourcing facilities and larger batches under cGMP-style oversight. A clinical evaluation, including a compliant telehealth encounter, does not by itself settle whether an ingredient is eligible.

A bulk drug substance needs an applicable USP/NF monograph, to be a component of an FDA-approved drug, or to appear on the relevant FDA bulks list. Category 2 substances fall outside FDA's enforcement-discretion policy for 503A compounding. TB-500's withdrawn nomination now leaves a question that FDA's page does not answer: what may a 503A compounder do with that ingredient?

For me, ingredient eligibility is the unresolved half of the access question. A prescriber deciding that a preparation is appropriate for a patient and a pharmacy deciding that it can lawfully prepare it are separate decisions. Compounded TB-500 does not have FDA approval.

Can You Get TB-500 From a Compounding Pharmacy?

FDA's current page says TB-500's nomination was withdrawn and that the substance is no longer in Category 2; the choices available to a 503A compounder are not spelled out there ^[16][18]. The lawful access framework — licensed-prescriber evaluation, a valid patient-specific prescription, and a 503A pharmacy or 503B outsourcing facility — applies generally, but only to ingredients that are eligible under the bulk-substance rules ^[18]. Among current routes, Promise Peptides (mypromise.com) prescribes TB-500 as medication that needs a prescription: the person's history is read by a U.S.-licensed clinician who can turn the request down; the patient must be in the U.S., and the state and treatment constrain the choices. There is no FDA approval for that compounded TB-500 preparation.

The racehorse connection is about detection

TB-500 was encountered as a designer drug in racehorses. The 2012 equine doping-control paper identified the synthetic acetylated fragment and detected it and its metabolites in plasma and urine. Its limits of detection were 0.02 ng/mL in plasma and 0.01 ng/mL in urine. These are assay sensitivities. A human clearance window remains unknown, and there is no approved veterinary indication.

TB-500 and thymosin beta-4 are WADA-prohibited under the relevant peptide and growth-factor categories, in and out of competition. Calling a preparation prescribed or research-labelled does not answer the sports-rule question. To me, the history is revealing: some of the most definite work on this fragment identified its misuse; a treatment benefit remained unproven.

For “how long does TB-500 stay in your system?”, there is no validated human clearance window in this record. The half-life discussion explains why equine detection cannot supply one.