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Moderate EvidencePeptide

Thymosin Alpha-1

Zadaxin / thymalfasin

Immune-Modulating Peptide

Last updated: August 2026
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Evidence Level: Moderate Evidence

Some human data, strong animal studies, plausible mechanism

Regulatory Status: Approved as Zadaxin in more than 35 countries, primarily for chronic hepatitis B. Not FDA-approved in the United States. Added to FDA 503A Category 2 in 2023 and removed from Category 2 in September 2024, without being added to the permitted list.

Thymosin alpha-1 is a 28-amino-acid peptide your thymus gland makes naturally to help T-cells mature. A synthetic version has been an approved prescription drug in more than 35 countries for decades, mostly for chronic hepatitis B. It is not approved in the US, which is why Americans encounter it through compounding pharmacies and gray-market vendors.

How It Works

Your thymus is a small gland behind the breastbone where T-cells learn to tell your own tissue apart from invaders. It is also one of the first organs to shrink as you age, a process called thymic involution, which is a major reason immune function declines with age.

Thymosin alpha-1 is one of the signaling peptides the thymus produces.

What it actually does. Rather than stimulating or suppressing immunity in a blunt way, thymosin alpha-1 acts as a modulator. It promotes the maturation and differentiation of T-cells, and it activates Toll-like receptors, particularly TLR2 and TLR9, which are part of how your innate immune system recognizes threats and coordinates a response. The practical effect is that it appears to help a sluggish immune response engage without pushing a hyperactive one further.

Where the evidence is strongest. Chronic hepatitis B. That is the indication it is approved for in most countries, and the one supported by multiple randomized trials showing improved sustained virological response, both alone and combined with interferon.

Where the evidence is weaker than you would think. Sepsis is the honest test case. Thymosin alpha-1 was an attractive candidate because sepsis involves immune paralysis, and earlier smaller trials looked promising. Then the TESTS trial enrolled 1,106 adults with sepsis across 22 centers in China and reported a 28-day mortality of 0.99, with a confidence interval running from 0.77 to 1.27 and a p-value of 0.93. Mortality was 23.4% on thymosin alpha-1 and 24.1% on placebo. That is about as flat a null result as a trial can produce.

This pattern is worth internalizing: promising small trials, then a large well-powered trial that finds nothing. It happens constantly, and it is the reason we grade by trial size and design rather than by how many positive studies exist.

On the anti-aging pitch. Thymosin alpha-1 is increasingly marketed for immune support and healthy aging in people with no diagnosed condition. There is no trial evidence for that use. The peptide is real, the approvals are real, and the specific claim that a healthy adult benefits from taking it has not been tested.

Evidence by Outcome

Each claim below is rated using a GRADE-aligned five-tier system. Same molecule, different outcomes, different evidence.

Evidence at a glance

How we grade

Want to see how this compares to every other peptide? Browse the full evidence matrix, where every graded claim is grouped by outcome and sorted by certainty.

Immune functionVery Low certainty

Improves sustained virological response in chronic hepatitis B.

Population: Adults with chronic hepatitis B infection

Why this rating

A 2026 Cochrane review of 10 randomized trials (1,349 participants; trials published 1991–2018) concluded that the evidence is very uncertain for nearly all patient-relevant outcomes and did not establish a reliable clinical benefit. For HBV-related morbidity, the pooled estimate was imprecise (RR 0.86, 95% CI 0.54–1.40; 3 studies, 854 participants). The review found no ongoing trials. Earlier reports of improved virological response come from old studies with substantial reporting limitations and inconsistent results; regulatory approvals in some countries do not resolve those evidence gaps.

GRADE downgrade factors applied

  • Risk of bias: Methodological flaws in the studies (e.g. lack of blinding, allocation concealment, high attrition).
  • Inconsistency: Heterogeneous results across studies that cannot be explained, or pending replication.
  • Imprecision: Wide confidence intervals or small sample size, the estimate could be substantially different.

Supporting studies

Reviewed October 2026· Designing Longevity EditorialHow we grade
Immune functionHigh certainty

Does not reduce 28-day mortality in severe sepsis.

Population: Adults with severe sepsis

Why this rating

The TESTS trial was a large, multicentre, double-blinded, trial of 1,106 patients that found no mortality benefit, with a of 0.99 and a confidence interval running from 0.77 to 1.27. This is a well-powered null result that supersedes earlier smaller positive trials, and we have high confidence in it. It is included here because a null finding of this quality is exactly as informative as a positive one.

Supporting studies

Reviewed October 2026· Designing Longevity EditorialHow we grade
Immune functionVery Low certainty

Improves immune function or supports healthy aging in adults without a diagnosed condition.

Population: Healthy adults. This is the population most wellness clinics prescribe it to.

Why this rating

We could not identify randomized controlled trials of thymosin alpha-1 in healthy adults for immune support, infection prevention, or aging outcomes. The argument rests on the peptide's role in T-cell maturation and the fact that the thymus shrinks with age. That is a mechanism, not a result, and the largest trial in a population with genuine immune dysfunction came back null.

GRADE downgrade factors applied

  • Indirectness: Studied population, intervention, comparator, or outcome differs from the question being answered.
  • Imprecision: Wide confidence intervals or small sample size, the estimate could be substantially different.

Supporting studies

Reviewed October 2026· Designing Longevity EditorialHow we grade

Potential Benefits

  • A genuinely approved prescription drug in more than 35 countries, with decades of clinical use
  • Multiple randomized trials support improved viral response in chronic hepatitis B
  • Modulates immune function rather than blunt stimulation or suppression
  • Long safety record from real-world clinical use, with a generally mild side effect profile
  • Has been studied in a wide range of conditions including sepsis, cancer adjuvant therapy, and vaccine response
  • Its mechanism, T-cell maturation and Toll-like receptor activation, is well characterized

Risks & Considerations

  • Not FDA-approved in the US, so American supply is or gray-market with variable quality
  • The largest trial ever conducted, in sepsis, was null. Earlier positive results did not hold up at scale
  • No trial evidence for the immune support or anti-aging uses it is most commonly marketed for today
  • Modulating immune function is not risk-free in people with autoimmune disease
  • Most of the strongest trial data comes from a small number of countries and research groups
  • Injection site reactions are the most common reported effect
  • Expensive when obtained as branded Zadaxin, and unverifiable when obtained otherwise

Dosing Information

Unlike most peptides in this category, thymosin alpha-1 has approved labeled dosing in the countries where it is registered. Off-label wellness dosing is clinic convention.

  • •Approved hepatitis B dosing: 1.6mg subcutaneously twice weekly
  • •Sepsis trials used higher and more frequent dosing regimens
  • •Wellness and immune support protocols commonly use 1.6mg once or twice weekly, which mirrors the hepatitis dose without the evidence
  • •Supplied as a powder requiring
  • •Cycles of several weeks with breaks are common clinic practice rather than a trial-derived schedule

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Practical Tips

  • 1If you have chronic hepatitis B, this is a conversation for a hepatologist, not a wellness clinic
  • 2If you have an autoimmune condition, discuss this with your specialist before starting. Modulating T-cell function cuts both ways
  • 3Understand that the immune support pitch is an extrapolation. The approvals are for a specific viral disease
  • 4Zadaxin is the branded product. and gray-market versions are not equivalent and are not verified
  • 5The null sepsis result is a useful reality check on how much this peptide can do in a seriously ill person

Key Research

Thymosin alpha-1 in chronic hepatitis B

Multiple randomized controlled trials, 1998

A pivotal randomized trial in 98 hepatitis B patients reported complete virological response in 40.6% on thymosin alpha-1 versus 9.4% of untreated controls. Additional randomized trials support improved sustained virological response alone and with interferon-alpha.

The efficacy and safety of thymosin alpha-1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial

BMJ, 2025

28-day all-cause mortality was 23.4% on thymosin alpha-1 versus 24.1% on placebo ( 0.99, 95% CI 0.77 to 1.27, P=0.93). The largest trial of thymosin alpha-1 ever conducted found no mortality benefit in sepsis, and no secondary or safety outcome differed significantly either.

View Study

The efficacy of thymosin alpha-1 as immunomodulatory treatment for sepsis: a systematic review of randomized controlled trials

BMC Infectious Diseases, 2016

A systematic review of earlier randomized trials found a possible mortality benefit but flagged small sample sizes and methodological limitations, calling for larger trials. The later TESTS trial did not confirm the benefit.

View Study

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Frequently Asked Questions

Is thymosin alpha-1 FDA-approved?

No, not in the United States. It is approved as Zadaxin in more than 35 countries, including China, Italy, and the Philippines, primarily for chronic hepatitis B. In the US it was placed in of the FDA's bulk substances list in 2023 and removed from Category 2 in September 2024, but it has not been added to the permitted list, leaving it in a gray zone.

What is thymosin alpha-1 used for?

Its main approved use is chronic hepatitis B, where multiple randomized trials support improved viral response. It has also been studied in sepsis, as an adjunct in cancer treatment, and for improving vaccine responses. In the US wellness market it is mostly sold for general immune support and healthy aging, which is not an indication anyone has actually tested.

Does thymosin alpha-1 boost your immune system?

It modulates immune function rather than boosting it in a blunt sense. It promotes T-cell maturation and activates Toll-like receptors that coordinate immune responses. Whether that translates into fewer infections or better health in someone without a diagnosed immune problem has not been studied in a randomized trial. The largest trial ever run, in sepsis patients with severe immune dysfunction, found no mortality benefit.

Why did the sepsis trial fail?

The TESTS trial enrolled 1,106 patients across 22 centers and reported a 28-day mortality of 0.99, with a confidence interval running from 0.77 to 1.27. Mortality was 23.4% on the drug and 24.1% on placebo. Earlier, smaller trials had looked promising. This is a very common pattern in medicine: small trials produce noisy, often flattering results, and larger properly powered trials correct them. It is one of the main reasons we weight trial size and design heavily in our grading.

What are the side effects of thymosin alpha-1?

It is generally well tolerated, with injection site reactions being the most commonly reported effect, which is consistent with its decades of use as an approved drug. The more meaningful caution is for people with autoimmune conditions: a drug that promotes T-cell activity is not obviously safe when your T-cells are already attacking your own tissue. Discuss it with your specialist first.

How is thymosin alpha-1 different from thymosin beta-4 (TB-500)?

They share a name and almost nothing else. Thymosin alpha-1 is a 28-amino-acid immune-modulating peptide with regulatory approvals in dozens of countries. Thymosin beta-4, which TB-500 is a fragment of, is an actin-regulating protein studied for tissue repair. Different molecules, different mechanisms, different evidence bases. The shared "thymosin" label reflects where they were originally isolated, not what they do.

Can I get thymosin alpha-1 in the US?

Not as an approved product. Some compounding pharmacies dispense it and gray-market vendors sell it, but its status is unsettled and quality is not verified. If you have a condition it is actually approved for elsewhere, such as chronic hepatitis B, that is a conversation for a hepatologist who can weigh it against the treatments that are approved here.

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Disclaimer: This information is for educational purposes only and is not medical advice. Many peptides discussed are not FDA-approved for human use. Always consult with a qualified healthcare provider before starting any treatment. Evidence levels and regulatory status can change, this content was last updated August 2026.

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