Thymosin Alpha-1
| Category | Other Peptides |
| Goals | Immune Support |
| Evidence level | Clinical (randomized human trials, mostly hepatitis B; approved abroad) |
| Legal status | Prescription drug in 30+ countries; not FDA-approved in the US |
| FDA status | Not approved; PCAC recommended against 503A bulks-list inclusion (Dec 4, 2024) |
| Half-life | ~2 hours after subcutaneous injection (human data) |
| Routes | Subcutaneous · Intramuscular |
| CAS / MW / Sequence | 62304-98-7 · ~3108.3 g/mol · Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN (28 aa, N-acetylated) |
| Last reviewed | 2026-06-07 |
In one line
An acetylated 28-amino-acid thymic peptide that modulates the immune system; sold abroad as the prescription drug Zadaxin (thymalfasin) for chronic hepatitis B and immune support, but not FDA-approved in the United States.
Evidence at a glance
Unlike many “research peptides,” Thymosin Alpha-1 has a substantial clinical record — it is an approved drug in 30+ countries with randomized trials (strongest for chronic hepatitis B and vaccine response). However, it is not approved by the FDA, US-marketed evidence for “wellness/longevity” uses is weaker, and quality of research-grade material varies. See Evidence Grading Explained and the Disclaimer.
Key Takeaways
- A naturally occurring thymic peptide (fragment of prothymosin alpha) made synthetically as the drug thymalfasin / Zadaxin.
- Acts as an immune modulator — works largely through Toll-like receptors (TLR2/TLR9) on dendritic cells to promote a Th1 / T-cell response.
- Approved in 30+ countries for chronic hepatitis B (and hepatitis C in some), as a vaccine enhancer, and as immune support; investigated for sepsis, cancer immunotherapy, and COVID-19.
- Not FDA-approved in the US, and the FDA’s advisory committee (PCAC, Dec 4, 2024) recommended against adding it to the 503A bulks list — so it is not eligible for traditional 503A compounding.
- Among peptides marketed for Immune Support, it has the most clinical backing — contrast with the research-tier Thymalin.
What Is It
Thymosin Alpha-1 (Tα1) is a 28-amino-acid peptide originally isolated from thymosin fraction 5 of calf thymus and corresponding to a naturally produced fragment of the precursor protein prothymosin alpha. The therapeutic version is a chemically synthesized, N-terminally acetylated peptide marketed as thymalfasin under the brand name Zadaxin. As a thymic peptide it plays a physiological role in maturation and regulation of T-cells, which is the basis for its use as an immune-modulating drug.
Mechanism of Action
Thymosin Alpha-1 is an immunomodulator rather than a simple stimulant — it tends to restore balanced immune function:
- TLR signaling (clinical/mechanistic) — binds TLR2 and TLR9 on myeloid and plasmacytoid dendritic cells, driving MyD88/TRIF signaling and dendritic-cell maturation.
- T-cell differentiation (clinical/mechanistic) — promotes maturation of T-cells and a Th1-polarized response, increasing CD4+ and CD8+ effector populations.
- Cytokine modulation (clinical) — increases IL-2, IL-12 and interferon production while helping down-regulate excessive inflammation in some settings.
- NK-cell and antiviral activity (clinical) — enhances natural-killer-cell activity and type-I interferon responses, supporting antiviral defense (the rationale for hepatitis use).
Limitations
Mechanisms are well described in immunology literature, but the exact contribution of each pathway to clinical benefit — and the optimal use outside approved indications — is still debated.
Evidence by Outcome
| Outcome | Evidence | Notes |
|---|---|---|
| Chronic hepatitis B | Approved / Clinical | Strongest indication; multiple RCTs and meta-analyses; approved in 30+ countries |
| Chronic hepatitis C (with/without antivirals) | Approved / Clinical | Approved in some countries; adjunct to standard antivirals |
| Vaccine response (e.g. influenza in immunocompromised/elderly) | Clinical | Used as a vaccine enhancer; improved seroprotection in trials |
| Sepsis / critical illness | Clinical (mixed) | Trials suggest possible mortality benefit; not definitive |
| COVID-19 | Clinical (mixed) | Studied during the pandemic; signals of benefit in some cohorts, not conclusive |
| Cancer immunotherapy (adjunct) | Clinical / Preclinical | Investigated alongside chemo/checkpoint inhibitors; ongoing trials |
| General “immune boosting” / longevity | Anecdotal | Popular off-label use; not the basis of approvals |
Reported Dosing
Not medical advice
The schedule below reflects the approved hepatitis B regimen plus protocols reported in community/clinical sources. Dosing should be directed by a qualified prescriber. See Reconstitution & Dosing Math.
| Route | Dose (reported) | Frequency | Cycle |
|---|---|---|---|
| Subcutaneous (approved HBV) | 1.6 mg | Twice weekly | 6–12 months |
| Subcutaneous (reported immune support) | ~0.5–1.6 mg | 1–3× weekly | Variable (weeks) |
| Intramuscular | Similar to SC | As above | As above |
Pharmacokinetics
After subcutaneous injection, Thymosin Alpha-1 is rapidly absorbed with peak serum levels at roughly 2 hours and an elimination half-life of about 2 hours. It is not extensively protein-bound and is cleared by tissue and circulating aminopeptidases (proteolytic degradation). The relatively short half-life is consistent with the intermittent (twice-weekly) approved dosing schedule. See Half-Life & Pharmacokinetics.
Side Effects & Risks
- Generally well tolerated in clinical trials; the most common issues are injection-site reactions (redness, discomfort).
- Occasional reports of transient fatigue, malaise, or flu-like symptoms consistent with immune activation.
- Theoretical autoimmune concern: as an immune modulator, caution is discussed in autoimmune disease, though this is not well established.
- Immunogenicity / impurity concern: the FDA has flagged that compounded Tα1 products may carry peptide-impurity and immunogenicity risks depending on route and source.
- Sourcing risk: US research-grade material is unregulated; identity/purity vary — see Sourcing and Red Flags & Scams.
- See Side Effects & Risk Management.
Cycling
The approved hepatitis schedule runs continuously for 6–12 months, not in short on/off cycles. For off-label immune-support use, anecdotal protocols run a few weeks during illness or seasonally; no evidence-based cycling standard exists. See Cycling.
Stacks It Appears In
- Often discussed alongside other immune peptides such as Thymalin for Immune Support.
- No standardized, evidence-based stack exists; combining immune modulators increases uncertainty.
Comparisons
- Thymosin Alpha-1 vs Thymalin — Tα1 is a single defined, approved peptide with clinical trials; Thymalin is a thymic peptide extract used mainly in research/anecdotal contexts.
Sourcing & Quality
Outside countries where Zadaxin is sold as a regulated drug, Tα1 is offered as a lyophilized “research chemical,” so identity and purity are not guaranteed. Evaluate quality before trusting a product: How to Read a CoA, HPLC vs Mass Spec, Third-Party Testing, Red Flags & Scams. Reconstitution and storage: Reconstitution & Dosing Math, Storage & Handling. No vendors are endorsed here.
Legal & Regulatory Status
(As of 2026-06-07.) Thymosin Alpha-1 (thymalfasin / Zadaxin) is an approved prescription drug in 30+ countries (notably for chronic hepatitis B), but it is not FDA-approved in the United States. On its US compounding status: the bulk-substance nominations were withdrawn by their nominators, but the FDA chose to evaluate thymosin alpha-1 (free base and acetate) on its own initiative and proposed not to add it to the 503A Bulks List. At its December 4, 2024 meeting, the FDA’s Pharmacy Compounding Advisory Committee (PCAC) voted against including thymosin alpha-1 on the 503A bulks list, citing concerns such as immunogenicity risk and peptide-impurity/characterization complexity. It is therefore not on the 503A bulks list — i.e., not eligible for traditional 503A compounding. (Note: thymosin alpha-1 is not among the seven peptides on the separate July 2026 PCAC agenda.) Status varies by country and it may be prohibited in tested sport. See Regulatory & Legal Status.
FAQ
Is Thymosin Alpha-1 FDA-approved? No. It is not FDA-approved in the US. It is an approved prescription drug (Zadaxin) in 30+ other countries.
Does it actually work? For chronic hepatitis B and as a vaccine enhancer, it has real randomized-trial support and regulatory approval abroad. For general “immune boosting” or longevity, the evidence is weaker and largely off-label.
How is it taken? By subcutaneous injection (the approved hepatitis regimen is 1.6 mg twice weekly); intramuscular use is also reported.
Is it the same as Thymalin? No. Thymalin is a different thymic peptide preparation with far less clinical evidence.
References
- King R., Tuthill C. (2016). “Immune Modulation with Thymosin Alpha 1 Treatment.” Vitamins and Hormones.
- Garaci E. et al. (2007/2012). Reviews on thymosin alpha 1 in immunomodulation, infection, and cancer. Annals of the New York Academy of Sciences / Expert Opinion on Biological Therapy.
- Camerini R., Garaci E. (2024). “Comprehensive Review of the Safety and Efficacy of Thymosin Alpha 1 in Human Clinical Trials.” PubMed PMID 38308608.
- SciClone Pharmaceuticals — Zadaxin (thymalfasin) prescribing information; approved indications for chronic hepatitis B.
- U.S. FDA (2024). “December 4, 2024: Meeting of the Pharmacy Compounding Advisory Committee” — FDA proposed thymosin alpha-1 (free base/acetate) not be included on the 503A Bulks List; PCAC voted against inclusion. fda.gov
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