Which is better, thymosin alpha-1 or thymulin?
Both technically sit at the human RCT grade, but thymosin alpha-1's base is broader and more recent: a 98 patient hepatitis B trial with 40.6% versus 9.4% virological response, a 361 patient sepsis trial with a 26.0% versus 35.0% mortality trend, and decades of use abroad with a benign safety record. Thymulin rests on one 1987 report of two trials. On regulation, thymosin alpha-1 is an approved drug in other countries while thymulin has no approval anywhere, though in the United States both lack any lawful compounding route. [1] [2] [4] [6] [11] [12]
How do thymosin alpha-1 and thymulin compare?
| Dimension | Thymosin alpha-1Favored | Thymulin |
|---|---|---|
| Structure [5] [7] | 28 amino acid N-acetylated peptide, a synthetic copy of a natural thymus derived immunomodulator. | 9 amino acid peptide hormone from thymic epithelial cells that is active only when bound to zinc. |
| Mechanism [3] [5] [9] | Acts on TLR2 and TLR9 on dendritic cells and T cell precursors, raising IL-2 and interferon, boosting natural killer and CD8 activity, and reducing T cell exhaustion markers. | Promotes T cell precursor differentiation and modulates T cell subsets and natural killer activity; linked to a thymus to neuroendocrine axis regulated by growth hormone and prolactin. |
| Human trial evidence [1] [2] [6] | Chronic hepatitis B RCT (98 patients): complete virological response 40.6% versus 9.4% at 18 months. Severe sepsis RCT (361 patients): 28 day mortality 26.0% versus 35.0% (relative risk 0.74, p = 0.062). | Two double blind, placebo controlled rheumatoid arthritis trials reported in 1987: 5 mg daily improved 56% of patients by global assessment versus 17% on placebo. No trial since. |
| Main clinical uses studied [3] [4] [6] [10] | Chronic hepatitis B and C, severe sepsis, severe COVID-19 (retrospective), vaccine adjuvant in dialysis and immunocompromised patients, and cancer immunotherapy. | Rheumatoid arthritis in humans; inflammation and neuropathic pain in rodents. |
| Approval status worldwide [4] [12] | Approved as Zadaxin (thymalfasin) in more than 30 countries; not FDA approved. | Not approved in any country. |
| US compounding status [11] [12] | Placed in 503A Category 2 in 2023; FDA now lists the nomination as withdrawn, so it is not in Category 2 and not on the bulks list, leaving no lawful compounding basis. Not part of the July 2026 PCAC review. | Never nominated: not on the 503A bulks list, Category 2, or the withdrawn list, and no USP monograph. |
| WADA status [13] | Unclear; not named on the list. Athletes should confirm with their anti-doping organization. | Prohibited at all times under S0 (non-approved substances). |
| Note: Thymosin alpha-1 is an approved medicine in some countries, which is why it is not automatically captured by S0 the way thymulin is. | ||
| Trial dosing [1] [2] [6] | 1.6 mg subcutaneously twice weekly for 6 to 12 months (hepatitis B); 1.6 mg twice daily for 7 days (sepsis). | 1, 5, or 10 mg daily in the 1987 trials, with 5 mg most effective. |
| Safety record [1] [2] [6] | Mostly injection site redness and transient aches; no significant side effects in the hepatitis B trial and no serious drug related events in the sepsis trial. | Minimal adverse effects in the 1987 trials; no modern human safety data. |
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 | Thymulin |
|---|---|---|
| Class | 28 amino acid N-acetylated thymic peptide (synthetic form of a natural thymus derived immunomodulator) | Zinc dependent nonapeptide (9 amino acids) hormone produced by thymic epithelial 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. | Zinc dependent thymus hormone (9 amino acids) with 1980s rheumatoid arthritis trials and rodent anti-inflammatory data; never approved, never nominated to FDA. |
| Evidence | Evidence: Human RCT evidence | Evidence: Human RCT evidence |
| FDA status | Regulatory: Removed from Category 2 (Apr 2026) | Regulatory: Unscheduled |
| 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. | Not FDA approved and not approved in any country. Thymulin does not appear on the FDA 503A bulks list, in Category 2, or among the withdrawn nominations as of the FDA pages current to April 2026, and it has no USP monograph, so there is no lawful basis for a US compounding pharmacy to dispense it. It was not part of the July 2026 advisory committee review. Products sold online are research chemicals. |
| WADA | WADA: WADA status unclear | WADA: WADA prohibited |
| Routes | Subcutaneous injection (all trials and foreign labeling) | Injection (1987 trials), Subcutaneous injection and intranasal (as sold, not from published trials) |
| 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. | Not available through licensed channels. 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 thymulin?
No published trial has compared thymosin alpha-1 and thymulin 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 thymulin cost?
| Dimension | Thymosin alpha-1 | Thymulin |
|---|---|---|
| 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. | Not available through licensed channels. 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 | Price index in progress |
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
Which thymic peptide has better evidence?
Thymosin alpha-1. It has randomized trials in hepatitis B and sepsis and decades of use as an approved drug abroad, while thymulin's human evidence is limited to two small 1987 rheumatoid arthritis trials that were never repeated. [1] [2] [4] [6]
Can I get either one legally in the United States?
No lawful US path exists for either as of the verification date. Thymosin alpha-1's 503A nomination is listed by FDA as withdrawn and it is not on the bulks list; thymulin has never been nominated. Neither is FDA approved. [11] [12]
Did thymosin alpha-1 lower sepsis deaths?
The ETASS trial of 361 patients found 26.0% mortality with thymosin alpha-1 versus 35.0% with standard care alone, but the difference narrowly missed statistical significance (relative risk 0.74, 95% CI 0.54 to 1.02). It is a promising signal, not proof. [2]
Why does thymulin need zinc?
Thymulin binds zinc in a one to one complex, and only the zinc bound form is biologically active, which was shown in 1982. That makes zinc status a factor in its activity. [7] [8]
Are they allowed in sport?
Thymulin is prohibited at all times under WADA section S0. Thymosin alpha-1 is not named on the list, so its status is unclear, and athletes should confirm with their anti-doping organization. [13]
Conditions studied
From the blog
- 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]Amor B et al. Nonathymulin in rheumatoid arthritis: two double blind, placebo controlled trials. Ann Rheum Dis 1987PubMed 3310925, 1987
- [7]Dardenne M et al. Contribution of zinc and other metals to the biological activity of the serum thymic factor. Proc Natl Acad Sci USA 1982PubMed 6957870, 1982
- [8]Bach JF, Dardenne M. Thymulin, a zinc-dependent hormone. Med Oncol Tumor Pharmacother 1989PubMed 2657247, 1989
- [9]Reggiani PC et al. Physiology and therapeutic potential of the thymic peptide thymulin. Curr Pharm Des 2014PubMed 24588820, 2014
- [10]Safieh-Garabedian B et al. Targeting inflammatory components in neuropathic pain: the analgesic effect of thymulin related peptide. Neurosci Lett 2019PubMed 30503917, 2019
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