Not approved in the U.S.No U.S. application for thymalfasin exists.[1] The FDA told its advisory committee in 2024 that it found a "lack of evidence of effectiveness" for all 13 uses it reviewed.[5] This page does arithmetic and reports the record; it does not suggest amounts.
What is thymosin alpha 1?
Thymosin alpha 1 is a 28-amino-acid peptide with an acetyl group on its first amino acid; its drug name is thymalfasin, and the FDA writes that "Ta1 and thymalfasin can be used interchangeably".[5]
- Sequence
- Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN (28 residues, N-terminal acetyl)[5]
- Formula and weight
- C129H215N33O55, 3,108.3 g/mol[23][5]
- Our engine
- C129H215N33O55, 3,108.32 g/mol average, 3,106.5041 monoisotopic
- Names
- Thymalfasin, Ta1, TA-1; Zadaxin is "a finished drug product available in other countries"[5]
Paste the sequence into the peptide molecular weight calculator with "N-terminal acetyl" ticked and you get the same formula; the page fails to build if it ever does not.
Thymosin alpha 1 vs thymosin beta 4
They share a name and nothing else: thymosin alpha 1 is a 28-amino-acid immune peptide, while thymosin beta 4 is a 43-amino-acid actin-binding protein, and the two were simply isolated from the same thymus extract.[23][24]
| Thymosin alpha 1 | Thymosin beta 4 | |
|---|---|---|
| Length | 28 amino acids | 43 amino acids[24] |
| Molecular weight | 3,108.3 g/mol | 4,963 g/mol |
| Drug name | Thymalfasin | Timbetasin |
| Parent protein | Prothymosin alpha | Made as a complete peptide |
| Known for | Immune signalling; studied in hepatitis and sepsis | Binding actin; studied for tissue repair |
| Fragment sold separately | None | TB-500, a 7-amino-acid piece[25] |
| U.S. approval | None | None |
| WADA 2026 | Not named[21] | Prohibited, S2.3, named with its derivatives[21] |
The practical difference for a reader: in sport they are treated oppositely. Thymosin beta 4 and TB-500 are named on the Prohibited List; thymosin alpha 1 is not.[21] "Thymosin" alone on a label therefore tells you nothing, and the status table lists each separately.
Is thymosin alpha 1 FDA-approved?
No. No U.S. application exists, and four U.S. orphan designations granted between 1991 and 2006 never led to an approval.[1][2]
| Designated | Proposed use |
|---|---|
| May 3, 1991 | Chronic active hepatitis B |
| Jan 8, 1998 | DiGeorge anomaly with immune defects |
| Mar 6, 2000 | Hepatocellular carcinoma |
| Mar 13, 2006 | Stage IIb–IV malignant melanoma |
- Orphan designation is not approval. It is an early status for a drug under study in a rare disease. The EU's 2002 designation for liver cancer is the same kind of status, not a marketing authorization.[2][3]
- Italy. The Italian medicines agency lists Zadaxin 1.6 mg as authorized and marketed, prescription only.[4]
- Other countries. The FDA notes claimed approvals in India, Hong Kong and elsewhere but says it "is unable to independently verify these claims"; we confirmed only Italy, on the regulator's own site.[5]
The Zadaxin label in Italy
Zadaxin's only labeled use in Italy is as an add-on to flu vaccination in immunocompromised adults: one 1.6 mg vial, into muscle or under the skin, twice a week for four weeks, repeated from week 8.[4]
| Item | Label |
|---|---|
| Strength | Each vial holds 1.6 mg; after adding the supplied 1 mL solvent, the solution is 1.6 mg/mL |
| Indication | "Adjuvant to anti-influenza vaccination in immunocompromised subjects" |
| Schedule | One vial twice a week for 4 weeks from the first vaccination; repeated from week 8 to week 12 (second vaccination) |
| Body-size wording | Gives 900 mcg/m² as the planned "weekly" dosage, yet its own table (1.75 m² → 1.58 mg) equals 900 mcg/m² per injection: inconsistent as printed |
| Children | Must not be used |
| Contraindications | Hypersensitivity, pregnancy, breastfeeding |
| Side effects | None clinically relevant, "except for the occasional possibility of modest and transient pain at the injection site" |
| Storage | 2–8 °C; after reconstitution use once, within 24 hours, and discard the rest |
The calculator above starts at the label's strength: 1.6 mg in 1 mL is 1.6 mg/mL, so the whole vial is 1 mL, which is 100 units on a U-100 scale. That is the arithmetic of a foreign label, not a U.S. dose. More on mixing in how to reconstitute peptides.
Compounding: the 2024 FDA committee vote
The FDA's Pharmacy Compounding Advisory Committee voted 17 to 4 against adding thymosin alpha 1 to the list of substances pharmacies may compound, separately for the free base and the acetate.[6]
| Date | What happened |
|---|---|
| Sept 29, 2023 | 503A Category 2: substances with "significant safety risks"[8] |
| Sept 27, 2024 | Removed from Category 2 because the nominator withdrew; FDA still took it to the committee[8] |
| Dec 4, 2024 | FDA proposes not adding either form; committee votes 4 yes, 17 no for each[5][6] |
| Mar 21, 2025 | 503B Category 3, "nominated without adequate support"[9] |
| Oct 30, 2026 | Not covered by the HHS enforcement-discretion policy[10] |
- FDA's reasons. Thymalfasin is not well characterized; its limited water solubility made it "unclear how it would be possible" to make the nominated 3 mg/mL injection; and the FDA raised immunogenicity and found no evidence of effectiveness for any of 13 uses, from hepatitis to COVID-19.[5]
- Safety page. The FDA says compounded thymosin alpha 1 "may pose significant risk for immunogenicity for certain routes of administration".[7]
- Adverse event reports. One U.S. report through June 2024: atrial fibrillation, anxiety and a brief drop in TSH in a hepatitis C trial patient also on peginterferon, which the FDA says confounds causality.[5]
- Enforcement. Two joint FDA–FTC warning letters in 2020 cited sellers offering thymosin alpha 1 with COVID-19 claims.[11][12]
The are peptides legal? page explains what these categories mean.
Thymosin alpha 1 in clinical trials
The largest and most rigorous trial, a 1,106-patient double-blind phase 3 study in sepsis published in 2025, found no reduction in deaths: 23.4% vs 24.1% at 28 days.[18]
| Trial | People | Amount and route studied | Result |
|---|---|---|---|
| Chien 1998[13] | 98, chronic hepatitis B | 1.6 mg SC twice weekly, 26 or 52 weeks | Response at 18 months 40.6% (26 wk) vs 9.4% untreated |
| Mutchnick 1999[14] | 97, hepatitis B, U.S., placebo-controlled | 1.6 mg SC twice weekly, 6 months | 14% vs 4% (p = 0.084); authors: results "do not confirm" efficacy |
| Sherman 1998[15] | 109, hepatitis C, placebo-controlled | 1.6 mg SC twice weekly with interferon | Sustained response 14.2% vs 8.1% with interferon alone |
| Ciancio 2012[16] | 552, hepatitis C non-responders | 1.6 mg SC twice weekly with peginterferon and ribavirin | Sustained response 12.7% vs 10.5% (p = 0.407) |
| ETASS 2013[17] | 361, severe sepsis, single-blind | 1.6 mg SC twice daily for 5 days, then daily for 2 | Deaths 26.0% vs 35.0% (p = 0.062) |
| TESTS 2025[18] | 1,106, sepsis, double-blind phase 3 | 1.6 mg SC every 12 hours, up to 7 days | Deaths 23.4% vs 24.1% (p = 0.93); no safety differences |
| Shehadeh 2023[19] | 49, hospitalized COVID-19, open-label | 1.6 mg SC daily, 7 days | Recovery not significantly different |
| Shetty 2022[20] | 105, COVID-19, placebo-controlled | 1.6 mg SC two or three times daily, 7 days | Lower deaths reported; the FDA notes limitations |
The pattern: the earlier, smaller trials looked promising, and the larger blinded ones did not confirm it. Amounts above are what researchers used under study conditions, not doses for anyone.
Thymosin alpha 1 math: micrograms and nanomoles
At 3,108.3 g/mol, 1 mg of thymosin alpha 1 is 321.7 nmol, and a 1.6 mg Zadaxin vial holds 515 nmol.
| Vial + liquid | mg/mL | mcg per unit | nmol per unit | Units per 100 mcg |
|---|---|---|---|---|
| 1.6 mg + 1 mL | 1.6 | 16 | 5.15 | 6.25 |
| 1.6 mg + 2 mL | 0.8 | 8 | 2.57 | 12.5 |
| 5 mg + 1 mL | 5 | 50 | 16.09 | 2 |
| 5 mg + 2 mL | 2.5 | 25 | 8.04 | 4 |
| 10 mg + 1 mL | 10 | 100 | 32.17 | 1 |
| 10 mg + 2 mL | 5 | 50 | 16.09 | 2 |
For how units relate to millilitres, see the peptide dosage calculator.
Is thymosin alpha 1 banned in sport?
WADA's 2026 list does not name thymosin alpha 1.[21] Its S0 class covers substances with no approval by any government health authority; because Italy approves thymalfasin, S0 would not seem to apply, but that is our reading and WADA has not ruled on it. See are peptides banned in sports?
Other peptide pages
The peptides most often looked up alongside this one. Each page gives that peptide's FDA status, compounding status and published human evidence, with sources; the status table has all 23 in one place.
- EpitalonNot approved AEDG tetrapeptide. FDA flags a theoretical cancer risk.
- KisspeptinNot approved Reproductive-hormone signal. Kisspeptin-10 is on FDA Category 2.
- BPC-157Not approved 15-amino-acid peptide. Human data: a few tiny, uncontrolled studies.
- TB-500Not approved A 7-amino-acid fragment of thymosin beta-4. No human trials of TB-500 itself.
- BPC-157 + TB-500 blendNot approved Two peptides in one vial: the arithmetic of a fixed ratio.
- TesamorelinFDA-approved Egrifta SV and Egrifta WR. Labeled doses, diluents and volumes.
Frequently asked questions
Is thymosin alpha 1 FDA-approved?
What is the thymosin alpha 1 dosage?
What is thymosin alpha 1 used for?
Is thymosin alpha 1 the same as thymalfasin?
Yes. The FDA says the two names can be used interchangeably; Zadaxin is the brand sold abroad.[5]
Sources
- openFDA Drugs@FDA searches for thymalfasin, thymosin, kisspeptin, emideltide, hexarelin, examorelin, GHRP, cathelicidin and Zadaxin (no match)
- Search orphan drug designations and approvals (thymalfasin: 4 designations; kisspeptin, hexarelin, emideltide, cathelicidin: none)
- EU/3/02/110: orphan designation of thymalfasin for hepatocellular carcinoma
- ZADAXIN 1.6 mg/ml (timosina alfa 1): Riassunto delle Caratteristiche del Prodotto (Italian label)
- FDA briefing document: thymosin alpha-1 (Ta1)-related bulk drug substances
- Pharmacy Compounding Advisory Committee, December 4, 2024: final summary minutes
- Certain bulk drug substances for use in compounding that may present significant safety risks
- Bulk drug substances nominated for use in compounding under section 503A (category lists)
- Bulk drug substances nominated for use in compounding under section 503B (category lists)
- Enforcement discretion policy regarding 503A compounding using certain peptide bulk drug substances (91 FR 68526)
- Warning letter (FDA and FTC): Center for Wellness and Integrative Medicine
- Warning letter (FDA and FTC): Paradigm RE LLC
- Chien RN et al. Thymosin alpha-1 in chronic hepatitis B: randomized controlled trial (Hepatology 1998;27:1383–1387)
- Mutchnick MG et al. Thymosin alpha-1 in chronic hepatitis B: U.S. multicenter trial (J Viral Hepat 1999;6:397–403)
- Sherman KE et al. Thymosin alpha-1 plus interferon in chronic hepatitis C (Hepatology 1998;27:1128–1135)
- Ciancio A et al. Thymosin alpha-1 with peginterferon/ribavirin in HCV non-responders (J Viral Hepat 2012;19 Suppl 1:52–59)
- Wu J et al. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind RCT (Crit Care 2013;17:R8)
- Wu J et al. Thymosin alpha 1 in sepsis (TESTS): multicentre, double-blind, placebo-controlled phase 3 trial (BMJ 2025;388:e082583)
- Shehadeh F et al. Thymosin alpha-1 in hospitalized COVID-19: pilot RCT (J Infect Dis 2023;227:226–235)
- Shetty A et al. Thymosin alpha-1 in moderate to severe COVID-19 (Indian J Crit Care Med 2022;26:913–919)
- World Anti-Doping Code: Prohibited List 2026
- UniProt P06454: prothymosin alpha, 111 residues (thymosin alpha-1 = residues 2–29, N-acetylserine)
- PubChem CID 16130571: thymalfasin
- PubChem CID 16132341: thymosin beta-4 (timbetasin)
- PubChem CID 62707662: TB-500 (Ac-LKKTETQ)