Also called Thymalfasin, Tα1, Zadaxin (brand, outside the US)
An immune peptide sold as a prescription drug in many countries, but not the US.
Thymosin alpha-1 is a small protein made of 28 building blocks (amino acids), first found in the thymus, the gland where T cells (key immune cells) grow up. A lab-made version called thymalfasin is a prescription drug in dozens of countries, mostly in Asia, for hepatitis B and C and as an immune booster. It is not FDA-approved.
Think of it as a coach for your immune system. It helps T cells mature and turns on signals your body uses to fight viruses and some tumors. It tends to nudge a weak immune system up rather than flood it.
Some human studies
It's one of the better-studied peptides in people, tested mostly outside the US in hepatitis B and C, cancer and severe infection, with mild side effects. Results are mixed: a 2013 trial in 361 people with sepsis (a body-wide infection) hinted at fewer deaths, but a bigger 2025 trial in 1,106 sepsis patients found no difference in deaths versus placebo. Nobody has tested whether it helps healthy people who just want stronger immunity.
Why: 1.6 mg twice a week is the dose from the hepatitis B trials and the approved product sold abroad. It only lasts a few hours in your blood. But the immune effects build up over weeks.
Why: The sepsis trials used frequent doses for one week in intensive care. The biggest one found it didn't help people survive. This is a hospital plan, not something to copy at home.
Start at the low end, and change one thing at a time. If you take medication or have a health condition, check with a doctor first.
Set up for a 5 mg vial. Change any number to match yours. New to this? Read how to mix a vial and mg vs mL vs units first.
Pick your dose and how long you'll run it. We'll count what you need.
Get help right away for swelling of the face or throat, trouble breathing, chest pain, or a spreading, hot, red injection site.
Powder: fridge, out of the light. Mixed with bacteriostatic water: fridge, use within about 4 weeks.
FDA: Not approved in the US, though approved as thymalfasin in many other countries. FDA removed it from its Category 2 compounding risk list in April 2026 after the nomination was withdrawn. It is not on the 503A bulks list.
Sport (WADA): Not named on the WADA list. Because it is approved as a medicine in some countries, the S0 catch-all may not apply; athletes should check with their anti-doping agency before using it.
Yes in many countries outside the US, where it's sold as thymalfasin for hepatitis B and other uses. No in the US.
Nobody has tested that in healthy people. The trials were in people who were already sick.
A 2013 trial looked hopeful. But a bigger 2025 trial found no difference in deaths compared with placebo.
Last checked 2026-10