Thymosin Alpha-1 Dosage: Zadaxin's Schedule vs US Status
Last updated September 27, 2026 · Reviewed by Grey Peptides Editorial Board · ✓ Primary-sourced
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Thymosin Alpha-1 (thymalfasin) is a genuine approved medicine in 35+ countries, marketed as Zadaxin for chronic hepatitis B and immune indications — one of the few peptides here with decades of pharmaceutical use and Phase 3 data. But it is not FDA approved (it holds only US orphan-drug designation), and in the US it is a research or compounded product. The dosing below is the regulated Zadaxin schedule, documented for education.
Educational information, not medical advice. The doses on this page come from approved labels, published studies and, where no study exists, amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
The short version
Thymosin Alpha-1 is a 28–amino-acid peptide that the thymus uses to help tune immune function; it activates T cells, NK cells, and dendritic cells. Its evidence base is unusually deep for this field — it has been studied in trials involving thousands of patients and used pharmaceutically abroad for decades. The honest framing is "approved there, not here," and the dosing that is actually anchored in regulatory data is the Zadaxin hepatitis B schedule.
The pharmaceutical (Zadaxin) schedule
The most established, regulatory-grade regimen is the chronic hepatitis B protocol:
Human study. Source: "Efficacy of thymosin alpha1 in patients with chronic hepatitis B: a randomized, controlled trial" (1998, PMID 9581695)
| Parameter | Zadaxin (hepatitis B) label |
|---|---|
| Dose | 1.6 mg (≈900 µg/m²) |
| Frequency | Twice weekly (≈3–4 days apart) |
| Route | Subcutaneous |
| Duration | ~6–12 months |
A useful "do not overdose" point from the clinical record: the 1.6 mg dose was optimized in Phase 3 trials, and higher amounts (3.2–6.4 mg) studied did not consistently show superior immune outcomes. More is not reliably better. Community immune-support protocols sometimes cite slightly lower maintenance amounts, but the figure with the strongest backing is the 1.6 mg twice-weekly hepatitis B schedule.
Evidence is indication-specific
It is worth being precise about what the evidence supports. The strong data are in chronic viral hepatitis and certain immune/oncology adjunct settings. The compound has also been studied in sepsis, COVID-19, and as a vaccine adjuvant — but a large 2025 Phase 3 sepsis trial was negative for its primary endpoint. So "well-studied" does not mean "proven for everything"; the benefit is tied to specific indications.
The US picture
In the US, Zadaxin is not available because it lacks FDA approval; Thymosin Alpha-1 circulates only as a research chemical. FDA placed it in 503A Category 2 (significant safety risks) in 2023; its nomination was later withdrawn, and FDA's category list (updated May 14, 2026) names it in no category, so 503A pharmacies cannot compound it. It was not on either 2026 PCAC agenda (as of September 27, 2026). See the Regulatory Status Tracker.
Frequently asked questions
Is it the same as TB-500 (thymosin beta-4)?
No. Both are thymosins by name, but Thymosin Alpha-1 (immune modulation) and thymosin beta-4 / TB-500 (tissue repair) are different peptides with distinct biology. See the TB-500 dosage page.
Why isn't it FDA approved if it's used in 35+ countries?
Much of its US history is orphan-drug designations and earlier development that did not culminate in commercial FDA approval, plus more recent compounding-quality scrutiny — not a simple "approved but hidden" situation.
Can a US clinician prescribe it?
Not as the approved drug Zadaxin, and not as a compounded prescription either: as of September 27, 2026 FDA's 503A list names it in no category, so pharmacies cannot compound it. What is sold in the US is an unregulated research chemical.
Sources
- Zadaxin (thymalfasin) prescribing information — chronic hepatitis B, 1.6 mg twice weekly; approvals in 35+ countries.
- Goldstein AL, et al. Thymosin alpha-1 immunology and clinical development.
- TESTS phase 3 sepsis trial (negative primary endpoint), 2025; U.S. FDA orphan-drug records and compounded-peptide statements.
Medical disclaimer: Education only, not medical advice. Thymosin Alpha-1 is not FDA approved; US-sourced product is an unregulated research chemical. Dosing reflects the regulated Zadaxin schedule, not a recommendation. Consult a licensed clinician.