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Thymosin alpha-1

Also known as Thymalfasin, Tα1, Zadaxin, TA-1

Synthetic 28-amino-acid thymus peptide, registered in dozens of countries for chronic hepatitis B and C.

clinically proven Immune & longevity approved drug

At a glance

Category
Immune & longevity
Status
approved drug
Route
subcutaneous
Half-life
approximately 2 hours in serum (human, subcutaneous)
Onset
immunological parameters within days; clinical effect in hepatitis only after months
Molecular weight
approximately 3108 g/mol
Sequence
Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN (N-terminally acetylated, 28 amino acids)

Registered as thymalfasin (brand name Zadaxin) in more than 35 countries for the treatment of hepatitis B and C and as an immune stimulant and adjuvant 7 — the countries commonly named include Italy, China, Singapore, Mexico and several other South-East Asian and Latin American markets, though PeptideX has not been able to open a regulator's own register confirming that country list, and it is reproduced here from a review rather than from a marketing authorisation. What is verifiable is the negative: it is not approved by the FDA and holds no central EMA marketing authorisation, so in the US and the EU it is an unapproved drug 7. The material sold online as a 'research peptide' falls outside any pharmaceutical registration.

Doping status: Not listed by WADA (unlike thymosin beta-4)

Checked against the WADA 2026 Prohibited List. For a specific product and country, Global DRO is the lookup athletes are expected to use — the List names substances, not brand names.

Mechanism of action

Thymosin alpha-1 is the acetylated N-terminal fragment of prothymosin alpha and is produced naturally by thymic epithelium. It promotes the differentiation and maturation of T lymphocytes and shifts the immune response towards a Th1 profile, with increases in interferon gamma and interleukin-2.

The main known receptor interaction runs via Toll-like receptor 9 (and to a lesser extent TLR2) on dendritic cells and monocytes, leading to maturation of dendritic cells and antigen presentation. Increased NK cell activity has also been described.

The effect is modulatory and context-dependent: with a suppressed immune system the response is amplified, whereas in sepsis models damping of excessive inflammation has been described. Why the agent would work in both directions has not been fully elucidated mechanistically.

What the research shows

Of all the agents in this category, thymosin alpha-1 has by far the strongest dossier: dozens of randomised trials, a registered indication in dozens of countries and decades of safety data. At the same time the evidence is not unambiguous — the largest placebo-controlled phase III trial in the US missed its primary endpoint 1, and the agent does not appear in the EASL or AASLD guidelines for hepatitis B. For the popular applications outside hepatitis (general 'immune support', ageing, chronic fatigue) there is hardly any controlled evidence.

Research in humans

Several randomised trials in chronic hepatitis B using 1.6 mg twice weekly for 6 months showed a higher sustained response than untreated controls, with effects that peaked only 6-12 months after stopping. A Japanese randomised trial reported around 36% ALT normalisation in the 1.6 mg group after 72 weeks 3. A US phase III trial (Mutchnick et al., 97 patients), however, found no statistically significant advantage over placebo (14% versus 4% complete response) and the authors explicitly concluded that their results did not confirm earlier positive findings 1. Combination studies with lamivudine and with peginterferon showed better HBeAg seroconversion than monotherapy in meta-analyses 5. The agent has further been studied as a vaccine adjuvant in dialysis patients and older people, and during COVID-19 in observational and small randomised studies with mixed outcomes.

Animal and lab research

Animal models consistently show restoration of T cell counts after thymectomy, chemotherapy or irradiation, and a survival benefit in sepsis models. These models are built around immunosuppression and say little about use in people with a normally functioning immune system.

Caveats. Many positive studies were conducted in Asia, in part with small numbers, short follow-up and no placebo group. A large share of the research was funded or initiated by the manufacturer. Heterogeneity between studies is considerable and the most rigorously designed Western trial was negative 1. For unregistered applications such as ageing, autoimmune disease or Lyme there is no controlled evidence.

What it is used for

  • Chronic hepatitis B and C (the registered indication in the countries where it is authorised) 7
  • Adjuvant to hepatitis B and influenza vaccination in older people and dialysis patients
  • Supportive treatment in sepsis and in chemotherapy-induced immunosuppression (investigational)
  • Hepatocellular carcinoma and melanoma as adjunctive treatment (investigational)
  • Off-registration: general 'immune support', chronic infections, ageing — without controlled evidence

Dosing

Dose
1.6 mg (900 mcg/m² body surface area) per administration — the registered dose for chronic hepatitis B
Frequency
twice weekly in hepatitis B; in hepatitis C, often twice weekly alongside (peg)interferon and ribavirin in trial settings
Route
subcutaneous, usually into the skin of the thigh or abdomen
Duration
6 to 12 months in chronic hepatitis B; the response is only assessed months after stopping
  • 1.6 mg twice weekly for 6-12 months is the dose from the approved product information and from the phase III trials 1; it is not simply transferable to other indications.
  • The approved product information gives a weight-based dose for patients under 40 kg: 40 mcg/kg, rather than the flat 1.6 mg. The 900 mcg/m² in the line above is the label's own body-surface-area equivalent of the 1.6 mg adult dose, not a separate dose to calculate and administer.
  • In sepsis, a more intensive schedule was used in the Chinese ETASS trial: 1.6 mg subcutaneously twice daily for five consecutive days, then once daily for two days — a seven-day course in 361 intensive care patients, not an ongoing regimen 6.
  • Doses circulating in user protocols for 'immune support' (for example 450-1600 mcg daily or every other day in courses of a few weeks) do not come from validated clinical studies.
  • Dose adjustment in renal impairment has not been systematically studied.

These figures describe what the literature and published protocols report. They are not advice and not a dosing instruction.

Protocols

Chronic hepatitis B (thymalfasin, Zadaxin schedule)

published trial

Source: Journal of Viral Hepatitis, 1999 - phase III multicentre randomised, double-blind, placebo-controlled study

6 months1.6 mg (900 mcg per m2 body surface area) by subcutaneous injection, twice weekly
After the last doseno treatment; response is assessed months later, not at the end of dosing

1.6 mg twice weekly is also the registered thymalfasin dose in the countries where Zadaxin holds an authorisation. The 900 mcg/m² in the step above is the label's own body-surface-area equivalent of that same 1.6 mg, not a second dose; the label's only genuinely separate figure is 40 mcg/kg for patients under 40 kg. It is specific to chronic viral hepatitis and does not transfer to general immune support. The Chinese ETASS sepsis trial used a far more intensive and much shorter schedule: 1.6 mg twice daily for five days, then once daily for two.

Grey-market 'immune support' use (user-reported)

user protocol — not validated

Source: The pattern repeated across peptide, longevity and post-viral-illness forums together with research-chemical vendor labelling

This is not a validated schedule. It is a pattern that circulates among users and sellers, reproduced because it is what people actually follow — not because it has been tested. No trial established these doses, this interval or this duration, and nobody is checking what is in the vial. Treat every number below as an assertion, not a finding.

Most commonly reported amount450 mcg to 1.6 mg subcutaneously, daily or every other day
Reported course lengthruns of a few weeks, sometimes repeated seasonally or at the onset of an infection

The registered schedule is 1.6 mg twice weekly for six to twelve months in chronic hepatitis B, where the response is judged months after dosing stops. The circulating use is different in both purpose and rhythm: general 'immune support', longevity and post-viral recovery — none of it tested — with research-chemical powder self-injected daily or every other day in short courses of a few weeks. Reported amounts run from roughly 450 mcg to the 1.6 mg trial dose, and the frequency does not settle. The trial safety profile was favourable, close to placebo, but that was in supervised hepatitis patients; it says nothing about repeated self-dosing of unverified powder for an untested indication, and the near-absence of reported problems from an unmonitored population is not evidence that there are none.

Schedules are reproduced as their source states them. Units, IU and milligrams explains why the figures are not interchangeable between products.

Reconstitution

Vial sizes
1.6 mg, 5 mg, 10 mg
Solvent
The registered product is supplied with sterile water for injection (1 ml per 1.6 mg vial). For unregistered powder, bacteriostatic water (0.9% benzyl alcohol) is used in practice.
Storage
Powder of the registered product: 2-8 °C. Use immediately after reconstitution; the registered product contains no preservative. Liquid reconstituted with bacteriostatic water is in practice kept at 2-8 °C and used within a few weeks.

Worked example

1.6 mg vial + 1 ml diluent = 1.6 mg/ml; the entire contents is a single dose. With a 5 mg vial + 2.5 ml water = 2 mg/ml; 1.6 mg then corresponds to 0.8 ml (80 units on a U100 insulin syringe).

Do not shake — let the diluent run down the wall of the vial and swirl gently. The solution should be clear and colourless.

Work it out for Thymosin alpha-1

Safety

Side effects

  • Local injection site reaction: redness, swelling, discomfort (most frequently reported)
  • Transient fatigue or malaise
  • Rare: temporary rise in ALT during treatment of hepatitis, possibly reflecting an immune response against infected liver cells
  • Rare: rash, muscle pain
  • The side effect profile is regarded as favourable; in randomised trials the number of adverse events generally did not differ substantially from placebo 1

Do not use if

  • Hypersensitivity to thymosin alpha-1 or any of the excipients
  • Organ transplant recipients and others deliberately using immunosuppression — immune stimulation may promote rejection
  • Active autoimmune disease: theoretical risk of exacerbation, not systematically studied
  • Pregnancy and breastfeeding: insufficient data

Interactions

May counteract the action of immunosuppressants (ciclosporin, tacrolimus, corticosteroids). It is frequently combined in studies with (peg)interferon, ribavirin and lamivudine without reported pharmacokinetic interactions. Further interactions have not been systematically studied.

Common questions

What is thymosin alpha-1 used for?
Thymosin alpha-1 is a synthetic thymus peptide registered in a number of countries (as Zadaxin) as an immune modulator, used alongside treatment for certain infections (such as hepatitis) and some cancers.
Is thymosin alpha-1 the same as TB-500 or thymosin beta-4?
No — despite the shared “thymosin” name, thymosin alpha-1 (immune modulation) and thymosin beta-4 / TB-500 (tissue repair) are different molecules with different jobs.
Is thymosin alpha-1 legal and banned in sport?
It is an approved medicine in several countries but an unapproved research chemical in others, so its status should be checked directly. See doping status and legal status.

Sources