A 28-amino-acid thymus peptide sold as Zadaxin in dozens of countries for hepatitis B; not approved in the US, mixed trial results.
Also called Thymalfasin, Zadaxin, Ta1, TA-1, Thymosin alpha 1
Thymosin alpha-1 is a 28-amino-acid peptide first isolated and sequenced from calf thymus in 1977 by Allan Goldstein's lab. The synthetic version, thymalfasin, is sold as Zadaxin, a 1.6 mg injection approved since the mid-1990s in China and later in dozens of other countries, mainly for chronic hepatitis B and as an add-on to vaccines and cancer treatment. It acts on dendritic cells, the immune cells that decide how strongly T cells respond, through Toll-like receptors such as TLR9.
The human data are larger than for almost any other peptide on this site, and they are mixed. A Taiwanese hepatitis B trial in 1998 found 40.6% of patients cleared the virus markers 18 months after a 6-month course versus 9.4% of untreated controls, but a US placebo-controlled trial the next year found 14% vs 4%, which was not statistically significant. A 2026 Cochrane review of 10 hepatitis B trials rated nearly every outcome as very low certainty. In sepsis, a 361-patient trial hinted at fewer deaths, then the 1,106-patient TESTS trial in 2025 found 28-day mortality of 23.4% vs 24.1% on placebo: no difference.
In the US it has never been approved. FDA's compounding advisory committee voted 17 to 4 in December 2024 against adding it to the list of substances pharmacies may compound, citing weak evidence of benefit, so compounded US supply sits outside the normal rules. It is well tolerated in trials, with few side effects beyond injection-site discomfort. The uses promoted online, such as long COVID, chronic Lyme disease and general immune support in healthy people, have not been tested in controlled trials.
What it binds to, and what happens inside the body after that, step by step.
Thymosin alpha-1 acts mainly on dendritic cells and other innate immune cells through Toll-like receptors, especially TLR9 and TLR2. That shifts T-helper cells toward a Th1, virus- and tumor-fighting response and raises interferon and IL-2 output. In mouse and cell work it also switches on the IDO enzyme in dendritic cells, which builds regulatory T cells, so it is described as an immune balancer rather than a pure stimulant.
Human results and animal results are kept apart below. A result in rats tells you a mechanism is plausible. It does not tell you what happens in a person.
Humans: many RCTs exist, mostly small and mostly from China, Taiwan and Italy, in hepatitis B, hepatitis C, sepsis, cancer and COVID-19. The better-designed and larger trials (US hepatitis B 1999, TESTS sepsis 2025) were negative on their main outcomes, and Cochrane rates the hepatitis B evidence as low to very low certainty. Safety in humans is well documented and good. The mechanism work (TLR9, IDO, dendritic cells) is from cells and mice. We grade it B rather than A because, despite approvals outside the US, no high-quality trial has confirmed a clinical benefit.
1,106 adults with sepsis in 22 Chinese ICUs
28-day mortality was 23.4% on thymosin alpha-1 vs 24.1% on placebo (HR 0.99, 95% CI 0.77 to 1.27); no secondary or safety outcome differed.
Read the study10 RCTs, 1,349 adults with chronic hepatitis B
Pooled all-cause mortality RR 0.53 (95% CI 0.29 to 0.96, 3 trials), but the authors rated the certainty very low and said they are not sure it helps.
Read the study98 patients with chronic hepatitis B in Taiwan
Complete virological response at 18 months was 40.6% after a 26-week course vs 9.4% in untreated controls (P=0.004).
Read the study97 US patients with HBeAg-positive chronic hepatitis B
Complete response was 14% on thymosin alpha-1 vs 4% on placebo (P=0.084); the authors said the results did not confirm earlier trials.
Read the study361 ICU patients with severe sepsis in China
28-day mortality was 26.0% vs 35.0% (log-rank P=0.049, RR 0.74, 95% CI 0.54 to 1.02); this signal was not confirmed by the larger TESTS trial.
Read the study488 patients with metastatic melanoma
Median overall survival was 9.4 months with thymosin alpha-1 vs 6.6 months on control (HR 0.80, P=0.08), not statistically significant, with no added toxicity.
Read the study8 studies, mostly retrospective, in hospitalized COVID-19 patients
Pooled mortality RR 0.59 (95% CI 0.37 to 0.93) with very high heterogeneity (I2 84%) and no change in ventilation or hospital stay; the authors called for RCTs.
Read the studyHuman and mouse dendritic cells; mouse infection and transplant models
Thymosin alpha-1 primed dendritic cells through TLR9 and induced IDO, producing both Th1 immunity and regulatory T cells.
Read the studyThese are questions the published research has not answered.
No FDA approval for any indication. A US Phase III hepatitis B trial in 1999 did not show a significant benefit. Zadaxin is approved in about 35 countries, mostly in Asia and Latin America; approvals and indications vary by country. By early 1998 it was also approved for hepatitis B in Singapore and the Philippines, and SciClone listed Italy as approved for the vaccine-adjuvant use by December 1998; the company documents we found do not give the original years of those approvals. Thymosin alpha-1 is not named on the WADA Prohibited List, and because thymalfasin is an approved drug in several countries it falls outside the S0 catch-all; this is a different molecule from thymosin beta-4 (TB-500), which WADA does prohibit. Athletes should confirm with their anti-doping agency.
How much of Thymosin Alpha-1 is left after a single dose
Simple first-order model from the published half-life. Real clearance varies with dose, route, kidney and liver function and formulation. This is not a dosing schedule.
This table records the doses that trials and approved labels used, with the source for each row. It is a record of research, not a recommendation, and it is open to free members.
Which symptoms are expected and which mean stop and get help: side effects and red flags.
Tap a claim to see what the studies found.
It has never been approved by FDA. Zadaxin is approved in other countries, mainly for hepatitis B. A US Phase III hepatitis B trial in 1999 did not show a significant benefit.
A 361-patient trial in 2013 suggested fewer deaths, but the 1,106-patient TESTS trial in 2025 found 23.4% vs 24.1% mortality, no difference. Meta-analyses that include small trials look positive, but high-quality trials alone do not.
Pooled mostly retrospective studies suggested lower death rates in hospitalized COVID-19, with very inconsistent results. There are lab studies in cells from long COVID patients but no controlled trial in long COVID.
Every trial enrolled sick or immune-compromised patients. Nobody has tested whether a healthy immune system gets sick less often on it.
They share the name 'thymosin' because both were found in thymus extract, but they are unrelated proteins. Thymosin alpha-1 acts on immune cells; thymosin beta-4 (the parent of TB-500) binds actin and is studied for tissue repair.
FDA's 2023 concern was mainly missing safety data for compounded versions and possible immunogenicity, not a pattern of harm. The 2024 advisory committee vote against it was mostly about weak evidence that it works.
These pairings come up often. Almost none have been tested together in a trial, and the evidence note on each says so plainly.
Promoted online as an 'immune plus repair' combination for injury or illness recovery.
EvidenceNever studied together in humans or animals. TB-500 and BPC-157 have no human efficacy trials of their own, and both are WADA-prohibited.
Combined in hepatitis B, hepatitis C and melanoma trials on the idea that it strengthens the interferon response.
EvidenceTested in several RCTs; the combination added no toxicity, but benefits over interferon alone were inconsistent. This is a prescription-drug combination used under specialist care, not a self-directed stack.
Zadaxin is supplied as a lyophilized 1.6 mg powder with its own diluent. Follow the national product label; FDA has no US label for it.
In the ETASS sepsis trial each vial was reconstituted with 1 mL sterile water (1.6 mg/mL) right before injection. No published stability data exist for research-grade powders after mixing.
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