Which is better, thymosin alpha-1 or TB-500 (thymosin beta-4)?
Thymosin alpha-1 wins on evidence grade (human RCT versus animal only) and on regulatory standing internationally, as an approved drug in more than 30 countries with a benign safety record across decades of trials. TB-500's only human randomized data are for a 0.1% eye drop in dry eye, where both primary endpoints missed, and the identity of injectable products sold as TB-500 is not verified. The US access picture runs the other way: thymosin alpha-1's compounding nomination was withdrawn, while TB-500 is awaiting a final FDA rule after the July 2026 PCAC recommendation. [1] [2] [4] [6] [8] [10] [12]
How do thymosin alpha-1 and TB-500 (thymosin beta-4) compare?
| Dimension | Thymosin alpha-1Favored | TB-500 (thymosin beta-4) |
|---|---|---|
| What it is [5] [9] | A 28 amino acid N-acetylated thymic peptide (thymalfasin). | Synthetic thymosin beta-4 (43 amino acids) or its 7 amino acid actin binding fragment LKKTETQ, depending on the product. |
| Note: The two are not related in structure or function; both were first isolated from thymus extracts, which explains the shared name. | ||
| Mechanism [3] [5] [7] [9] | Immune modulation through TLR2 and TLR9 on dendritic cells: more IL-2 and interferon, stronger natural killer and CD8 activity, fewer exhausted T cells. | Sequesters G-actin to regulate cell migration; in animals promotes keratinocyte, endothelial, and cardiomyocyte migration and activates Akt through a PINCH and ILK complex. |
| Evidence grade [1] [2] [6] [7] | Human RCT: hepatitis B trial (98 patients, 40.6% versus 9.4% virological response) and sepsis trial (361 patients, 26.0% versus 35.0% 28 day mortality, p = 0.062). | Animal only: rat wound reepithelialization up 42% at day 4 and 61% at day 7; improved cardiac function after infarction in mice. No human trial of the injectable. |
| Main uses [4] [8] | Chronic viral hepatitis, sepsis, vaccine adjuvant in immunocompromised patients, and cancer immunotherapy. | Marketed for tendon, muscle, and soft tissue recovery; studied as eye drops for dry eye (primary endpoints missed in a 72 patient trial). |
| Safety [1] [2] [9] | Mostly injection site redness and transient aches; no serious drug related events in the sepsis trial. | No systematic human safety data for the injectable; theoretical concern about supporting tumor growth because it drives cell migration and angiogenesis. |
| US compounding status [10] [11] [12] | Placed in 503A Category 2 in 2023; FDA now lists the nomination as withdrawn, so it is not on the bulks list and has no lawful compounding basis. | Removed from 503A Category 2 on April 15, 2026; PCAC recommended adding it to the 503A bulks list on July 23 to 24, 2026; no final FDA rule at verification. |
| WADA status [13] | Unclear; not named on the list. | Prohibited at all times under S2, which names thymosin beta-4 and TB-500. |
| Dosing reference [1] [2] [8] | Trials used 1.6 mg subcutaneously twice weekly for 6 to 12 months (hepatitis B) or twice daily for 7 days (sepsis). | Not established for the injectable; the only human regimen is 0.1% eye drops for 28 days. |
Evidence grade and regulatory status
Pulled from each peptide’s own record, so it stays in step with the peptide pages.
| Attribute | Thymosin alpha-1 | TB-500 (thymosin beta-4) |
|---|---|---|
| Class | 28 amino acid N-acetylated thymic peptide (synthetic form of a natural thymus derived immunomodulator) | Synthetic version or fragment of thymosin beta-4, a 43 amino acid actin sequestering peptide present in nearly all human cells |
| What it is | Immune modulating thymus peptide approved abroad (not the US) for hepatitis B and as a vaccine booster; mixed sepsis trials; FDA nomination withdrawn. | Copy or fragment of the repair protein thymosin beta-4: strong animal data, small human trials of the full protein, no trial of injected TB-500; WADA banned. |
| Evidence | Evidence: Human RCT evidence | Evidence: Animal-only evidence |
| FDA status | Regulatory: Removed from Category 2 (Apr 2026) | Regulatory: Under FDA review |
| Compounding | Not FDA approved, although approved as Zadaxin (thymalfasin) in more than 30 countries. FDA placed thymosin alpha-1 in 503A Category 2 in 2023, citing immunogenicity risk and inadequate safety information for compounded product. FDA's Category 2 page (current as of April 22, 2026) now lists it under bulk drug substances nominated but withdrawn, so it is no longer in Category 2 but has no active nomination and is not on the 503A bulks list. That leaves US compounding pharmacies without a lawful basis to make it. It was not among the peptides reviewed by the advisory committee in July 2026. | Removed from the FDA 503A Category 2 list (substances nominated for compounding that raise significant safety risks) on April 15, 2026. On July 23 to 24, 2026 the Pharmacy Compounding Advisory Committee recommended adding TB-500 to the 503A bulks list. As of the last verification date FDA has not published a final rule listing it, so it is not on the 503A bulks list and a 503A pharmacy has no federal basis to compound it from bulk; state boards cannot authorize what federal law does not. It is not an FDA approved drug and is not eligible for 503B outsourcing. Products sold as research chemicals are not lawful for human use. |
| WADA | WADA: WADA status unclear | WADA: WADA prohibited |
| Routes | Subcutaneous injection (all trials and foreign labeling) | Subcutaneous or intramuscular injection (as sold), Topical eye drops (human dry eye trial, investigational), Topical to wound or intraperitoneal (animal studies) |
| Typical cost | Not available through licensed US channels since the 2023 Category 2 placement. Zadaxin is sold by prescription in countries where it is approved. | No lawful compounded price: TB-500 is not on the 503A bulks list, and PCAC's July 2026 recommendation is advisory until FDA publishes a final rule. Advertised clinic prices are not a comparable price. Products sold as research chemicals are not lawful for human use and are not verified for identity or purity. |
| Last verified |
Has thymosin alpha-1 been tested directly against TB-500 (thymosin beta-4)?
No published trial has compared thymosin alpha-1 and TB-500 (thymosin beta-4) directly. The dimensions above come from separate studies and labels, and cross-trial comparisons are less reliable than a direct trial.
What do thymosin alpha-1 and TB-500 (thymosin beta-4) cost?
| Dimension | Thymosin alpha-1 | TB-500 (thymosin beta-4) |
|---|---|---|
| Typical cost | Not available through licensed US channels since the 2023 Category 2 placement. Zadaxin is sold by prescription in countries where it is approved. | No lawful compounded price: TB-500 is not on the 503A bulks list, and PCAC's July 2026 recommendation is advisory until FDA publishes a final rule. Advertised clinic prices are not a comparable price. Products sold as research chemicals are not lawful for human use and are not verified for identity or purity. |
| Price detail | Thymosin alpha-1 price by channel | TB-500 (thymosin beta-4) price by channel |
Where a price index exists, the range is the cash price per month across branded, compounded, and telehealth channels, without insurance. The cost pages give each channel separately with its sources. Otherwise the typical cost from the peptide record is shown. Legal access depends on your state: see peptide legality by state.
Frequently asked questions
Are thymosin alpha-1 and TB-500 the same thing?
No. Thymosin alpha-1 is a 28 amino acid immune modulator. TB-500 refers to thymosin beta-4 or a fragment of it, an actin binding peptide involved in cell migration and tissue repair. They share a name because both were first found in thymus extracts. [5] [9]
Which has better human evidence?
Thymosin alpha-1, which has randomized trials in hepatitis B and severe sepsis and is approved as Zadaxin abroad. TB-500 has no human trial of the injectable form; its only randomized human data are for eye drops in dry eye, where the primary endpoints were not met. [1] [2] [8]
Is TB-500 proven to heal injuries?
Not in humans. Rodent studies show faster wound closure and improved heart repair, but no human trial has tested injected TB-500 for tendon, muscle, or other musculoskeletal injuries. [6] [7] [8]
Which can a US pharmacy compound?
Currently neither has a final bulks listing. TB-500 is closer: FDA removed it from Category 2 in April 2026 and PCAC recommended it for the bulks list in July 2026, so some pharmacies may fill it where state boards permit. Thymosin alpha-1's nomination is listed as withdrawn, leaving no lawful basis. [10] [11] [12]
Can athletes use them?
TB-500 is prohibited at all times under WADA section S2, which names thymosin beta-4. Thymosin alpha-1 is not named, so its status is unclear and athletes should check with their anti-doping organization. [13]
Conditions studied
From the blog
- TB-500 vs thymosin beta-4: is it the same peptide?
TB-500 is not the same molecule as thymosin beta-4. What each one is, which has human data, what is known about capsules and sprays, and what the evidence says about cancer.
- Thymosin alpha-1 access: approved abroad, not in the US
Thymosin alpha-1 is approved as Zadaxin in more than 30 countries but has never been FDA approved, and US compounding has no lawful basis. What that means for access, forms, and thymalin.
Sources
Numbered citations above point to these primary sources. PubMed entries link to the indexed abstract.
- [1]Chien RN et al. Efficacy of thymosin alpha1 in patients with chronic hepatitis B: a randomized, controlled trial. Hepatology 1998PubMed 9581695, 1998
- [2]Wu J et al. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Crit Care 2013PubMed 23327199, 2013
- [3]Liu Y et al. Thymosin alpha 1 reduces the mortality of severe COVID-19 by restoration of lymphocytopenia and reversion of exhausted T cells. Clin Infect Dis 2020PubMed 32442287, 2020
- [4]Camerini R, Garaci E. Historical review of thymosin alpha 1 in infectious diseases. Expert Opin Biol Ther 2015PubMed 26098768, 2015
- [5]Dominari A et al. Thymosin alpha 1: A comprehensive review of the literature. World J Virol 2020PubMed 33362999, 2020
- [6]Malinda KM et al. Thymosin beta4 accelerates wound healing. J Invest Dermatol 1999PubMed 10469335, 1999
- [7]Bock-Marquette I et al. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature 2004PubMed 15565145, 2004
- [8]Sosne G et al. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, phase II clinical trial. Clin Ophthalmol 2015PubMed 26056426, 2015
- [9]Goldstein AL et al. Thymosin beta4: a multi-functional regenerative peptide. Basic properties and clinical applications. Expert Opin Biol Ther 2012PubMed 22074294, 2012
- [10]FDA: Certain bulk drug substances for use in compounding may present significant safety risks (Category 2 list and withdrawn nominations, content current as of April 22, 2026)FDA
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