Thymosin Alpha-1: Approved in 35+ Countries, Not by the FDA
A 28-amino-acid thymic peptide approved in 35+ countries as Zadaxin for immune support — but never FDA-approved in the United States.
What is Thymosin Alpha-1?
Thymosin Alpha-1 is a naturally occurring 28-amino-acid peptide fragment of prothymosin alpha, produced by the thymus gland and involved in T-cell maturation and innate immune signaling (notably through TLR9). Under the brand name Zadaxin, it's an approved drug in more than 35 countries — including Italy and China — for chronic hepatitis B and C and as a general immune-modulating agent. It has never been FDA-approved in the United States; as of a February 2026 reclassification, it holds FDA 503A Category 2 status, meaning US patients can access it only by prescription through licensed compounding pharmacies.
Quick facts
- Molecular Formula
- C129H215N33O55
- Molecular Weight
- 3108.3 g/mol
- CAS Number
- 62304-98-7
- Half-Life
- ~2 hours
- Class
- 28-amino-acid thymic peptide, TLR9 agonist
- Brand Names
- Zadaxin (approved in 35+ countries; not FDA-approved in the US)
- Legal Status
- Not FDA-approved in the US. FDA 503A Category 2 status (Feb 2026) — prescription-only via licensed compounding pharmacies
- Category
- Immune Support
- Storage
- Store the lyophilized vial refrigerated (2-8°C) or at -20°C, away from light. Once reconstituted, keep refrigerated at 2-8°C and use promptly.
Dosing at a glance
1.6mg vial reconstituted with 1mL bacteriostatic water per the Zadaxin clinical dosing convention, where available. Run the numbers in the reconstitution calculator.
Doses shown are those reported in published research, not a recommendation.
Thymosin Alpha-1 Mechanism of Action
A Thymic Immune-Signaling Peptide
Thymosin Alpha-1 is a fragment of prothymosin alpha, naturally produced by the thymus gland, where it plays a role in T-cell maturation and differentiation. It also acts as a TLR9 (Toll-like receptor 9) agonist, engaging innate immune pathways — a dual innate/adaptive immune-signaling profile that underlies its use as a general immune-modulating agent.
Broad Immunomodulatory Applications
Because it touches both innate and adaptive immunity, Thymosin Alpha-1 has been studied across a wide range of applications — chronic viral hepatitis, as an adjunct to vaccines and cancer immunotherapy, and general immune support — rather than one narrow mechanism-to-outcome pathway.
Thymosin Alpha-1 Dosage and Protocols
Zadaxin Clinical Dosing Convention
Where Zadaxin is approved, standard dosing is 1.6mg subcutaneous twice weekly. This is the most widely cited protocol in both clinical and research-vendor contexts, reflecting the international drug's actual approved label dosing rather than an informal research-community estimate.
Thymosin Alpha-1 Side Effects and Safety
Established International Safety Record
Given decades of use as an approved drug abroad, Thymosin Alpha-1 has a comparatively well-characterized safety profile for a peptide in this catalog — commonly reported effects include mild injection-site irritation. As an immune-activating compound, it's contraindicated in organ transplant recipients on immunosuppressive therapy (it could counteract the immunosuppression) and treated with caution in autoimmune conditions. Pregnancy is a precautionary contraindication.
Thymosin Alpha-1 Research Evidence
Approved Abroad, Not in the US
As Zadaxin, Thymosin Alpha-1 is an approved drug in more than 35 countries — including Italy, China, South Korea, and India — for chronic hepatitis B and C and as an immunomodulatory agent, with decades of international clinical use and post-marketing data. It has never completed FDA approval in the United States.
Regulatory and Legal Status
Thymosin Alpha-1 is not FDA-approved in the US. As of a February 2026 reclassification, it holds FDA 503A Category 2 status, meaning it can legally be dispensed in the US only by prescription through a licensed compounding pharmacy — a different (and more regulated) pathway than an unapproved research chemical, but still short of full FDA drug approval. Verify any US source is a licensed compounding pharmacy operating under a valid prescription.
Thymosin Alpha-1 Stacking and Combinations
Standalone Immune Support
Thymosin Alpha-1 is typically used as a standalone immune-support compound, sometimes alongside standard-of-care treatment for chronic viral hepatitis in countries where it's approved, rather than combined with other peptides in a stacking protocol.
Thymosin Alpha-1 Pharmacokinetics
Short Half-Life Supporting Twice-Weekly Dosing
Thymosin Alpha-1 has a plasma half-life of approximately 2 hours, though its immunomodulatory effects — driven by downstream immune-cell signaling rather than sustained receptor occupancy — persist well beyond the compound's direct circulating presence, consistent with its twice-weekly clinical dosing schedule.
Contraindications
- pregnancy
- organ transplant recipients on immunosuppressive therapy
- known hypersensitivity
Frequently asked questions
What is a typical Thymosin Alpha-1 dose?
Published research protocols report 1.6 mg, 2x weekly. This is the range described in the literature, not a recommendation.
What is the half-life of Thymosin Alpha-1?
~2 hours.
Is Thymosin Alpha-1 backed by strong evidence?
Thymosin Alpha-1 carries a REGEN research grade of A. REGEN Research Tier A — human RCTs, plus approval in roughly 35 countries as Zadaxin — but not by the FDA, and not for general immune support.
How is Thymosin Alpha-1 administered?
Routes reported in the literature: subcutaneous.
Who should avoid Thymosin Alpha-1?
Contraindications noted in the literature include pregnancy, organ transplant recipients on immunosuppressive therapy, known hypersensitivity.
References
- Thymosin-α1 for people with chronic hepatitis B (Cochrane Review CD014610) · Htet NH, et al. · 2022