The Immune Modulator
Thymosin alpha-1
Also known as Tα1 · TA1 · Thymalfasin · Zadaxin
At a glance
- What it is
- The best-evidenced immune peptide here, and it is not FDA-approved.
- Evidence
- Clinical trialsHow the tiers workThymosin alpha-1 is the best-evidenced immune peptide in this library and it is not FDA-approved. Why this tier
- Routes
- Subcutaneous
- Category
- Immune, Longevity
- WADA
- We do not state a status
- Amino acids
- 28
Putting Thymosin alpha-1 on a schedule, with the vial it comes from and a log of each dose, happens in the app: Claritide for iPhone.
Why this tier
Thymosin alpha-1 is the best-evidenced immune peptide in this library and it is not FDA-approved. It holds orphan-drug designation in the US and marketing approval in a number of other countries, but there is no US label. A DailyMed search returns nothing. Its hepatitis B data is genuinely positive. Its two biggest modern trials are both negative: a 1,089-patient Phase 3 in sepsis found no mortality benefit (23.4% vs 24.1%, P=0.82), and a 5,352-patient COVID-19 meta-analysis found no effect. We include those because a library that only publishes the wins is a catalogue.
What it is
Thymosin alpha-1 is a 28-amino-acid peptide from thymic epithelial cells, isolated in the 1970s. It has genuine positive trial data in chronic hepatitis B and marketing approval in a number of countries outside the United States. It is not FDA-approved: it holds orphan-drug designation, which is not the same thing, and a search of the US drug label database for thymalfasin returns nothing. Its two largest modern trials, in sepsis and in COVID-19, are both negative.
How it works
Thymosin alpha-1 is an immunomodulator rather than an immune stimulant, and the distinction matters. It promotes maturation of T cells, activates natural killer cells and dendritic cells, and restores HLA-DR expression on monocytes, a marker that falls in severe illness and is used as a readout of immune paralysis. In sepsis studies it improves the ratio of CD4-positive to CD8-positive T cells. The through-line is that it pushes an under-responding immune system back toward a normal configuration, which is why the trials that exist are in conditions of immune exhaustion rather than in healthy adults.
Commonly researched ranges
- Chronic hepatitis B (Phase 3)1.6 mg
The dose used in Chien 1998. This is a trial dose from a published study, not a community figure.
- Sepsis (TESTS, Phase 3, negative trial)1.6 mg
The dose used in the 1,089-patient TESTS trial, which found no mortality benefit. Included so the schedule is on the record alongside its result.
- Under 40 kg40 mcg/kg
× body weight in kg
Weight-based dosing reported for patients under 40 kg.
| Phase | Amount | Frequency | Timing | Note |
|---|---|---|---|---|
| Chronic hepatitis B (Phase 3) | 1.6 mg | Twice weekly, 3–4 days apart, for 26 weeks | Not established in available literature | The dose used in Chien 1998. This is a trial dose from a published study, not a community figure. |
| Sepsis (TESTS, Phase 3, negative trial) | 1.6 mg | Every 12 hours for 7 days | Not established in available literature | The dose used in the 1,089-patient TESTS trial, which found no mortality benefit. Included so the schedule is on the record alongside its result. |
| Under 40 kg | 40 mcg/kg× body weight in kg | Twice weekly | Not established in available literature | Weight-based dosing reported for patients under 40 kg. |
- Cycle guidance
- The hepatitis B protocol runs 26 weeks continuously. We publish no community cycling schedule: the widely circulated 5-on/2-off, 8-weeks-on/8-weeks-off pattern rests on a vendor blog, and the derived claim that it delivers several times the trialled weekly dose inherits that source. The trial-side figure stands on its own.
- Goals
- Immune, Longevity
Where this sits with the FDA
FDA 503A bulks · nominated but withdrawn
Do not read "FDA-approved" anywhere on this record. Thymosin alpha-1 holds US orphan-drug designation, which is a development incentive rather than a marketing authorisation, and a query of the US drug label database for thymalfasin returns zero results. There is no marketed US label. It is approved in a number of other countries. It sits on FDA’s nominated-but-withdrawn table, was not in the July 2026 advisory batch, and is not among the five compounds FDA has queued for a later advisory committee meeting.
Last change · Placed in Category 2 in September 2023 and removed when the nominator withdrew; the exact withdrawal date is not verified in our sources.
Primary sourceWADA status
We searched the 2026 Prohibited List text for "thymosin"; the only hit is the thymosin-β4 and TB-500 line, which is a different molecule. Thymosin alpha-1 holds human therapeutic approval in multiple countries, so the S0 catch-all does not obviously capture it. We do not state a status, and specifically we do not say "banned".
We looked and could not resolve it. This is not the same as "not prohibited". Treat it as unresolved and check with your federation before competing.
Mixing recipes our sources document
- 32 units1.6 mg
- 16 units0.8 mg
No source we hold states a beyond-use window for this pairing, so we publish none. Go by what is printed on your own vial, and read the storage note below.
Week by week
Not established in available literature.
Side effects
Common
- Injection-site reactions
- Transient fatigue
- Mild flu-like symptoms
- Overall adverse-event rate in the largest trial66.4% in trials
Uncommon
- Transient ALT flare
Rare
- Headache, GI discomfort, joint pain with hand swelling
Contraindications and interactions
Do not use if
- Organ transplant recipients on immunosuppressants: mechanistically opposed to anti-rejection therapy
- Active autoimmune flare
- Pregnancy and breastfeeding: Zadaxin was pregnancy Category C
- Under 18
- Known hypersensitivity
Interactions
- Immunosuppressants and anti-rejection therapyavoid
Thymosin alpha-1 works by restoring T-cell function, which is the exact thing anti-rejection therapy is suppressing.
Absence from this list does NOT mean safety. It means lack of research.
Storage and cost
- Storage
- Lyophilised powder, refrigerated. None of our sources flag thymosin alpha-1 as benzyl-alcohol sensitive, so the sterile-water-only rule does not apply to it, but no source states a beyond-use window for the reconstituted vial either, so we publish none.
- Cost
- Not established in available literature
Stacks with
Curated stacks including it
Thymosin alpha-1 in full
The correction first
Thymosin alpha-1 is routinely described as FDA-approved. It is not.
What it holds in the United States is orphan-drug designation, which is an incentive granted to encourage development of treatments for rare diseases: tax credits, fee waivers, a period of market exclusivity if approval ever arrives. It is not approval. It says nothing about whether a drug works. The dispositive check is the label: a query of the US structured product label database for thymalfasin returns zero results and the message "No Drug Package Labels found". There is no marketed US product.
It is approved in a number of other countries, where it is sold as Zadaxin. Two figures that circulate widely, "4,400+ published articles" and "prescribed in 35+ countries", trace only to vendor dosing blogs and a single aggregator, and we do not publish either as fact.
What it is
A 28-amino-acid peptide produced by thymic epithelial cells, isolated in the 1970s by Allan Goldstein's group at George Washington University. The thymus is where T cells mature, and it involutes with age; a peptide from that tissue that modulates T-cell function is a reasonable thing to be interested in.
Mechanistically it promotes T-cell maturation, activates natural killer and dendritic cells, and restores monocyte HLA-DR expression, a surface marker that collapses in severe illness and is used clinically as a measure of immune paralysis. In sepsis studies it improves CD4/CD8 ratios. Note the pattern: every one of those is a correction of a suppressed immune system, not an amplification of a healthy one.
What the evidence actually supports
This record is the reason we split evidence by indication, because a single tier would lie in both directions.
Chronic hepatitis B: positive. Chien 1998, 98 patients: 40.6% achieved a complete virological response at 26 weeks against 9.4% untreated. A second trial, Mutchnick 1999 with 97 patients, showed a trend in the same direction that did not reach significance: 25% against 13%. A 2022 trial in hepatitis B acute-on-chronic liver failure, 120 patients, found 75.0% against 53.4% transplant-free survival, P=0.030.
Sepsis: negative, and this is the trial worth knowing about. TESTS, published in the BMJ in January 2025, is a multicentre, double-blind, randomised, placebo-controlled Phase 3 trial. 1,089 patients in the modified intention-to-treat analysis. 1.6 mg subcutaneously every 12 hours for seven days. 28-day all-cause mortality was 23.4% on thymosin alpha-1 and 24.1% on placebo, hazard ratio 0.97, 95% confidence interval 0.76 to 1.24, P=0.82. That is not a near miss. It is a well-powered null result in exactly the population where the mechanism should have shown up.
COVID-19: negative. A 2022 meta-analysis across 5,352 patients: risk ratio 1.03, 95% CI 0.60 to 1.75, P=0.92. Early observational studies looked positive and are best explained by confounding by indication.
Immune support in healthy adults: no trials. This is what it is mostly bought for.
Running it
The trial doses are on the record and they are real numbers from published studies: 1.6 mg subcutaneously twice weekly, three to four days apart, for 26 weeks in hepatitis B; 40 mcg/kg twice weekly under 40 kg. Reconstitution reported at 10 mg in 2 mL of bacteriostatic water gives 5 mg/mL, putting 1.6 mg at 0.32 mL, or 32 units on a U-100 syringe.
We deliberately publish no community protocol. The widely circulated 1.5 mg daily, five days on, two off, eight weeks on and eight weeks off pattern comes from a vendor blog, and the tidy editorial hook that follows from it, that community practice runs several times the trialled weekly dose, inherits that source. The comparison would be worth making if the numerator were sourced. It is not.
Half-life is null. A "2 to 3 hours, peaking 1 to 2 hours after injection" figure circulates from a community dosing site and could not be traced to any pharmacokinetic paper.
The honest summary
Thymosin alpha-1 has the best evidence base of any immune peptide in this library. Its tolerability data is genuinely good: in the TESTS trial, adverse events ran 66.4% against 67.6% on placebo. And its two biggest modern trials both failed. A library that only publishes the wins is a catalogue, and the sepsis trial is the single most informative document about this compound in existence.
Sources
- 1TESTS trial, BMJ, 15 January 2025 · thymosin alpha-1 in sepsis, n=1,089, negative for 28-day mortality
- 2DailyMed query for thymalfasin · total_elements: 0, no US drug package labels found
- 3RxList · Zadaxin (thymalfasin) monograph
- 4FDA · Certain bulk drug substances for use in compounding that may present significant safety risks
Every record in this library is free to read, and it stays free. Your own schedule, vials and log live in the app: Claritide for iPhone.