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Immune peptides

Thymosin Alpha-1 (thymalfasin)

Also known as Tα1, Ta1, thymalfasin, thymosin α1, Zadaxin

Controlled human trials

Synthetic thymic peptide, immune modulator

Not FDA-approved. Not on FDA's 503A category lists (May 14, 2026). Source, May 14, 2026

Identity. Thymosin alpha-1 is a synthetic 28-amino-acid peptide (thymalfasin; brand Zadaxin). It is a different molecule from thymosin beta-4 / TB-500. FDA treats thymosin alpha-1 free base and acetate as separate bulk substances. The formulary product name does not state which form is supplied, so ask the partner pharmacy for the salt form.

At a glance

These dosages are a helpful guide, not a prescription. Every clinician is expected to do their own due diligence before prescribing.

  1. Start

    1.6 mg subcutaneously twice a week (hepatitis B or C label dose)1

    Approved label
Frequency
twice a week1
Route
subcutaneous (not intravenous)1
Course
6 to 12 months (hepatitis B); 12 months (hepatitis C)1
Label vial is 1.6 mg; the 10 mg formulary vial is 6.25x stronger, so write dose in mg and mL.
Calculate this dose

Dosing detail

Approved label

Approved label

  • Chronic hepatitis B, adults, monotherapy or combination with interferon (Singapore Zadaxin product information)

    1.6 mg (labeled with 900 mcg/m2 equivalent) mg1

    Route
    subcutaneous (not intravenous)
    Frequency
    twice a week
    Duration
    6 to 12 months
    n
    not stated in the label text
    Design
    Product labeling (Singapore NDF)
    Source quote
    1.6 mg (900 mcg/m2) administered subcutaneously twice a week for 6 to 12 months.

    Finding. Labeled dose for chronic hepatitis B. The label also notes a delayed therapeutic response, seen 12 months or longer after completion of therapy. Adults receive the flat 1.6 mg dose. Do not scale by body surface area (900 mcg/m2 equals 1.6 mg only at about 1.78 m2, arithmetic).

  • Chronic hepatitis B, patients weighing less than 40 kg (Singapore Zadaxin product information) (adults under 40 kg; label does not extend this to children)

    40 mcg/kg mcg/kg1

    Route
    subcutaneous
    Frequency
    not separately stated in the label; the adult schedule is twice a week
    Duration
    not separately stated in the label
    n
    not stated in the label text
    Design
    Product labeling (Singapore NDF)
    Source quote
    Patients weighing less than 40 kg should receive a ZADAXIN dose of 40 mcg/kg.

    Finding. Weight-based dose for patients under 40 kg. The same rule is stated for hepatitis C.

  • Chronic hepatitis C, adults, combination therapy with interferon (Singapore Zadaxin product information)

    1.6 mg (labeled with 900 mcg/m2 equivalent) mg1

    Route
    subcutaneous (not intravenous)
    Frequency
    twice a week
    Duration
    12 months
    n
    not stated in the label text
    Design
    Product labeling (Singapore NDF)
    Source quote
    1.6 mg (900 mcg/m2) administered subcutaneously twice a week for 12 months.

    Finding. Labeled dose for chronic hepatitis C, only in combination with interferon.

Human studies

Human study

  • Chronic hepatitis B, 98 patients; 26-week (T6) and 52-week (T12) courses vs untreated control

    1.6 mg2

    Route
    subcutaneous injection
    Frequency
    two times a week
    Duration
    26 weeks (T6) or 52 weeks (T12)
    n
    98
    Design
    Randomized, controlled, three-arm, open to no treatment in control
    Source quote
    group A received a 26-week course of Talpha with a 1.6-mg subcutaneous injection two times a week (T6 group); 2) group B received the same regimen as group A, but Talpha therapy extended for 52 weeks (T12 group)

    Finding. Complete virological response 18 months after entry: 40.6% (T6), 26.5% (T12), 9.4% control (T6 vs control P=.004; T12 vs control P=.068, not significant). The authors concluded a 26-week course is effective. The 52-week course was not shown to be better. No significant side effects.

  • HBeAg-positive chronic hepatitis B, US multicentre phase III

    1.6 mg3

    Route
    not stated in the abstract (the label route is subcutaneous)
    Frequency
    twice weekly
    Duration
    6 months, then 6 months follow-up
    n
    97 (49 Talpha1, 48 placebo)
    Design
    Multicentre, randomized, double-blind, placebo-controlled phase III
    Source quote
    Forty-nine patients received Talpha1 (1.6 mg) and 48 patients received placebo, twice weekly for 6 months, and were followed-up for an additional 6 months.

    Finding. Complete response 14% vs 4% (P=0.084). The authors state the results do not confirm efficacy reported in other studies.

  • HBeAg-positive chronic hepatitis B, thymosin alpha-1 added to peginterferon alfa-2a vs peginterferon alone

    1.6 mg4

    Route
    not stated in the abstract
    Frequency
    twice a week
    Duration
    first 12 weeks (peginterferon continued 48 weeks)
    n
    51 (26 combination, 25 monotherapy)
    Design
    Prospective, multicentre, randomized, open-label
    Source quote
    26 patients; 180 μg of peginterferon α-2a weekly for 48 weeks and 1.6 mg of thymosin α-1 twice a week for the first 12 weeks

    Finding. Combined response at end of follow-up 23.1% vs 20.0% (p=0.789). Short-term addition was not superior.

  • Severe sepsis, ICU, six Chinese hospitals (ETASS)

    1.6 mg5

    Route
    subcutaneous injections
    Frequency
    twice per day for five consecutive days, then once per day for two consecutive days
    Duration
    7 days
    n
    361 (181 Tα1, 180 control)
    Design
    Multicenter, single-blind, randomized, controlled (saline placebo)
    Source quote
    patients received subcutaneous injections of 1.6 mg Tα1 (ZADAXIN™, SciClone Pharmaceuticals, Foster City, CA, USA) twice per day for five consecutive days, then once per day for two consecutive days.

    Finding. 28-day mortality 26.0% vs 35.0% (P=0.062; RR 0.74, 95% CI 0.54 to 1.02). No Tα1-related severe adverse event reported.

  • Sepsis (Sepsis-3), 22 Chinese centres (TESTS)

    1.6 mg6

    Route
    subcutaneous injection
    Frequency
    every 12 hours
    Duration
    seven days unless discontinued (ICU discharge, death, withdrawal)
    n
    1089 modified intention-to-treat (542 Tα1, 547 placebo)
    Design
    Multicentre, double-blind, placebo-controlled phase 3
    Source quote
    Participants in the intervention group received a subcutaneous injection of 1.6 mg of lyophilised thymosin α1 powder dissolved in 1 mL of sterilised water every 12 hours.

    Finding. 28-day all-cause mortality 23.4% vs 24.1%. No clear evidence of mortality benefit. Higher mortality in a prespecified under-60 subgroup.

  • Severe sepsis, combined with ulinastatin (not thymosin alone), 26 Chinese hospitals

    1.6 (treatment group 1); 'double dosage of the above agents' (treatment group 2) mg per day7

    Route
    not stated in the abstract
    Frequency
    per day
    Duration
    7 days
    n
    433
    Design
    Multiple-center, prospective, randomized, controlled (combination therapy)
    Source quote
    Ulinastatin (urinary trypsin inhibitor) 300 K units per day+thymosin alpha1 (Maipuxin) 1.6 mg per day for 7 days (as treatment group 1, adopted in the first trial), or double dosage of the above agents (as treatment group 2, adopted in the second trial)

    Finding. Group 1 (n=91 trial): no 28-day difference. Group 2 (n=342 trial): 28-day mortality 38.32% to 25.14% (P=0.0088). The effect cannot be attributed to thymosin alone.

Clinical practice

Clinical practice

Outside hepatitis B/C (label) and sepsis (trials), no controlled trial supports any practice schedule. The clinic and reference pages are not peer reviewed, give no references for their numbers, and disagree on the starting dose (1 to 3 mg vs a fixed 1.6 mg). Only the peer-reviewed pharmacy review matches the label. Treat 1.6 mg twice weekly as the evidence-anchored dose and label any other schedule as clinician judgment.

Start
No standardised starting dose or ramp was found. The labeled and trial adult dose is 1.6 mg (40 mcg/kg under 40 kg per label). One clinic web page (no trial support) reports clinicians 'frequently starting within a range of 1 mg to 3 mg'; 3 mg is about 1.9x the labeled single dose.
Titration
None sourced. A telehealth page states the dose is fixed, not weight-based; the Singapore label adds 40 mcg/kg for patients under 40 kg.
Typical
1.6 mg subcutaneously twice weekly (3.2 mg/week; label and hepatitis B trials). One clinic web page with no trial support reports 1.5 mg twice or three times a week; three times weekly is 4.5 mg/week, about 1.4x the labeled weekly total.
Ceiling
No outpatient ceiling has been established. Separately, inpatient sepsis trial exposure (TESTS) was 1.6 mg every 12 hours for 7 days: 3.2 mg/day or 22.4 mg/week, about 7x the labeled weekly dose, in ICU patients. It is not an outpatient maximum.
Frequency
Twice weekly; sepsis trials used every 12 hours for 7 days.
Route
Subcutaneous. The label states it should not be given intravenously.
Timing
No time of day or interval between twice-weekly doses is specified in the sourced label or trials.
Cycle
Hepatitis B per label: 6 to 12 months; trials compared 26 vs 52 weeks. A reference web page lists 3-6 months for immune support and per chemo cycle for cancer, with no cited trial.

Sources

  • Clinical practicePeer-reviewed review8
    For hepatitis B and C, TA1 1.6 mg (900 micrograms/m2) should be administered subcutaneously twice a week.

What clinics and vendors publish

These ranges come from clinic and vendor publications, not studies.

Clinic-reportedCommunity-reported
  • Clinic-reportedPharmacy or clinic protocol9
    The most common dosing regimen in 1.5mg twice or three times a week.
  • Clinic-reportedPharmacy or clinic protocol9
    frequently starting within a range of 1 mg to 3 mg.
  • Community-reportedOther web source10
    Twice weekly (standard); daily during chemotherapy cycles (cancer adjuvant)
  • Community-reportedOther web source10
    6-12 months (hepatitis B); 3-6 months (immune support); per chemo cycle (cancer).

Safety

Serious warnings

  • No boxed warning appears in the Singapore Zadaxin labeling text. A drug-information site's reproduction of the label states 'None' under Warnings.1
  • Label warning: 'A transient increase in ALT to more than twice baseline value (flare) can occur during ZADAXIN therapy.' The label says therapy should generally continue 'unless signs and symptoms of liver failure are observed'.1
  • FDA (page current as of 04/22/2026): 'Compounded drugs containing thymosin-alpha-1 (Ta1) may pose significant risk for immunogenicity for certain routes of administration' and 'may have complexities with regard to peptide-related impurities and API characterization.' FDA also states the safety-related information is inadequate for the agency to sufficiently understand the extent of any safety issues.11
  • TESTS (sepsis, 1.6 mg every 12 h): prespecified subgroup analysis showed higher 28-day mortality in participants under 60 years (hazard ratio 1.67, 1.04 to 2.67; P for interaction 0.01). Hypothesis-generating only, in critically ill patients.6
  • TESTS prespecified diabetes subgroup went the opposite way (diabetes HR 0.58, 0.35 to 0.99; no diabetes HR 1.16, 0.87 to 1.53; P for interaction 0.04). Hypothesis-generating, ICU population.6
  • Pregnancy: label says it should be given to a pregnant woman only if the benefits clearly outweigh the risks. Nursing: caution should be exercised. Pediatric: safety and effectiveness not established below age 18 (the 40 mcg/kg figure applies to adults under 40 kg).12
  • The label reports no instances of deliberate or accidental overdosage in humans. This reflects absence of data and does not define a safe upper dose.12

Contraindications

  • Label: 'history of hypersensitivity to thymosin alpha 1 or any component of the injection.'1
  • Label: should be considered contraindicated in patients who are being deliberately immunosuppressed, 'such as organ transplant patients', unless the potential benefits clearly outweigh the potential risks.1

Adverse effects

  • Label text: 'Adverse experiences have been infrequent and mild, consisting primarily of local discomfort at the injection site.' Rarer reported effects include redness, temporary muscle wasting, joint pain with hand swelling, and rash.12
  • Review (AJHP 2001): 'Most studies observed only local irritation at the injection site.'8
  • Hepatitis B RCT (1.6 mg twice weekly, 26 or 52 weeks): 'No significant side effects were observed.'2
  • ETASS (sepsis): 'no Tα1-related severe adverse event (SAE) was reported and no treatment was discontinued due to intolerance or adverse events.'5
  • TESTS (sepsis): adverse events 66.4% thymosin vs 67.6% placebo; serious adverse events 26.8% vs 29.3%. Most common events across both arms were anaemia (10.7%), fever (9.6%), abdominal distension (5.4%), coagulation disorders (4.8%). 'No unexpected serious adverse events related to thymosin α1 occurred.' Population was critically ill, so rates do not transfer to outpatient use.6

Monitoring

  • Label monitoring is ALT in the hepatitis B setting: a transient flare above twice baseline can occur and therapy generally continues unless signs of liver failure appear. No label-based monitoring exists for non-label outpatient use; any other labs are clinician judgment.1

Interactions

  • Label: 'Caution should be exercised when administering ZADAXIN (thymalfasin) therapy in combination with other immunomodulating drugs.' Interferon combination is the labeled use for hepatitis B and C. Deliberate immunosuppression is a labeled contraindication unless benefit clearly outweighs risk.12

Handling

Reconstitution

Warning: the label rule '1.0 mL diluent = 1.6 mg/mL' applies only to the 1.6 mg Zadaxin vial. The formulary vial is 10 mg (6.25x stronger). Injecting 1 mL of a 10 mg vial reconstituted in 1 mL would deliver about 10 mg. Always write the dose in mg and mL together and read the concentration from the pharmacy label. Worked arithmetic (not a published protocol): 10 mg + 2 mL = 5 mg/mL, so 1.6 mg = 0.32 mL, about 6 doses per vial; 10 mg + 3 mL = 3.33 mg/mL, so 1.6 mg = 0.48 mL. Label (1.6 mg vial): 'It should be reconstituted with 1.0 ml of the diluent provided'; use immediately. No diluent volume or beyond-use date is published for a compounded 10 mg vial. Follow the dispensing pharmacy's labeling.

Storage before mixing

Label text: 'Store ZADAXIN (thymalfasin) between 2° and 8°C (36° to 46° F).' Storage and beyond-use date for a compounded vial: per the dispensing pharmacy.

Storage after mixing

Label: use immediately. A beyond-use date for any compounded or pre-reconstituted product must come from the dispensing pharmacy.12

Evidence

A labeled product (Zadaxin, 1.6 mg vial) exists outside the US for chronic hepatitis B and C. No US label was found. Tier B applies only to 1.6 mg subcutaneous twice weekly in hepatitis B (Chien 1998: 26-week course P=.004 vs control; 52-week course P=.068, not better than 26 weeks) and to 1.6 mg sepsis regimens in hospitalized patients. The US placebo-controlled hepatitis B trial did not confirm efficacy (complete response 14% vs 4%, P=0.084). Two sepsis RCTs conflict: ETASS (n=361) showed borderline benefit, and TESTS (n=1089) showed no mortality difference, with a higher-mortality signal in patients under 60. A 2025 meta-analysis pooled OR 0.73, but high-quality trials gave OR 0.82 (0.65-1.03). The cited trials were short-course hepatitis or hospital-based sepsis trials, and they reported no serious drug-related adverse events. FDA states that safety information on compounded Ta1 is inadequate (immunogenicity concern). Any outpatient dose other than 1.6 mg twice weekly rests on clinic web pages, not trials.

Mechanism

Reviews describe thymosin alpha-1 as a thymic peptide immune modulator. It binds Toll-like receptors (TLR3/4/9, also TLR2 and TLR7), activates IRF3 and NF-κB pathways, and promotes activation of T cells, B cells, macrophages and natural killer cells. This mechanism comes from reviews. It is not established clinical pharmacology for the compounded product.1314

Availability

Amounts offered

  • 10 mg

Custom strengths on request.

References

According to PubMed

  1. 1ZADAXIN INJECTION [SIN07483P] product informationSingapore Ministry of Health, National Drug Formulary
  2. 2Efficacy of thymosin alpha1 in patients with chronic hepatitis B: a randomized, controlled trial.Hepatology, 1998
  3. 3Thymosin alpha1 treatment of chronic hepatitis B: results of a phase III multicentre, randomized, double-blind and placebo-controlled study.Journal of Viral Hepatitis, 1999
  4. 4Efficacy of thymosin α-1 plus peginterferon α-2a combination therapy compared with peginterferon α-2a monotherapy in HBeAg-positive chronic hepatitis B: a prospective, multicenter, randomized, open-label study.Scandinavian Journal of Gastroenterology, 2012
  5. 5The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial.Critical Care, 2013
  6. 6The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial.BMJ, 2025
  7. 7Clinical trial with a new immunomodulatory strategy: treatment of severe sepsis with Ulinastatin and Maipuxin (Chinese-language article)Zhonghua Yi Xue Za Zhi, 2007
  8. 8Thymosin alpha-1.American Journal of Health-System Pharmacy, 2001
  9. 9Thymosin Alpha-1 Best Practices for Optimal Results (Srinivas Reddy, MD, Desert Mobile Medical; posted 06/12/2026)Desert Mobile Medical, 2026
  10. 10Thymosin Alpha-1 dosing (updated January 29, 2026; no references given for dosing claims)Peptide Protocol Wiki, 2026
  11. 11Certain bulk drug substances for use in compounding that may present significant safety risks (content current as of 04/22/2026)U.S. Food and Drug Administration, 2026
  12. 12Zadaxin (thymalfasin) drug monograph (label text reproduction; the page does not state approval status)RxList
  13. 13Thymosin α1 and Its Role in Viral Infectious Diseases: The Mechanism and Clinical Application.Molecules, 2023
  14. 14Thymalfasin for the treatment of chronic hepatitis B.Expert Review of Anti-infective Therapy, 2004
  15. 15Efficacy of thymosin α1 for sepsis: a systematic review and meta-analysis of randomized controlled trials.Frontiers in Cellular and Infection Microbiology, 2025
  16. 16PCAC to discuss 503A bulks list substances (thymosin alpha-1 on the December 4 agenda)Alliance for Pharmacy Compounding, 2024

Last reviewed 2026-10-14. 49 figures checked against their sources. How this works