Is Thymosin Alpha-1 FDA-approved?

No. Thymalfasin (Zadaxin) is approved in over 35 countries for chronic hepatitis B, hepatitis C, and as an immune adjuvant, but it has not received FDA approval in the United States. Research-grade Thymosin Alpha-1 sold domestically is intended for laboratory research use only.

What is the standard clinical dose?

The approved Zadaxin protocol is 1.6 mg subcutaneously twice weekly, typically for 6–12 months, based on hepatitis B and C clinical trials.

Is Thymosin Alpha-1 detectable or banned in sport?

As of 2025, Thymosin Alpha-1 does not appear on the WADA prohibited substances list. It is an immunomodulatory peptide, not a performance-enhancing agent in the traditional sense.

How does Thymosin Alpha-1 differ from TB-500 (Thymosin Beta-4)?

Despite the shared 'thymosin' name, they are structurally unrelated peptides with distinct mechanisms. Thymosin Alpha-1 primarily modulates adaptive immunity via T-cell and dendritic cell pathways; Thymosin Beta-4 (TB-500) is involved in actin sequestration, tissue repair, and angiogenesis.

Longevity

Thymosin Alpha-1

Also known as: Tα1, Ta1, Thymalfasin, Zadaxin

Thymosin Alpha-1 (Tα1) is a 28-amino-acid thymic peptide with well-documented immunomodulatory activity. It is approved in over 35 countries as Zadaxin (thymalfasin) for...

Reviewed by the Helica Labs research team · Last updated August 10, 2026

Buy Thymosin Alpha-1

Overview

Thymosin Alpha-1 is a 28-amino-acid peptide originally isolated from bovine thymus fraction 5 by Allan Goldstein and colleagues in 1977. A synthetic version, thymalfasin (brand name Zadaxin, developed by SciClone Pharmaceuticals), has since been studied extensively in clinical settings and is approved in more than 35 countries for the treatment of chronic hepatitis B, as an adjunct to interferon therapy for chronic hepatitis C, and as an immune adjuvant in oncology contexts. Despite this robust international clinical track record, Thymosin Alpha-1 has not received FDA approval in the United States. Its regulatory status in the US means that any material available domestically is sold strictly as research-grade for laboratory research use only and is not authorized for human therapeutic administration. Thymosin Alpha-1 occupies a distinctive position among immune-modulating peptides: it is one of the few peptides in this class with a substantial body of published randomized clinical trial data, multiple systematic reviews, and documented real-world use across approved indications spanning decades. Its profile is meaningfully different from preclinical-only peptides, though US research users should remain aware of the unapproved status domestically.

How it works

Thymosin Alpha-1 exerts its effects primarily through the innate and adaptive immune systems. Its most well-characterized signaling action is activation of Toll-like receptor 9 (TLR9) on plasmacytoid dendritic cells, which triggers downstream type I interferon production and amplifies antiviral and antitumor immune responses. Beyond TLR9, it promotes maturation and antigen-presenting function of dendritic cells, driving a shift toward Th1-dominant immune responses characterized by increased IFN-γ and IL-2 production. Thymosin Alpha-1 also stimulates natural killer (NK) cell activity, enhancing cytotoxic responses against virally infected and malignant cells. Within the thymus, it supports T-cell maturation and differentiation, including the development of regulatory T cells, thereby contributing to immune homeostasis rather than unopposed immune activation. In immunocompromised states—such as critical illness, advanced cancer, or aging-related immune senescence—it has been shown to restore peripheral lymphocyte counts and functional T-cell responses. This combination of innate immune priming and adaptive immune restoration underpins its efficacy across its approved and investigational indications.

Researched effects

Clinical Antiviral immune support in chronic hepatitis B
Clinical Adjunct to interferon in chronic hepatitis C
Clinical Immune adjuvant in cancer settings
Clinical Reduced mortality in severe COVID-19 via T-cell restoration
Clinical Restoration of lymphocyte counts in critical illness
Anecdotal General immune optimization and longevity support

Evidence levels: Clinical (human trials) · Preclinical (animal/lab) · Anecdotal (community-reported).

Dosing reference

For research reference only — not a recommendation.

Clinical / studied dosing

The approved Zadaxin regimen is 1.6 mg subcutaneously twice weekly for 6–12 months. Dosing studied in hepatitis and critical illness contexts follows this schedule. Protocols outside approved indications lack standardized clinical guidance.

Community-reported dosing (anecdotal)

Community protocols typically mirror the clinical regimen: 1.6 mg SC twice weekly, with some users running shorter cycles of 4–8 weeks for immune support. Some report lower doses of 0.5–1.0 mg twice weekly. These uses are outside any approved indication and are unsupported by controlled evidence.

Half-life: Approximately 2 hours following subcutaneous administration, based on pharmacokinetic studies in humans.
Routes: Subcutaneous injection, Intramuscular injection

Safety & side effects

Thymalfasin (Zadaxin) has a well-characterized safety profile from decades of clinical use in approved countries. Common adverse effects are mild and include injection-site reactions. No serious immune-mediated adverse events have been consistently identified in clinical trials. Autoimmune conditions represent a theoretical caution. Thymosin Alpha-1 is not listed on the WADA prohibited substances list as of 2025. Research-grade material available in the US is not manufactured to pharmaceutical-grade standards and is not approved for human use.

Research summary

Thymosin Alpha-1 has the most robust clinical evidence base of any peptide in the longevity/immune category. Its efficacy for chronic hepatitis B has been demonstrated in multiple randomized trials and a meta-analysis. Adjunct use in HCV alongside interferon showed improved response rates. A 2020 retrospective clinical study demonstrated reduced mortality in severe COVID-19 patients. Systematic reviews of critical illness support its immune-restorative effects, though larger randomized controlled trials are warranted. Longevity and age-related immune decline applications remain preclinical or anecdotal.

FAQ

Is Thymosin Alpha-1 FDA-approved?: No. Thymalfasin (Zadaxin) is approved in over 35 countries for chronic hepatitis B, hepatitis C, and as an immune adjuvant, but it has not received FDA approval in the United States. Research-grade Thymosin Alpha-1 sold domestically is intended for laboratory research use only.
What is the standard clinical dose?: The approved Zadaxin protocol is 1.6 mg subcutaneously twice weekly, typically for 6–12 months, based on hepatitis B and C clinical trials.
Is Thymosin Alpha-1 detectable or banned in sport?: As of 2025, Thymosin Alpha-1 does not appear on the WADA prohibited substances list. It is an immunomodulatory peptide, not a performance-enhancing agent in the traditional sense.
How does Thymosin Alpha-1 differ from TB-500 (Thymosin Beta-4)?: Despite the shared 'thymosin' name, they are structurally unrelated peptides with distinct mechanisms. Thymosin Alpha-1 primarily modulates adaptive immunity via T-cell and dendritic cell pathways; Thymosin Beta-4 (TB-500) is involved in actin sequestration, tissue repair, and angiogenesis.

References

All content is for research and educational use only and is not medical advice. Products are sold for laboratory research only and are not for human consumption.