Underground Biohacking
Immune

Thymosin Alpha-1

Chris Hallewell, founder of Underground Biohacking

By , Founder, Underground Biohacking

Last reviewed

Approved pharmaceutical (Zadaxin) in 35+ countries (not US/UK). Research use elsewhere.

Thymosin alpha-1 has more real human evidence behind it than almost any other research peptide. It is an approved prescription medicine in more than 30 countries outside the United States, mostly for chronic hepatitis B. Trials in hepatitis B, sepsis and COVID-19 have reported benefits. But the largest and best designed sepsis trial, published in 2025, found no effect on deaths. The picture is genuinely mixed. Nobody has tested it in healthy people looking for an immune boost.

What is Thymosin Alpha-1?

Thymosin alpha-1 is a peptide made of 28 amino acids. Your body makes it naturally in the thymus. The thymus is the small gland behind your breastbone where T cells learn their job. The lab made copy is called thymalfasin. It is not a hormone and it is not an antibiotic. It belongs to a class called immune modulators. That means it does not attack a virus or a bacterium itself. Instead it changes how your immune cells behave. Research shows it binds to Toll-like receptors, the sensors immune cells use to spot trouble. Switching those on pushes dendritic cells and T cells to mature and respond. Because it is a modulator, it can calm an overreacting immune system or wake up a flattened one, depending on the situation. Structure matters here. Twenty-eight amino acids is far too big to survive the stomach, so it is given as an injection under the skin. Its front end carries an acetyl group, a small chemical cap that slows breakdown in the blood. The drug is chemically identical to the peptide your own thymus makes. Its safety record in patient trials has been mild.

What is Thymosin Alpha-1 used for?

  • In 361 people with severe sepsis, a hospital trial found fewer deaths within 28 days on thymosin alpha-1 than on standard care alone.[1]Human study
  • A 2025 pooling of 11 randomised sepsis trials found it may cut 28 day deaths, with the benefit differing by patient group.[2]Human study
  • In people with chronic hepatitis B, response rates six months after treatment ended were better with thymosin alpha-1 than with interferon alpha.[3]Human study
  • In people who have hepatitis B with cirrhosis, adding it to the antiviral entecavir raised clinical response rates compared with the antiviral alone.[4]Human study
  • In people with moderate to critical COVID-19, pooled data from eight studies pointed to a lower death rate.[5]Human study
  • In people on dialysis given a pandemic flu vaccine, adding thymosin alpha-1 raised the antibody response to the shot.[6]Human study
  • In a records review of lung cancer patients matched pair by pair after surgery, five year survival was higher in those who received it.[7]Human study
Thymosin Alpha-1 dosage chart
Typical dose1.6mg subcutaneous twice weekly (clinical). 800mcg-1.6mg per injection, 2-3 times weekly (research).
Cycle length12-24 weeks; pulsed cycling per protocol
FormInjectable (subcutaneous)

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How does Thymosin Alpha-1 work?

A 28-amino-acid peptide hormone. Modulates T-cell function, dendritic cell activity, and Th1/Th2 balance. Approved as a pharmaceutical (Zadaxin) in 35+ countries for chronic hepatitis B/C, immune deficiency, and cancer adjunct therapy.

How much Thymosin Alpha-1 should you take?

1.6mg subcutaneous twice weekly (clinical). 800mcg-1.6mg per injection, 2-3 times weekly (research).

How long should you run Thymosin Alpha-1?

12-24 weeks; pulsed cycling per protocol

How is Thymosin Alpha-1 supplied?

Injectable (subcutaneous)

What are the side effects of Thymosin Alpha-1?

EffectHow seriousWhat to watch
There is no human safety record for healthy adults taking it outside medical care. Every safety figure below comes from patients being treated for infection, liver disease or cancer.UnknownNone. You cannot monitor a risk nobody has measured.
Mild swings in liver enzymes were the main reported reaction in a hepatitis B trial. The authors read this as the immune system reacting, not as damage.[8]MildLiver blood tests, before and during treatment.
In the severe sepsis trial of 361 patients, no serious reaction was traced to the drug.[1]None recordedNone specific in that trial.
In a placebo controlled trial of 1,106 adults with sepsis, safety results were no different from placebo.[9]No signal versus placeboNone specific in that trial.
A hospital records study in COVID-19 patients found worse recovery in those given it, and the effect was largest in the sickest patients. This was not a randomised trial, so sicker patients may have been the ones chosen for it.[10]Potentially serious in critically ill patientsTalk to the treating doctor before using it during an acute severe illness.

Is Thymosin Alpha-1 legal in 2026?

Checked 6 August 2026. In the United States there is no FDA approved thymosin alpha-1 product. FDA lists thymosin-alpha 1 in category 2 of its bulk drug substances review, the group it says may present significant safety risks in compounding. The agency's stated concerns are immunogenicity for certain injection routes, plus peptide impurities and difficulty characterising the substance. FDA says the safety information it holds is not adequate to understand how big those issues are. That page was current as of 22 April 2026. Outside the United States the position is different. The synthetic form, thymalfasin, is an approved prescription medicine in more than 30 countries, mainly for chronic hepatitis B. Approval abroad does not change the United States position.

Status checked against the primary source.

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Deep dives on Thymosin Alpha-1

Frequently Asked Questions

Does thymosin alpha-1 actually work?
It depends entirely on the condition. In chronic hepatitis B it is an approved medicine in many countries, and the trials back that. In sepsis the story is messier. An early trial in 361 patients found fewer deaths. A much larger placebo controlled trial in 1,106 patients, published in the BMJ in 2025, found no difference. So it does something in some conditions and nothing in others. No trial has tested it in healthy people.
Is it approved as a medicine anywhere?
Yes. The synthetic form is called thymalfasin. It is an approved prescription medicine in more than 30 countries outside the United States. Most of those approvals cover chronic hepatitis B, and some cover improving vaccine response in people with weakened immune systems. It is not approved by the FDA in the United States. Approval abroad means a regulator reviewed real trial data. It does not mean the peptide works for everything sold under its name.
Can I get it legally in the United States?
There is no FDA approved thymosin alpha-1 product in the United States. FDA has also placed it in category 2 of its bulk drug substances review. That is the group flagged for possible significant safety risks in compounding, and FDA says its safety information is not adequate. Compounding it is not a route the agency has cleared. Anything sold outside a prescription is unregulated, and what is actually in the vial is unverified.
Will it boost my immune system if I am healthy?
Nobody knows, because nobody has tested it. Every trial on this page studied sick people: hepatitis B, sepsis, cancer, COVID-19. Those immune systems were damaged, suppressed or overwhelmed. Thymosin alpha-1 is a modulator, which means it pushes an unbalanced response back toward normal. There is no evidence that pushing an already normal immune system does anything useful. Treat healthy immune boosting claims as untested marketing.
What did the COVID-19 research actually show?
Mixed results, pulling in both directions. A 2023 pooling of eight studies found lower death rates in moderate to critical COVID-19 patients. The same analysis found no difference in ventilation needs or hospital stay. Pulling the other way, a study across five hospitals in Hubei found worse recovery in patients given it, especially the sickest. That study was not randomised, so those patients may have been sicker to begin with. The question is not settled.
What should I ask a doctor about it?
Ask what condition you would be treating, and whether a trial exists in that exact condition. Ask about liver blood tests, since mild enzyme swings turned up in hepatitis B trials. Ask where the product would come from, since there is no FDA approved version in the United States. Ask what happens if nothing changes. And ask whether an approved treatment for your condition already exists, because one usually does.

References

  1. 1.Wu J, et al. (2013). The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Critical Care. PMID: 23327199. DOI: 10.1186/cc11932.
  2. 2.Gu B, et al. (2025). Efficacy of thymosin α1 for sepsis: a systematic review and meta-analysis of randomized controlled trials. Frontiers in Cellular and Infection Microbiology. PMID: 40969554. DOI: 10.3389/fcimb.2025.1673959.
  3. 3.Yang YF, et al. (2007). Comparison of the efficacy of thymosin alpha-1 and interferon alpha in the treatment of chronic hepatitis B: a meta-analysis. Antiviral Research. PMID: 18078676. DOI: 10.1016/j.antiviral.2007.10.014.
  4. 4.Peng D, et al. (2020). The clinical efficacy and adverse effects of Entecavir plus Thymosin alpha-1 combination therapy versus Entecavir Monotherapy in HBV-related cirrhosis: a systematic review and meta-analysis. BMC Gastroenterology. PMID: 33076834. DOI: 10.1186/s12876-020-01477-8.
  5. 5.Soeroto AY, et al. (2023). The efficacy of thymosin alpha-1 therapy in moderate to critical COVID-19 patients: a systematic review, meta-analysis, and meta-regression. Inflammopharmacology. PMID: 37845598. DOI: 10.1007/s10787-023-01354-2.
  6. 6.Carraro G, et al. (2011). Thymosin-alpha 1 (Zadaxin) enhances the immunogenicity of an adjuvated pandemic H1N1v influenza vaccine (Focetria) in hemodialyzed patients: a pilot study. Vaccine. PMID: 22178096. DOI: 10.1016/j.vaccine.2011.12.014.
  7. 7.Guo CL, et al. (2021). Impact of thymosin α1 as an immunomodulatory therapy on long-term survival of non-small cell lung cancer patients after R0 resection: a propensity score-matched analysis. Chinese Medical Journal. PMID: 34732663. DOI: 10.1097/CM9.0000000000001819.
  8. 8.Iino S, et al. (2005). The efficacy and safety of thymosin alpha-1 in Japanese patients with chronic hepatitis B; results from a randomized clinical trial. Journal of Viral Hepatitis. PMID: 15850471. DOI: 10.1111/j.1365-2893.2005.00633.x.
  9. 9.Wu J, et al. (2025). The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ. PMID: 39814420. DOI: 10.1136/bmj-2024-082583.
  10. 10.Liu J, et al. (2021). Efficacy of Thymosin Alpha 1 in the Treatment of COVID-19: A Multicenter Cohort Study. Frontiers in Immunology. PMID: 34408744. DOI: 10.3389/fimmu.2021.673693.

This content is for educational purposes only. These compounds are intended for research use. Nothing here is medical advice.