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Thymosin alpha-1

Thymosin alpha-1 is a 28-amino-acid peptide first found in thymus extract. The lab-made version, thymalfasin (brand Zadaxin), is approved in some countries, mainly for hepatitis B. A large 2025 sepsis trial found no clear benefit.

Approved in more than 35 countries as thymalfasin (Zadaxin), for hepatitis B and C (per a 2020 review)Not approved as a medicine by the US FDASepsis: no clear benefit in a 1,106-person phase 3 trial
OverviewDosingMixingResearchCommunitySafetyFAQSources

Thymosin alpha-1 quick reference

Type28-amino-acid thymic peptide
Studied forHepatitis B, sepsis, vaccines
EvidenceApproved in some countries
RouteInjection under the skin
Half-lifeAbout 1.5 hours

What is Thymosin alpha-1?

Natural thymus peptide; the lab-made form is thymalfasin (Zadaxin).

People use it for immune support and infection recovery.

Approved in some countries, not the US; the big sepsis trial found no benefit.

How Thymosin alpha-1 works

T-cell maturation (lab)

Influences T-cell production and maturation.

Cytokine signals (lab)

Raises interferon-gamma and interleukin-2 signals.

Natural killer cells (lab)

Activates NK cells that hunt infected cells.

Modulator, not booster

Effects depend on the immune state and the condition.

What Thymosin alpha-1 is studied for

PeopleHepatitis B

98 people: virus response 40.6% vs 9.4% untreated.

PeopleSepsis

1,089 adults: 28-day deaths 23.4% vs 24.1%, no benefit.

PeopleFlu vaccine response

90 older men: antibody response improved with 8 doses.

PeopleMelanoma

488 patients: more responses; survival gain not significant (P = .08).

Lab / cellsT-cell and NK activity

Immune-cell activation in lab studies.

Thymosin alpha-1 dosing protocol

Use studiedAmount and schedule
Chronic hepatitis B (trial, 98 people)1.6 mg under the skin twice a week for 26 weeks (a second group got 52 weeks)
Sepsis (phase 3 TESTS, 1,106 adults)1.6 mg under the skin every 12 hours for up to 7 days

Chien 1998 (Hepatology) and Wu 2025 (BMJ, NCT02867267). The BMJ paper calls 1.6 mg twice a week the conventional dose for cancers and viral hepatitis. Hepatitis B use dates from before today's antiviral drugs.

HEPATITIS B TRIAL1.6 mg twice weekly, 26 weeksTrial
SEPSIS TRIAL1.6 mg every 12 hours, up to 7 daysTrial
VACCINE TRIAL900 mcg/m² twice weekly, 8 dosesTrial
MELANOMA TRIAL1.6, 3.2 or 6.4 mg per doseTrial
COMMUNITY RANGE1.6 mg twice weeklyCommunity
CYCLE4–8 weeks, often seasonalCommunity

Label and trial amounts come from the sources below. Community amounts are what people report; they have not been tested.

Mixing

Vial10 mg
+
BAC water2 mL
=
Strength5 mg per mL

Bacteriostatic water. Refrigerate. A 1.6 mg dose is 0.32 mL at this strength.

Open the peptide calculator

What the research found

Two trials. Hepatitis B: 98 people, 26-week course, measured 18 months after starting. Sepsis: 1,089 adults analysed, measured as deaths within 28 days.

40.6%hepatitis B complete virus response, 26-week course
9.4%same measure in the untreated group
23.4%died within 28 days, sepsis, thymosin alpha-1
24.1%died within 28 days, sepsis, placebo (no clear difference)

Chien RN et al. Hepatology 1998 (PMID 9581695); Wu J et al. BMJ 2025 (PMID 39814420, hazard ratio 0.99, 95% CI 0.77 to 1.27).

Thymosin alpha-1 community experience

What users report. Not tested in trials.

Positive reports

  • Fewer sick days Fewer seasonal infections and shorter colds.
  • Better stamina Post-viral fatigue users report more stamina.
  • Bloodwork changes Some track higher lymphocyte or CD4 counts.
  • Seasonal cycles 4–8 week blocks during cold and flu months.

Negative & mixed

  • Fades after stopping Perceived benefit fades after the cycle ends.
  • No change Some finish a cycle without noticing anything.
  • Minor effects Brief fatigue, headache, insomnia or site irritation.

Thymosin alpha-1 stacking

TA-1 + BPC-157

Immune plus healing idea. Not studied together.

TA-1 + KPV or KLOW

Immune plus inflammation idea. Not studied together.

TA-1 + chemotherapy

Studied with dacarbazine and interferon in melanoma, under supervision.

Thymosin alpha-1 side effects & safety

Common

  • Hepatitis B trial: no significant side effects.
  • Sepsis trial: no safety outcome differed from placebo.
  • User reports: redness or tenderness at the injection site.
  • User reports: tiredness, headache or insomnia.

Less common

  • Severe multi-organ immune reaction reported with a PD-1 drug (case report).
  • Long-term safety in healthy anti-aging use is unstudied.

Precautions

  • Avoid with an organ transplant on immunosuppressants.
  • Cancer immunotherapy (PD-1 drugs): only with oncologist approval.
  • Autoimmune disease: immune stimulation needs medical advice.
  • Avoid in pregnancy or breastfeeding; safety not established.
  • Known allergy to thymalfasin.

Our verdict on Thymosin alpha-1

Best-studied peptide here, with real trials. Benefits are condition-specific and the big sepsis trial was negative.

Thymosin alpha-1 FAQ

Is thymosin alpha-1 approved?

In some countries as thymalfasin (Zadaxin). Not in the US.

Does it prevent infections?

Not proven in healthy people. Trials were in disease settings.

Is it the same as TB-500?

No. TB-500 relates to thymosin beta-4, a different peptide.

What dose did trials use?

1.6 mg under the skin twice weekly in hepatitis B.

Sources

  • Chien RN et al. Hepatology 1998, PMID 9581695
  • Wu J et al. BMJ 2025 (TESTS phase 3), PMID 39814420, NCT02867267
  • Dominari A et al. World J Virol 2020 (review: approved in more than 35 countries), PMID 33362999
  • Expert Rev Anti Infect Ther 2004 (thymalfasin for hepatitis B; Zadaxin, SciClone; 28 amino acids), PMID 15482167
  • Drugs@FDA / openFDA search: no thymalfasin product
  • Gravenstein S et al. J Am Geriatr Soc 1989 (influenza vaccine, 90 older men), PMID 2642497
  • Maio M et al. J Clin Oncol 2010 (melanoma, 488 patients), PMID 20194853
  • Li T et al. Clin Case Rep 2026 (immune reaction with sintilimab plus thymosin alpha-1), PMID 41669704