Peptifact

Thymosin Alpha 1 Dosage: The Best-Evidenced Compound on FDA's Withdrawn List

1.6 mg subcutaneously twice a week, from trials in 357 hepatitis patients. Thymosin alpha 1 has 66 registered studies and 39 randomised trials behind it — and no US label at all. We counted the file and read the numbers.

Robert F · Edited by Caroline S · Published 2026-09-13

Illustration: A precision scale with a clear glass vial of lyophilized powder on a cool grey lab bench.
Illustration

Thymosin alpha 1 sits in an odd position. It is sold in the research-peptide market alongside compounds with four preclinical papers to their name, and it has one of the deepest clinical files of anything in that catalogue — hundreds of human studies, dozens of randomised trials, a named dose used consistently for twenty-five years. It also has no US label, which means no regulator in this market has reviewed that dose.

This page reports what the published sources state, with the dates they were read. It is not a protocol and contains no recommendation.

The dose, and where it comes from

1.6 mg, subcutaneously, twice a week.

The figure is stated in a 2001 review of the compound's pharmacology and clinical trials in Am J Health Syst Pharm, where it is given as "1.6 mg (900 micrograms/m2)" for hepatitis B and C — a body-surface-area calculation that resolves to 1.6 mg for an average adult and has been carried forward as a flat figure ever since.

It recurs across the literature with unusual consistency. A 2021 study of COVID-19 patients in Wuhan gave 78 patients 1.6 mg subcutaneously alongside supportive treatment for 15 days. Where most compounds in this catalogue have a dose figure that drifts between sources, this one does not.

What it is not is a labelled dose. DailyMed returned no record for thymalfasin, thymosin or ZADAXIN on 13 September 2026. The distinction matters and is set out in full in our guide to how to read a dosing claim: a trial dose is a quantity investigators chose and tested, and a labelled dose is a quantity a regulator reviewed and approved. This compound has the first and not the second.

The pharmacokinetics, and the mismatch in them

From the same review: rapid absorption, peak serum concentration within two hours, serum half-life approximately two hours, blood levels back at baseline within 24 hours.

Set that against a twice-weekly schedule and the arithmetic is strange. The compound is gone from the circulation within a day, and the next dose arrives three or four days later. For most of the treatment period there is no measurable drug present.

The explanation offered is mechanistic rather than pharmacokinetic. The proposed action is on thymocytes and T-cell function — stimulating differentiation, converting thymocytes to active T cells — which is a change to a cell population rather than occupancy of a receptor. A population change persists after the molecule that triggered it has cleared.

Worth naming because it cuts both ways: the same reasoning that justifies infrequent dosing also means plasma levels cannot be used to check whether a schedule is working, and there is no established biomarker that can.

The evidence, counted

Measure Count Read
PubMed records 621 13 Sep 2026
Human-tagged records 373 13 Sep 2026
Randomised-controlled-trial publication type 39 13 Sep 2026
ClinicalTrials.gov registrations 66 13 Sep 2026
DailyMed label records 0 13 Sep 2026

This is the best-evidenced compound on FDA's nominated-but-withdrawn list, and the margin is not close. That list holds sixteen substances, all peptides, indexed on our page covering FDA's category 2 record. Of the other fifteen, most return single-digit or low-double-digit human counts. Epitalon returns zero registered trials. DSIP returns zero. Thymosin alpha 1 returns 66.

The registrations are also current rather than archival: sepsis, hepatitis B in combination with entecavir, metastatic melanoma with interferon and dacarbazine, and a 2025 study of it as immunotherapy in advanced microsatellite-stable colorectal cancer.

What the hepatitis trials actually found

This is where the page has to be careful, because a large evidence base is not the same as a positive one.

Hepatitis B — 195 patients across four trials.

  • One study: HBV DNA clearance at six months in 9 of 17 patients on thymosin alpha 1, against 10 of 16 on interferon alfa-2b and 4 of 15 historical controls.
  • An open-label trial: clearance in 53 percent at six months.
  • A randomised controlled trial: clearance in 40.6 percent treated for six months and 25.6 percent treated for twelve, against 9.4 percent untreated.

That last line is the one worth pausing on. Both treated arms beat no treatment, and the twelve-month arm performed worse than the six-month arm — 25.6 against 40.6. A simple more-is-better dose-response does not produce that. It may reflect the populations, the endpoints or the trial design rather than the drug, but it is in the record and it is not what a summary reading "40 percent clearance" conveys.

Hepatitis C — 162 patients across three trials.

  • One trial: no significant difference from placebo in the number of patients reaching a normal serum ALT.
  • Two trials of combination therapy with interferon alfa-2b against interferon alone: ALT normal at six months in 71 percent versus 35 percent in one; 37.1 percent versus 16.2 percent for interferon alone and 2.7 percent for placebo in the other. HCV RNA clearance 65 percent versus 29 percent in the first.

So the hepatitis C picture is that the one monotherapy comparison against placebo was null, and the benefit appears in combination. The review's own conclusion, twenty-five years ago: the compound "may be useful as monotherapy for hepatitis B or in combination with IFN-alpha 2b for hepatitis C, but its effects on morbidity and mortality remain to be seen."

The adverse-event record, which for once is not empty

The 2001 review describes the compound as well tolerated, with most studies observing only local irritation at the injection site.

FDA's adverse event reporting system returned 37 reports naming thymosin alpha on 13 September 2026. That is more than any other research-market compound in this catalogue — by comparison, BPC-157 returns 18, ipamorelin 11, and epitalon, MOTS-c, KPV, selank and AOD-9604 return none at all.

The coded terms, in order:

Reaction term Reports
Off label use 8
Fatigue 3
Anaemia 2
Abortion spontaneous 2
Blindness 2
Interstitial lung disease 2

The most frequently coded term is not a symptom. "Off label use" heads the list, which tells you something about how the compound reaches people rather than about what it does to them.

These are spontaneous reports. A report records that somebody associated an event with a product; it establishes no causal link, carries no denominator, and cannot support a rate. Why that database is nearly empty for almost every compound in this catalogue, and why the emptiness is a fact about reporting channels rather than about safety, is the subject of our page on how few reports these compounds generate.

What is not established

  • No US approved product, therefore no labelled dose, indication, route or schedule reviewed by a regulator in this market.
  • No demonstrated effect on morbidity or mortality — the 2001 review's own caveat, and no source opened here supersedes it.
  • No established biomarker for whether a schedule is achieving the proposed immunological effect.
  • No evidence base for any research-market use. The trial programme is hepatitis, oncology, sepsis and infection. Nothing opened here studies it for recovery, wellness or longevity, which is what it is largely sold for.
  • No published rationale for the twice-weekly interval beyond the mechanistic argument above.

What a reader can check in five minutes

Search ClinicalTrials.gov for "thymalfasin" and read the count and the conditions — it takes about thirty seconds and immediately separates this compound from the rest of its shelf. Then search DailyMed for the same term and confirm it returns nothing. The contrast between those two results is the whole regulatory position.

Then open PMID 11381492, the 2001 review, and read the hepatitis B paragraph to the end rather than stopping at the first percentage.

Sources and dates

  • Ancell CD, Phipps J, Young L. Thymosin alpha-1. Am J Health Syst Pharm. 2001;58(10):879–85. PMID 11381492. Source for the 1.6 mg / 900 µg/m² twice-weekly dose, the pharmacokinetics, and the hepatitis B and C trial results.
  • Li X, et al. Gender-associated difference following COVID-19 virus infection: implications for thymosin alpha-1 therapy. Int Immunopharmacol. 2021;90:107022. PMID 33160854. Source for the 1.6 mg subcutaneous dose over 15 days in 78 patients.
  • PubMed record counts via NCBI E-utilities, title/abstract searches across thymalfasin and thymosin alpha 1. Run 13 September 2026.
  • ClinicalTrials.gov API v2, query "thymalfasin" — 66 registrations, individual records inspected. Run 13 September 2026.
  • DailyMed SPL database, queries for thymalfasin, thymosin and ZADAXIN. Run 13 September 2026, all returning zero.
  • openFDA drug adverse event endpoint, product name "thymosin alpha" — total count and MedDRA reaction term frequencies. Run 13 September 2026.
  • FDA, Certain Bulk Drug Substances for Use in Compounding That May Present Significant Safety Risks, nominated-but-withdrawn section. Content current as of 22 April 2026; read 13 September 2026.

Frequently asked questions

What is the standard thymosin alpha 1 dose?

The figure used throughout the hepatitis trial programme is 1.6 mg, expressed as 900 micrograms per square metre of body surface area, given subcutaneously twice a week. It is reported in a 2001 review of the compound in Am J Health Syst Pharm and recurs in later work — a 2021 COVID-19 study, for example, gave 78 patients 1.6 mg subcutaneously over 15 days. It is a trial dose rather than a labelled one: there is no US approved-product label for this compound, so no regulator has reviewed a dose for it in that market. This page reports the figure and its sources; it does not recommend it.

Is thymosin alpha 1 FDA approved?

No. The DailyMed label database returned no record for thymalfasin, thymosin or ZADAXIN when searched on 13 September 2026. The 2001 review describes it as then in Phase III trials for hepatitis C and Phase II for hepatitis B, and no US label has followed. It also appears on FDA's list of bulk drug substances nominated for use in compounding whose nominations were later withdrawn — a procedural status that is neither a restriction nor a clearance.

How much clinical evidence is there for thymosin alpha 1?

More than for anything else on FDA's withdrawn list. 621 PubMed records as of 13 September 2026, 373 of them human-tagged, 39 carrying the randomised-controlled-trial publication type, and 66 registered studies on ClinicalTrials.gov. For comparison, epitalon returns zero registered studies and DSIP returns zero. This is a compound with a genuine clinical research programme behind it, which is unusual in this catalogue and worth stating plainly.

Does thymosin alpha 1 work for hepatitis?

The results are mixed and the review that collects them says so. For hepatitis B, one randomised controlled trial found HBV DNA clearance in 40.6 percent of patients treated for six months and 25.6 percent treated for twelve, against 9.4 percent of untreated controls — better than no treatment, but with the longer course performing worse than the shorter, which is not the pattern a straightforward dose-response would produce. For hepatitis C, one trial found no significant difference from placebo in normalising ALT, and the two trials reporting benefit both used it in combination with interferon alfa-2b rather than on its own. The 2001 review's own conclusion was that effects on morbidity and mortality remained to be seen.

What is thymosin alpha 1's half life?

Approximately two hours in serum, with peak concentrations reached within two hours of administration and levels returning to baseline within 24 hours, as reported in the 2001 Am J Health Syst Pharm review. The interesting feature is the gap between that and the twice-weekly schedule used in trials: a compound cleared within a day is being dosed every three to four days. The explanation offered is mechanistic — the proposed action is on T-cell differentiation and function rather than on a receptor requiring continuous occupancy — so the biological effect is expected to outlast the plasma exposure.

What side effects have been reported for thymosin alpha 1?

The 2001 review describes it as well tolerated, with most studies observing only local irritation at the injection site. FDA's adverse event reporting system holds 37 reports naming thymosin alpha, which is more than any other research-market compound in this catalogue. The most frequently coded term among them is 'off label use' at 8 reports, followed by fatigue at 3, then anaemia, spontaneous abortion, blindness and interstitial lung disease at 2 each. These are spontaneous reports, not trial findings — a report records that someone associated an event with a product, not that the product caused it.

Is thymosin alpha 1 the same as thymosin beta-4?

No. They are different peptides with different sequences, different proposed mechanisms and entirely different evidence bases. Thymosin alpha 1 is a 28-residue immunomodulatory peptide with a hepatitis trial programme behind it. Thymosin beta-4 is an actin-binding peptide whose fragment is sold in the research market as TB-500, and which returned a single adverse event report and no comparable trial programme. Both appear on FDA's nominated-but-withdrawn list, and the shared family name is the only thing they have in common for dosing purposes.