Thymosin alpha-1

Not FDA-approved Evidence grade B — human randomised trials, not approved for that use Updated 2026-10-20

This page describes published research and public regulatory records. It is not medical advice. SoluneFit does not sell, supply, or recommend any compound. The SoluneFit app includes a private, on-device tracker for people who want a record of what they and their clinician have already decided — the app neither suggests compounds nor doses.

Also known as: thymalfasin, Zadaxin, Ta1, Tα1, thymosin α1, thymosin alfa 1.

Regulatory status

Not FDA-approved (four US orphan-drug designations only). At the 4 Dec 2024 PCAC meeting the committee voted 17–4 against adding thymosin alpha-1 (free base and acetate) to the 503A bulk-substances list, as FDA had proposed; the nomination now appears under 'nominated but withdrawn' on FDA's safety-risk page [1][2][3]. Abroad, FDA notes approvals in Italy (influenza-vaccine adjuvant), India, Indonesia, Hong Kong and others, which FDA says it could not independently verify [1].

What it is

Immunomodulatory peptide (synthetic copy of a 28-amino-acid, N-terminally acetylated thymic peptide).

A synthetic version of a small peptide originally isolated from the thymus, marketed abroad as Zadaxin, mainly for chronic hepatitis B and as an add-on to vaccination in people with weak immune systems. It is not approved in the US.

What the published research shows

Many human RCTs, mostly small and from China/Asia; a 1,106-patient phase 3 RCT in sepsis was negative, and a 2026 Cochrane review of 10 RCTs (1,349 people) in hepatitis B rated the evidence very low to low certainty.

Published human studies and reports (these describe trial populations, not SoluneFit claims):

Chronic hepatitis B (Zadaxin label, Indonesia, as quoted by FDA): Label: 1.6 mg subcutaneously twice weekly, doses 3–4 days apart, for six months (52 doses).
Influenza-vaccine adjuvant in immunocompromised people (Zadaxin label, Italy, as quoted by FDA): Label: one 1.6 mg vial SC or IM twice weekly for 4 weeks from the first vaccination, repeated from week 8 (second vaccination) to week 12.
Sepsis (TESTS phase 3 RCT, BMJ 2025): 1,106 adults in 22 Chinese centres given SC thymosin alpha-1 or placebo every 12 h for 7 days; 28-day mortality 23.4 % vs 24.1 % (HR 0.99), no difference.
Chronic hepatitis B (Cochrane review, 2026): 10 RCTs, 1,349 participants (1991–2018); possible lower mortality (RR 0.53) but very-low-certainty evidence; reviewers could not say whether it helps.

Mechanism, as reported

Thought to boost T-cell function, promoting maturation of thymocytes into active T cells; the exact molecular target is not settled.

Practical record

Elimination half-life: 2.0 h (confidence: medium). Human subcutaneous PK: serum half-life about 2 h (means 1.8–2.6 h across formulations); peak at 0.7–2 h, back to near baseline by 24 h.
Studied routes: subcutaneous, intramuscular.
Typical supply: Zadaxin: 1.6 mg lyophilised powder vials (abroad, Rx). In the US only as research-market or compounded product, neither FDA-sanctioned.
Reconstitution: Zadaxin labels (as quoted by FDA): 1.6 mg powder, 1.6 mg/mL when reconstituted.

Safety signals on record

Reported adverse events: Usually mild: local irritation, redness and discomfort at the injection site; transient ALT (liver enzyme) flares were reported in hepatitis B trials (22 of 316 patients in one study); thyroid (TSH) abnormalities reported in combination with interferon.
Contraindications: No US label. Italian label: not for children; contraindicated in pregnancy and breastfeeding; caution in atopic or previously allergic people and in autoimmune disease.
Interactions: Italian label advises caution with other immunomodulating medicines. FDA flagged a theoretical risk in deliberately immunosuppressed patients (e.g. after stem-cell transplant: graft-versus-host disease, graft failure).
Notes: Peptides sold as 'thymosin alpha-1' in the US are unregulated research chemicals of unknown purity.

Sources

General wellness education, not medical advice. Unapproved compounds have no established dosing, purity standard, or safety profile.

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