Immune & antimicrobial

Thymosin alpha-1

Also called thymalfasin, Zadaxin, Tα1, Ta1

Thymosin alpha-1 is a small protein fragment first found in the thymus, the organ that trains immune T cells. A lab-made version called thymalfasin (Zadaxin) is sold as a prescription drug in many countries outside the US, mainly for hepatitis B and as an immune booster. In the US it is not approved, and a large 2025 sepsis trial found it did not save lives.

Late-stage trialsTested in large human trials (phase 2b/3) but not yet approved in the US.

All printable flashcards

Drawn from its published sequence: 28 residues, one bead each.

Overview

How it works

Proposed and partly established: thymosin alpha-1 appears to act as an immune modulator, influencing T-cell maturation and activity and signaling through innate immune pathways such as Toll-like receptors on dendritic cells. Effects reported in lab and clinical studies include changes in cytokine release and T-cell counts. How much these effects translate into clinical benefit is uncertain and depends on the condition studied.

What research shows

Sepsis (TESTS trial, multicentre double-blind placebo-controlled phase 3 RCT at 22 centres in China, n=1,106, BMJ 2025): 28-day all-cause mortality was 23.4% with thymosin alpha-1 versus 24.1% with placebo (hazard ratio 0.99, p=0.93) — no mortality benefit. Rates of adverse events were similar in both groups.
Chronic hepatitis B: multiple human trials and the Zadaxin labeling outside the US support use as monotherapy or with interferon; results across trials have been mixed.
FDA reviewed published evidence for many proposed uses (hepatitis B and C, HIV, COVID-19, vaccine adjuvant, melanoma, liver cancer, lung cancer, sepsis, infections after stem-cell transplant, COPD, ME/CFS) before its December 2024 advisory meeting.
FDA's Pharmacy Compounding Advisory Committee voted 4-17 against adding either thymosin alpha-1 or its acetate salt to the 503A bulks list, with most members citing a lack of convincing effectiveness and safety evidence (Dec 4, 2024).

Doses studied in people

Study or label	Dose used	Source
Zadaxin (thymalfasin) approved labeling outside the US for chronic hepatitis B	1.6 mg (900 micrograms per square metre of body surface area) subcutaneous twice weekly for 6 to 12 months; 40 micrograms per kg for people under 40 kg	³
TESTS phase 3 RCT in sepsis (n=1,106)	1.6 mg subcutaneous every 12 hours for up to 7 days	¹

Doses listed come from approved labeling outside the US and a published phase 3 trial; thymosin alpha-1 is not FDA-approved at any dose.

Read this first: these are the amounts given to participants in published studies or written on official prescribing labels, reported for reference. They are not recommendations. Trial doses were chosen for specific patients under medical supervision, and a dose that is safe in one person can be harmful in another.

Weekly protocol

How the schedule looks week by week. Each chart says where it comes from: a prescribing label, a clinical trial, or a protocol commonly reported online.

Zadaxin label: chronic hepatitis B (Singapore)

Prescribing label

One week

D1D2D3D4D5D6D7

Twice weekly · Subcutaneous injection

Dose by week

1.6 mgwk 1–26

Course: 6 to 12 months
Note: Approved outside the US only; label dose is 1.6 mg (900 mcg/m²), or 40 mcg/kg for people under 40 kg. Not FDA-approved.
Source: Zadaxin Injection (thymalfasin) product information — Singapore National Drug Formulary

TESTS phase 3 trial in sepsis

Clinical trial

One week

D1D2D3D4D5D6D7

Every 12 hours (twice daily) · Subcutaneous injection

Syringe units (U-100)

Vial Water added

Dose by week

1.6 mg every 12 hourswk 1

Supplies for one course (estimate)

Counts only. This site does not sell or link to supplies.

Course: Up to 7 days
Note: Hospital use in adults with sepsis.
Source: The efficacy and safety of thymosin alpha1 for sepsis (TESTS): multicentre, double-blinded, randomised, placebo-controlled, phase 3 trial

PeptideDosages.com daily protocol

Commonly reported, not studied

One week

D1D2D3D4D5D6D7

Once daily · Subcutaneous injection

Syringe units (U-100)

Vial Water added

Dose by week

300 mcgwk 1

500 mcgwk 2–8

Supplies for one course (estimate)

Counts only. This site does not sell or link to supplies.

Course: 8 weeks (8–16 weeks reported)
Note: Other public guides follow the label pattern instead: 1.6 mg twice weekly.
Source: PeptideDosages.com: Thymosin Alpha-1 (5 mg vial) dosage protocol
Also: Peptide Protocol Wiki: Thymosin Alpha-1 dosing protocols

Reference only, not a recommendation. Charts marked "commonly reported" describe what public sources say people use. They have not been tested in trials, and their safety and effect are unknown. Label and trial schedules were set for specific patients under medical supervision. Use the calculator only to understand the arithmetic.

Vial math

Many research vials hold 5 or 10 mg of powder. This table shows how much ends up in each mark on a U-100 insulin syringe for common amounts of added liquid. It is arithmetic only, not a dose.

Vial	Liquid added	Concentration	Per unit (0.01 mL)
5 mg	1 mL	5 mg/mL	50 mcg	Open in calculator
5 mg	2 mL	2.5 mg/mL	25 mcg	Open in calculator
5 mg	3 mL	1.67 mg/mL	16.7 mcg	Open in calculator
10 mg	1 mL	10 mg/mL	100 mcg	Open in calculator
10 mg	2 mL	5 mg/mL	50 mcg	Open in calculator
10 mg	3 mL	3.33 mg/mL	33.3 mcg	Open in calculator

More sizes: all vial sizes · How this works: Syringes and units · Reconstitution guide.

Safety

Generally well tolerated in published trials; injection-site discomfort is the most commonly reported effect (Zadaxin labeling, clinical trials).
No mortality benefit in sepsis in the largest RCT to date (TESTS, BMJ 2025).
FDA had cited immunogenicity and product-characterization concerns when it placed the substance in Category 2 in 2023.
Long-term safety of non-pharmaceutical (gray-market) products is unknown; purity and identity are not verified.

Report side effects of any drug or product to the FDA through MedWatch. For a possible poisoning in the US, call Poison Control at 1-800-222-1222.

Legal status

FDA: Not FDA-approved in the US. Approved as Zadaxin (thymalfasin) in a number of countries outside the US.
Compounding: 503A: placed in Category 2 in Sept 2023; removed effective Sept 27, 2024 after the nominator withdrew the nomination. FDA's PCAC voted 4-17 on Dec 4, 2024 against adding it to the 503A bulks list. Not on FDA's May 2026 503A categories list, so not eligible for 503A compounding.
Sports (WADA): Not named on the 2026 Prohibited List. Because it is approved for human use in some countries, its S0 status is not clear-cut; S2 also covers substances with similar biological effects to listed ones. Athletes should confirm with their anti-doping organization.
Other: Prescription medicine where approved; regulatory status varies by country.

Rules change often. See Legal status of peptides for how these categories work.

Storage and stability

Pharmaceutical product is supplied as a lyophilized powder for injection; follow the approved product labeling.

General lab handling, not instructions for personal use. See Storage and stability.

History

Isolated by Allan Goldstein's group in the 1970s from a thymus extract. A synthetic form, thymalfasin, was developed by SciClone Pharmaceuticals as Zadaxin and approved in a number of countries (mainly in Asia, Latin America and parts of Europe) for chronic hepatitis B and as an immune adjuvant. In the US it was nominated for compounding, placed in FDA's 503A Category 2 in September 2023, removed in September 2024 after the nomination was withdrawn, and then rejected by FDA's compounding advisory committee in December 2024.

Common questions

Is thymosin alpha-1 approved anywhere?

A synthetic form, thymalfasin (Zadaxin), is a prescription medicine in some countries outside the US, including for chronic hepatitis B ³⁸. It is not FDA-approved in the US ².

Does thymosin alpha-1 help in sepsis?

No benefit was shown. In a phase 3 trial of 1,106 patients, 28-day death rates were 23.4% with thymosin alpha-1 and 24.1% with placebo, a difference that was not significant ¹.

What doses were studied?

Zadaxin labeling outside the US lists 1.6 mg under the skin twice weekly for 6 to 12 months for hepatitis B ³. The sepsis trial used 1.6 mg under the skin every 12 hours for up to 7 days ¹. Neither is an FDA-approved dose ².

Can thymosin alpha-1 be compounded in the US?

No. FDA's compounding advisory committee voted 4-17 in December 2024 against adding it to the 503A bulks list, and it is not on FDA's May 2026 categories list ²⁴⁶.

Is thymosin alpha-1 safe?

It was generally well tolerated in published trials, with similar adverse event rates to placebo in the sepsis trial ¹³.

Is thymosin alpha-1 banned in sport?

It is not named on WADA's 2026 list, but its status is not clear-cut, so athletes should check with their anti-doping organization ⁷.

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Thymulin is a small hormone made only by epithelial cells of the thymus, a gland in the chest that helps T cells (a type of white blood cell) develop [4][5].

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LL-37

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LL-37 is the only cathelicidin-type germ-fighting peptide made by the human body.

Sources

Last reviewed 2026-10-25 · How we research · Found an error? See our corrections policy.