The questions readers actually arrive with

Can TB-500 be prescribed by a doctor in the United States?

No. A prescription presupposes an FDA-approved product, either as an on-label prescription for the indication the agency cleared or as an off-label prescription for an unapproved use of an approved product. No TB-500 product is FDA-approved. No full-length Thymosin Beta-4 product is FDA-approved either; the investigational programs RGN-259 (ophthalmic) and RGN-352 (intravenous, halted) remain investigational ^[15][16][17]. With no approved product anywhere in the regulatory record, there is no on-label prescription and no off-label path. A licensed prescriber who writes for TB-500 is not writing a prescription for an approved drug; they are ordering an investigational substance ^[26].

Is TB-500 available by prescription anywhere?

Not in any major regulated market. 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.

Why can't a compounding pharmacy make TB-500 for me?

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.

On September 29, 2023, the FDA placed Thymosin Beta-4, Fragment (LKKTETQ) — TB-500 by its chemical identity — in Category 2 of the Interim 503A Bulks List. Category 2 is the agency's designation for substances it has determined raise significant safety concerns and that may not be used in pharmacy compounding while the agency completes its review ^[26]. The same placement effectively closed the Section 503B route. State boards of pharmacy and major outsourcing facilities have since stopped accepting orders for the substance.

What is the FDA Category 2 / 503A bulks list, and how does it affect TB-500?

The Interim 503A Bulks List is the FDA's working categorization of bulk drug substances that have been nominated for use in pharmacy compounding. The agency sorts nominated substances into three categories.

Category 1 substances may be used in compounding while the FDA continues to consider them. Category 2 substances may not be used in compounding because the agency has identified significant safety concerns. Category 3 substances are those nominated without sufficient supporting information.

TB-500 — under the chemical identity Thymosin Beta-4, Fragment (LKKTETQ) — was placed in Category 2 on September 29, 2023 ^[26]. That placement is the operative regulatory fact for any United States question about whether a pharmacy can prepare TB-500.

When did TB-500 lose its compounding-pharmacy access, and is that decision under review?

The September 29, 2023 Category 2 placement was the operative restriction ^[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].

As of 2025, TB-500 remains in Category 2 — still ineligible for 503A or 503B compounding — with no PCAC vote on the fragment yet scheduled.

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 the September 2023 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.

Is TB-500 legal to possess if it's not prescribable?

TB-500 is not a federally scheduled controlled substance in the United States. Its regulatory restriction is medical-product law, not drug-scheduling law: it cannot be lawfully prescribed because no approved product exists ^[26], and it cannot be lawfully compounded under 503A or 503B because of its Category 2 placement on the Interim 503A Bulks List ^[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.

Why does this site use the word 'prescribed' in its name if TB-500 isn't prescribable?

Because the word prescribed is the search term readers actually use, and the editorial position of this site is that the term names a category absence — a thing the United States regulatory system has not yet created — rather than a product that exists.

The domain modifier sits on the front of the site the way an empty plinth sits in a gallery: as a position relative to the absent object, not a claim that the object is there. The /about page explains the editorial framing in more detail.