At a glance

What is thymosin alpha-1?

Thymosin alpha-1 (abbreviated Tα1) is a 28-amino acid peptide originally isolated from thymus-gland extract, where immune cells mature. It acts as an immune modulator and is an approved drug in more than 35 countries¹ under the name thymalfasin (brand Zadaxin), used for chronic hepatitis and as an immune adjunct. In the United States, it is not approved and is sold through wellness and compounding channels for immune support.

How does it work?

Thymosin alpha-1 modulates the immune system in several ways:

• Influencing the maturation and function of T-cells
• Boosting signals such as interferon-gamma and interleukin-2
• Enhancing natural killer-cell activity, partly through Toll-like receptor pathways

This mechanism is supported by laboratory and clinical immune data, such as improved immune-cell markers in sepsis studies. This describes how it acts on the immune system, separate from whether it improves outcomes in a given condition.

Is it FDA-approved?

No, not in the United States. It is approved and marketed as thymalfasin (Zadaxin) in many other countries for chronic hepatitis B and C and as an immune/vaccine adjunct. It was not among the peptides reviewed favorably at the 2026 FDA compounding advisory meetings. Superpower does not prescribe, sell, or facilitate access to thymosin alpha-1.

What does the research show?

It has real human trials, with mixed results, in specific immune conditions, not general wellness. Three trials report outcome figures.

In chronic hepatitis B, a separate trial reported modest effects on viral and liver markers⁵. In sepsis, a 2025 meta-analysis⁶ found the apparent benefit disappeared in higher-quality studies. So real human efficacy exists for some immune and antiviral uses but is mixed, and none of it supports a general wellness or longevity use.

Frequently asked questions

Is thymosin alpha-1 FDA-approved?

No. As of September 2026, thymosin alpha-1 is not FDA-approved for any indication in the United States. It is approved internationally as Zadaxin in more than 35 countries¹, including for chronic hepatitis B. In the US, it is not eligible for 503A compounding as of September 2026: FDA lists its Category 2 nomination as withdrawn⁷, and it is not on the 503A bulks list or in Category 1.

What is the difference between thymosin alpha-1 and thymosin beta-4?

Thymosin alpha-1 and thymosin beta-4 were both first isolated from thymosin fraction 5⁸, a calf thymus extract⁹, but they have distinct structures and mechanisms. Thymosin alpha-1 is a 28-amino-acid N-acetylated peptide¹⁰ that acts on dendritic cells and T-cell maturation, partly through TLR9 signaling¹¹. Thymosin beta-4 is a 43-amino-acid peptide¹² that acts primarily on actin polymerization, tissue repair, and anti-inflammatory pathways at injury sites. Their clinical applications are largely non-overlapping: Tα1 is studied for immune-modulating effects in infection and oncology; thymosin beta-4 is studied for wound healing and tissue regeneration.

How long does thymosin alpha-1 take to work?

Response kinetics depend on the indication. In hepatitis B trials, the characteristic response pattern includes a delayed peak effect: a meta-analysis¹³ published in Alimentary Pharmacology and Therapeutics in 2001 found that the virological response grew after treatment stopped and was clearest about 12 months after therapy ended, rather than during the treatment period itself.

What are the side effects of thymosin alpha-1?

Thymosin alpha-1 has a low reported adverse-event burden in clinical trials. In a hepatitis B trial¹⁴, all adverse drug reactions were mild, and most involved fluctuating liver enzymes. In the phase 3 TESTS sepsis trial¹⁵, no safety outcome differed significantly from placebo.

What happened to thymosin alpha-1 on FDA's compounding list?

FDA lists thymosin alpha-1⁷ among bulk substances that were previously in its 503A Category 2 list and whose nominations were withdrawn by the nominators. That was not a finding that it is safe, and thymosin alpha-1 is not on the 503A bulks list or in Category 1, so it is not eligible for 503A compounding as of September 2026. This does not affect thymosin alpha-1's international approval status as Zadaxin, which remains unchanged in the more than 35 countries¹ where it holds regulatory approval.

Are peptides legal in 2026?

The legality of specific peptides in the United States depends on their individual regulatory status. Some peptides are FDA-approved drugs, available by prescription through standard pharmacy channels. Others are available through licensed 503A compounding pharmacies by prescription, which requires a patient-specific prescription from a licensed provider. As of September 2026, thymosin alpha-1 is not in that group: FDA lists its Category 2 nomination as withdrawn, and it is not eligible for 503A compounding. No peptide in this class is available over the counter in the United States.

Can you get thymosin alpha-1 without a prescription?

No. Thymosin alpha-1 is not available over the counter in any legally recognized formulation in the United States. As of September 2026, it is also not eligible for 503A compounding, so no licensed compounding pharmacy can legally prepare it. Any product marketed as thymosin alpha-1 in the US is operating outside the legal framework and should be approached with caution.

What did the 2025 TESTS trial find about thymosin alpha-1 in sepsis?

The TESTS trial, published in the BMJ in 2025 by Wu, Pei, and colleagues, was a multicenter, double-blind, placebo-controlled phase 3 RCT that enrolled 1,106 adults with sepsis¹⁵ (1,089 analyzed) across 22 Chinese medical centers. Patients received subcutaneous Tα1 every 12 hours for seven days or placebo. Twenty-eight-day all-cause mortality was 23.4% in the Tα1 group versus 24.1% in the placebo group (hazard ratio 0.99; 95% CI 0.77-1.27; p = 0.93). No secondary or safety outcome differed significantly. The authors concluded there was no clear evidence that Tα1 reduces 28-day mortality in sepsis.

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