The questions readers actually arrive with

Is TB-500 available by prescription anywhere?

U.S. prescription access for compounded TB-500 is distinct from a drug's marketing authorization. No TB-500 product holds marketing authorization from the FDA in the United States, the EMA in Europe, the MHRA in the United Kingdom, the TGA in Australia, the PMDA in Japan, or any other comparable agency ^[26]. The full-length parent peptide has been studied in registered human trials — including in China — but has not received marketing authorization in any indication in any country.

Has any TB-500 or Thymosin Beta-4 product been FDA-approved?

No. The most-developed clinical program for the parent peptide is RGN-259, a 0.1% Thymosin Beta-4 ophthalmic solution developed by RegeneRx and its partners (most recently HLB Therapeutics and the ReGenTree joint venture). Phase III ARISE-3 in dry eye missed its prespecified co-primary endpoints ^[15]. The Phase III neurotrophic-keratopathy trial (NCT02600429, n=18) showed statistically significant healing at day 43 (p=0.036) ^[16], but neither trial has produced a regulator-grade efficacy result that could anchor an approval pathway. The cardiac program RGN-352 was halted in 2011 over manufacturing issues and never read out ^[17]. The synthetic seven-residue TB-500 fragment has never been the subject of a registered human IND.

What separates TB-500 compounding from FDA approval?

Compounding pharmacies operate under specific federal authority. A traditional, patient-specific compounding pharmacy works under Section 503A of the Food, Drug, and Cosmetic Act. A larger outsourcing facility works under Section 503B. Either route, when compounding from a bulk drug substance, requires that the substance appear on the relevant FDA Bulks List or otherwise meet specific statutory criteria.

FDA records Thymosin Beta-4, Fragment (LKKTETQ) — TB-500 by its chemical identity — in the Bulk drug substances nominated but withdrawn table, while keeping its safety concerns beside the entry ^[26]. Withdrawal neither places the fragment in Category 1 nor grants approval for compounding. Sections 503A and 503B set distinct requirements for compounding. A prescription alone does not resolve those requirements ^[26].

What does TB-500's withdrawn nomination mean for the 503A bulks list and compounding?

The 503A bulks list and the interim nomination categories are different regulatory records. The numbered categories describe the older framework; FDA does not assign new nominations to them ^[26].

Category 1 is the interim enforcement-discretion category, subject to the policy's conditions. Category 2 identifies substances with significant safety risks outside that discretion. Category 3 denotes nominations lacking sufficient information ^[26].

TB-500 was formerly in Category 2; FDA now records its withdrawn nomination, with the safety concerns retained ^[26]. That record establishes neither Category 1 status nor compounding permission.

How does TB-500's FDA record bear on present access?

TB-500's earlier Category 2 entry recorded FDA safety concerns ^[26]. The Pharmacy Compounding Advisory Committee (PCAC) — the FDA federal advisory committee that reviews bulk drug substances for the 503A Bulks Regulation — held meetings on October 29 and December 4, 2024 to begin formal review of several Category 2 peptides. TB-500 was not among the substances voted on at those December 2024 sessions, which focused on AOD-9604, CJC-1295, and Thymosin Alpha-1 ^[27].

FDA staff proposed against adding TB-500 to the 503A bulks list; a staff recommendation, committee discussion and final agency rulemaking are separate steps ^[27].

How is TB-500 different from full-length Thymosin Beta-4, and does that matter for prescribing?

TB-500 is a synthetic seven-amino-acid peptide, N-acetyl-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH, with the central LKKTET actin-binding motif of the parent peptide. Full-length Thymosin Beta-4 is a 43-amino-acid endogenous peptide encoded by the TMSB4X gene ^[1][25].

The fragment retains the actin-binding sequence. It does not retain most of the parent peptide's downstream interaction surfaces, including the C-terminal helix that mediates inflammation-related signaling ^[11], the regions involved in PINCH-ILK-Akt complex formation ^[6], and the N-terminal sequence that is enzymatically cleaved to release AcSDKP, a separately bioactive antifibrotic and proangiogenic tetrapeptide.

This matters for prescribing in two ways. First, vendor literature for TB-500 routinely cites animal and clinical studies of the parent peptide as if the data applied to the fragment, which it does not without separate study ^[24]. Second, the FDA explicitly cited identity ambiguity — fragment marketed, parent peptide studied — among the safety concerns supporting its historical Category 2 placement ^[26].

What are the human clinical trials of Thymosin Beta-4 (RGN-259, RGN-352) and did any succeed?

Three Phase III ophthalmic trials and one Phase II cardiac trial define the human record for the parent peptide.

ARISE-3 (NCT03937882) tested 0.1% RGN-259 ophthalmic solution BID in roughly 700 dry-eye patients. It missed its prespecified co-primary endpoints but produced statistically significant improvement in ocular grittiness and significant central corneal fluorescein staining improvement at two weeks in a defined subpopulation ^[15]. The European SEER-3 neurotrophic-keratitis Phase III missed its primary endpoint.

NCT02600429 tested the same 0.1% RGN-259 solution at six applications per day for 28 days in 18 neurotrophic-keratopathy patients. Complete corneal healing was observed at day 29 in 60% of treated subjects versus 12.5% in placebo (p=0.066, narrow miss) and statistically significant healing was reached at day 43 (p=0.036), with sustained healing two weeks after stopping treatment ^[16].

RGN-352 (NCT01311518) was the Phase II cardiac trial designed to enroll approximately 75 post-AMI patients at 450 mg or 1,200 mg IV daily for three days followed by weekly for four weeks. The trial was placed on FDA clinical hold in 2011 over contract-manufacturer cGMP non-compliance and never reported efficacy data ^[17].

In short: positive signals in ophthalmology, no clean primary-endpoint success, and an unfinished cardiac story.

Is TB-500 banned by WADA for athletes?

Yes. TB-500 is prohibited at all times under the World Anti-Doping Code, listed under section S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics) and the catch-all S0 (Non-Approved Substances) of the WADA Prohibited List in the 2024, 2025 and 2026 editions ^[22][26]. Multiple athlete sanctions and four-year ineligibility periods have been issued under both S0 and S2.

Validated LC-MS detection methods for TB-500 in equine plasma and urine have been published ^[22], and analogous human-sport detection methods are in routine use at WADA-accredited laboratories. The compound is not difficult to detect after recent administration.

How do prescription status and possession law differ for TB-500?

TB-500 is not a federally scheduled controlled substance in the United States. Prescription access, drug approval and possession law are separate questions: compounded TB-500 has no FDA approval, and its withdrawn nomination does not establish authority to compound it ^[26]. State laws and professional-licensure rules add further constraints on practitioners.

Research use of TB-500 as a research chemical is a separate question that depends on the institutional setting, the source's licensing, and applicable state and federal research-chemical regulation. This site does not advise on possession or use; it documents the regulatory record.

Are 'underground' TB-500 doses safe to use?

Underground- and research-chemical-market TB-500 is sold without GMP manufacturing, lot release testing, endotoxin control, or sterility assurance. The contamination and purity risks of unregulated peptide injectables are not theoretical — they were among the stated FDA bases for the 503A Category 2 placement ^[26]. The commonly quoted '2 to 10 mg per week subcutaneous' regimens have no peer-reviewed safety basis ^[22] and are not derived from any published clinical trial.

The pharmacology adds context. Tβ4 biology is not uniformly tissue-supportive: in hepatic stellate cells, conditional knockout of Tβ4 ameliorates liver fibrosis in mouse CCl4 models, suggesting that the molecule is pro-fibrotic in that compartment ^[21]. The largest registered cardiac efficacy study in pigs was negative for global infarct-size reduction ^[23]. These are the parts of the published record that vendor pages tend to leave out.

Does TB-500 cause cancer?

No clinical signal of tumor promotion has been reported in the published human safety data to date ^[13][14]. Theoretical concerns have been raised in the literature because Tβ4 promotes angiogenesis and cell migration — two activities that could in principle accelerate the progression of occult or pre-existing tumors. The Phase I human trials of recombinant Tβ4 enrolled small numbers of healthy adult volunteers under short observation periods and were not powered or designed to detect such a signal. The published record neither confirms nor refutes the theoretical concern.

Does TB-500 work for hair regrowth?

The hair-regrowth interest traces to a 2004 paper by Philp and colleagues showing that full-length Tβ4 at nanomolar concentrations increased clonogenic hair-follicle keratinocyte migration in rat vibrissa follicles and accelerated hair regrowth in mice ^[9]. The work used the parent peptide, not the seven-residue fragment, and was conducted in rodent models. No registered human clinical trial of either Tβ4 or TB-500 for hair regrowth has been published. The vendor-marketing claim is grounded in a real preclinical observation; it is not grounded in human evidence.