What is thymosin alpha-1? What it does, benefits and what the evidence shows

Thymosin alpha-1 (also called thymalfasin, sold as Zadaxin) is a 28-amino-acid peptide first isolated from calf thymus tissue. It is approved as a prescription drug in a number of countries, mainly for hepatitis B and as an immune adjunct, and we found no US approval for it. The human evidence is mixed: a Cochrane review of hepatitis B trials rates almost every finding as very low certainty, and the largest sepsis trial, published in 2025, found no reduction in deaths. In December 2024 an FDA advisory committee voted 17 to 4 against adding it to the list of bulk substances pharmacies may compound.

What thymosin alpha-1 is and how it works

Thymosin alpha-1 (Tα1) is a 28-amino-acid peptide that was first described and characterized from calf thymus in 1977 (Li et al., Peptides, 2010). The thymus is the organ where T cells mature, and the compound was originally isolated as the thing responsible for restoring immune function in mice whose thymus had been removed (King and Tuthill, Vitamins & Hormones, 2016). The version used in clinics and sold in vials is made by chemical synthesis, and the synthetic form is called thymalfasin (Dominari et al., World Journal of Virology, 2020).

The proposed mechanism is broad rather than precise. Tα1 is described as acting through Toll-like receptors on dendritic cells, which triggers signaling and immune-related cytokine production (King and Tuthill, 2016), and as enhancing T-cell, dendritic-cell and antibody responses (Li et al., 2010). In plain terms, the idea is a nudge that makes a weak immune response stronger. That pleiotropic (many-effects) profile is also why it has been tried in so many conditions.

One naming trap: thymosin alpha-1 is not thymosin beta-4 or its fragment sold as TB-500. They are different molecules from different families. Our TB-500 guide covers that one, and FDA's compounding pages list the two separately.

What people use it for

What the research actually shows

Approval status

A 2009 review states that Tα1 is approved in over 35 countries for hepatitis B and C and as an immune stimulant and adjuvant (Goldstein, 2009), and a 2020 review repeats the figure for thymalfasin (Dominari et al., 2020). Both are reviews, not regulator listings, so treat "35 countries" as a reported figure, not one we could check country by country. We searched DailyMed for thymalfasin, thymosin and Zadaxin on September 29, 2026 and found no US label, and an openFDA search of Drugs@FDA returned no match for thymalfasin, thymosin or Zadaxin. One of the reviews above says FDA "approved" thymalfasin as an orphan drug; we did not repeat that, because we found no FDA approval in either database and an orphan designation is not an approval.

Hepatitis B

The best-known trials are old. A 2005 Japanese trial randomized 316 people with chronic hepatitis B to two dose levels of Tα1 alone, with no placebo group. A year after treatment ended, about 23% of participants at the higher dose had cleared the hepatitis B e-antigen, and side effects were mild, mostly fluctuations in liver enzymes (Iino et al., Journal of Viral Hepatitis, 2005). A 2008 meta-analysis of four small trials (199 patients) compared Tα1 with interferon alpha. Interferon did as well or better at the end of treatment (the complete-response measure favored interferon), while Tα1 looked better six months after treatment ended (Yang et al., Antiviral Research, 2008).

The most recent and most cautious summary is a 2026 Cochrane review of 10 randomized trials with 1,349 participants, published between 1991 and 2018. It found that Tα1 may lower all-cause mortality and serious adverse events, but it rated the certainty of that evidence as very low and low respectively, and concluded that "we are not sure" whether Tα1 changes those outcomes. It also found little to no effect on quality of life and very uncertain effects on hepatitis B-related illness (Naing et al., 2026). One of the larger trials in the field, 690 people with hepatitis B cirrhosis, tested Tα1 added to entecavir and found no significant difference in liver decompensation, liver cancer or death, though the authors noted a tendency toward fewer liver cancers (Wu et al., Expert Opinion on Biological Therapy, 2018).

Sepsis

A 2013 Chinese trial of 361 people with severe sepsis found 28-day mortality of 26.0% with Tα1 versus 35.0% in the control group, a difference the authors called marginal (P = 0.062) (Wu et al., Critical Care, 2013). The same group then ran a much larger, double-blind, placebo-controlled Phase 3 trial (TESTS) in 22 Chinese hospitals with 1,106 adults. Mortality at 28 days was 23.4% with Tα1 and 24.1% with placebo (hazard ratio 0.99, 95% CI 0.77 to 1.27), and no secondary or safety outcome differed. The authors' conclusion was that the trial "found no clear evidence" that Tα1 lowers mortality in sepsis. Subgroup signals went in both directions, with possible harm in people under 60 and possible benefit in people with diabetes, and those are hypotheses, not findings (Wu et al., BMJ, 2025).

A 2025 meta-analysis pooled 11 randomized trials and found lower 28-day mortality overall, but the benefit disappeared in the high-quality and multicenter subgroups, and its trial sequential analysis suggested the total sample was still too small (Gu et al., Frontiers in Cellular and Infection Microbiology, 2025).

COVID-19 and cancer

In a retrospective review of 76 severe COVID-19 patients in Wuhan, mortality was 11% among those given Tα1 and 30% among those who were not (P = 0.044), alongside recovery of T-cell counts (Liu et al., 2020). Retrospective comparisons like this are prone to bias, because doctors decide who gets treated. A small randomized, open-label pilot of 49 hospitalized patients found faster recovery of CD4 T-cell counts but no significant difference in clinical recovery (Shehadeh et al., 2023).

For cancer, a 2026 scoping review mapped 26 clinical publications from 2016 to 2026. Most were from China, retrospective studies and case reports predominated, Tα1 was used as an add-on to chemotherapy, radiation, targeted therapy or checkpoint inhibitors, and the authors said heterogeneity limited comparison and asked for prospective controlled studies (Kim et al., 2026).

FDA's December 2024 advisory committee vote

This is the one to understand if you have seen "FDA reviewed thymosin alpha-1" in a forum thread. On December 4, 2024, FDA's Pharmacy Compounding Advisory Committee met to consider whether thymosin alpha-1 acetate and thymosin alpha-1 (free base) should be placed on the 503A bulks list, the list of bulk drug substances that can be used in pharmacy compounding under section 503A. FDA's proposal was that neither be included. The uses FDA evaluated ranged from hepatitis B and C, HIV and COVID-19 to melanoma, liver cancer, lung cancer, sepsis, COPD and chronic fatigue syndrome (FDA final summary minutes, December 4, 2024).

The recorded votes, from the minutes:

• Vote on treating the two forms together: 20 yes, 1 no, 0 abstain. Because one member voted no, the minutes report separate votes on each form.
• Should thymosin alpha-1 (free base) go on the list? 4 yes, 17 no, 0 abstain.
• Should thymosin alpha-1 acetate go on the list? 4 yes, 17 no, 0 abstain.

The minutes summarize the reasoning. Several members who voted no said there was no compelling evidence of clinical effectiveness and safety in the uses reviewed, and the majority on the acetate form cited a lack of convincing effectiveness data. Members who voted yes said the substance should be accessible to patients as an option (FDA, 2024).

Two limits on what this means. It was an advisory vote, and the minutes do not record a final agency decision. And it concerned pharmacy compounding of the substance, not the safety of a lab-made vial. FDA's current page on bulk substances that may present significant safety risks (content current as of April 22, 2026) lists thymosin alpha-1 in its section of nominated substances withdrawn by their nominators, and says compounded thymosin-alpha-1 may pose significant risk for immunogenicity for certain routes of administration and may have complexities with peptide-related impurities and API characterization (FDA, 2026). We did not find any FDA action later than these documents.

Side effects and risks

• In trials. Reported adverse events in the hepatitis B and sepsis trials were mostly mild or no different from placebo. The Japanese hepatitis B trial reported mild reactions, mainly liver-enzyme fluctuations (Iino et al., 2005), and the 2013 sepsis trial recorded no serious drug-related adverse event (Wu et al., 2013). In the 2025 Phase 3 sepsis trial no safety outcome differed from placebo (Wu et al., 2025).
• Immune stimulation. The cancer scoping review found safety reporting inconsistent and described one case of severe multisystem immune-related toxicity during combination treatment, without establishing that Tα1 was the cause (Kim et al., 2026).
• Immunogenicity and impurities. FDA's stated concerns about the compounded substance are immunogenicity and peptide-related impurities (FDA, 2026). Those are why purity and identity testing of a vial matter.
• Populations studied. The trials above enrolled patients being treated for a disease, in trials run largely in Asia. We found no long-term safety data in healthy people.

What the lab tests show

As of September 30, 2026, VendorVerify's database holds 9 thymosin alpha-1 vials from 7 vendors, drawn from 8 lab reports dated April 2025 to September 2026. Purity was measured on 9 of the 9: 8 came in at 99% or higher and the median was 99.11%. Dose versus label had a median of 12.8% over. As of September 29, 2026, the lowest purity was 98.38%, and only 1 vial landed within 10% of its label; one was about 27% over and one was about 29% under. By our failure definition (empty, under 95% pure, more than 20% under label or more than 50% over), 1 of the 9 failed, the vial that was about 29% under.

Read that as a small sample, not a verdict on any vendor: a single vendor can move these numbers, and the pattern so far is decent purity with fill amounts that drift from the label in both directions. Here is how the ranked vendors look across all compounds right now, since we don't have enough thymosin alpha-1 data to rank vendors for this compound alone. A vendor's overall record is a signal, not a guarantee for a compound they have had tested only a few times.

Buying smarter

Start with the thymosin alpha-1 results page and the vendor's full record, including how many reports sit behind the score and how recent they are. A vendor-posted COA is a starting point, and how to read a peptide COA and fake peptide COAs explain what to check. Because the samples so far show fill amounts drifting from the label, paying for a test on your own vial is worth considering: how to get a peptide tested walks through it, and endotoxin in research peptides covers when to add that result. If a vial fails, stop using that batch, tell anyone who bought from the same run, bring the report to the vendor, and submit it to VendorVerify.

Readers researching immune-related peptides often also look at thymulin, LL-37 and KPV. The peptide hub lists every compound we track.

Database figures without a date update automatically from VendorVerify's lab results; dated figures are snapshots. This isn't medical advice; thymosin alpha-1 is approved in some countries, has no US approval we could find, and its largest sepsis trial was negative.