Quick answer: the standard thymosin alpha-1 dosage on Zadaxin labels outside the US is 1.6 mg injected under the skin twice a week. For chronic hepatitis B, the doses are spaced 3 to 4 days apart for 6 months (52 doses).[1] Trials have used 0.8 mg to 6.4 mg per dose, and some short hospital trials gave 1.6 mg once to three times a day for about a week.[4][6][7][1] Thymosin alpha-1 is not FDA-approved, so there is no US label dose.[1]
Thymosin alpha-1 (thymalfasin, TA1) is a 28-amino-acid immune peptide sold as Zadaxin; its maker reported approvals in over 30 countries.[12] For its full evidence review, see our thymosin alpha-1 profile and the longevity and immune peptides hub.
Thymosin alpha-1 dosage on labels outside the US
FDA’s 2024 review summarized two public Zadaxin labels. Both are for a 1.6 mg powder for injection.[1]
| Country and use | Dose on the label |
|---|---|
| Indonesia: chronic hepatitis B in adults 18 and older | 1.6 mg under the skin twice weekly, 3 or 4 days apart, for 6 months (52 doses) without interruption[1] |
| Italy: add-on to flu vaccination in people with weak immune systems | 1.6 mg into a muscle or under the skin twice weekly for 4 weeks from the first vaccination, then again from week 8 to week 12[1] |
The Italian label says Zadaxin should not be used in children or during pregnancy or breastfeeding.[1] The Indonesian label treats people on deliberate immune suppression, such as organ transplant recipients, as a contraindication unless the benefit clearly outweighs the risk.[1] A clinician sets the dose and length of treatment where it is approved.
Thymosin alpha-1 doses used in clinical trials
FDA found that trials gave doses from roughly 1 to 16 mg, usually under the skin twice a week, for 1 day to 12 months, and that 1.6 mg was the most common dose.[1]
| Condition and trial | People | Dose and schedule |
|---|---|---|
| Hepatitis B, Taiwan (1998) | 98 | 1.6 mg twice weekly for 26 or 52 weeks[2] |
| Hepatitis B, US phase 3 (1999) | 97 | 1.6 mg twice weekly for 6 months[3] |
| Hepatitis B, Japan (2005) | 316 | 0.8 or 1.6 mg six times a week for 2 weeks, then twice weekly for 22 weeks[4][1] |
| Hepatitis C with interferon (1998) | 109 | 1.6 mg twice weekly for 26 weeks[9] |
| Severe sepsis, ETASS (2013) | 361 | 1.6 mg twice daily for 5 days, then once daily for 2 days[5] |
| Sepsis, TESTS (2025) | 1,106 | 1.6 mg every 12 hours for 7 days[6] |
| Metastatic melanoma (2010) | 488 | 1.6, 3.2 or 6.4 mg with chemotherapy[7] |
What the doses achieved
- Hepatitis B: in Taiwan, 40.6% of people on the 26-week course had a complete response 18 months after starting, versus 9.4% of untreated controls.[2] The US phase 3 trial saw 14% versus 4% on placebo, which was not statistically significant.[3]
- Dose comparison: in Japan, 0.8 mg and 1.6 mg gave similar overall results, though people with advanced liver scarring did better on 1.6 mg.[4]
- Sepsis: ETASS reported 26.0% versus 35.0% mortality, a borderline result.[5] The larger, double-blind TESTS trial found 23.4% versus 24.1%, no meaningful difference.[6]
- Melanoma: the 3.2 mg groups had more tumor responses than control, but survival differences were not statistically significant, and 6.4 mg did not do better than 3.2 mg.[7][1]
Doses used with flu vaccines
Besides the Italian label schedule, FDA reviewed five published studies that tested thymosin alpha-1 alongside flu vaccines, mostly in older adults or people on dialysis. Four gave 0.9 mg per square meter of body surface (roughly 1.5 mg for an average adult) twice weekly for 2 to 5 weeks after vaccination. A 2012 dialysis study gave 3.2 mg or 6.4 mg a week before and again on the day of vaccination.[1] FDA said these studies described side effects too briefly to judge safety with US vaccines, and that adding an immune-active drug to any vaccine needs proper trials.[1]
Who should not follow these doses
The labels abroad set limits on who should take thymosin alpha-1 at all:[1]
- Children: the Italian label says it should not be used.
- Pregnancy and breastfeeding: contraindicated on the Italian label.
- Transplant recipients and others on deliberate immune suppression: treated as a contraindication unless the benefit clearly outweighs the risk.
- Autoimmune disease and allergies: use is decided case by case, with caution.
Thymosin alpha-1 dosage “when sick”
People often search for a dose to use during a cold, flu or other infection. No trial has tested thymosin alpha-1 for everyday infections in otherwise healthy people.
The closest data come from hospital trials in very sick patients:
- Sepsis: 1.6 mg every 12 hours for 7 days in intensive care, with no reduction in 28-day deaths in the largest trial.[6]
- COVID-19: an Indian trial gave 1.6 mg two or three times a day for 7 days alongside standard care and reported fewer deaths in severe cases.[10][1] Another open-label trial of 49 hospitalized adults gave 1.6 mg daily for 7 days and found no difference in recovery or deaths.[1]
These were short courses given and monitored in hospital. They do not support taking thymosin alpha-1 at home for an infection, and an infection serious enough to worry about needs medical care.
How the body handles a dose
- Absorption: after an injection under the skin, blood levels peaked in about 1 to 2 hours.[8]
- Half-life: under 3 hours, with averages of 1.8 to 2.6 hours across three products.[8][1]
- No build-up: in 9 healthy volunteers dosed daily for 5 days, levels did not accumulate.[8]
- Formulation matters: one product reached higher blood levels than the other two.[8]
Thymosin alpha-1 protocols sold online
Online “protocols” often suggest daily injections or longer cycles for fatigue, long COVID or general immune support. Those schedules do not come from the approved labels or from trials in those conditions. For what the research shows in those areas, see our guides to peptides for long COVID, peptides for chronic fatigue and the best peptides for immune support.
Our peptide calculator explains vial math in general, but it cannot make an off-label schedule safe.
Side effects at studied doses
At these doses, thymosin alpha-1 has generally been well tolerated. The most common problem is discomfort at the injection site, and hepatitis B patients may have a temporary rise in liver enzymes.[1] See our full guide to thymosin alpha-1 side effects.
US status: why there is no US dose
Thymosin alpha-1 has never been FDA-approved.[1] In December 2024, FDA’s compounding advisers voted 17 to 4 against adding it to the list of substances pharmacies may use.[11] It does not appear in any of FDA’s 503A compounding categories as of May 14, 2026.[13] For how it compares with another immune peptide, see thymosin alpha-1 vs LL-37.
Questions to ask a clinician
- Is there an approved treatment with stronger evidence for my condition, such as antiviral drugs for hepatitis B?
- Do I have an autoimmune disease, a transplant or another reason not to stimulate my immune system?
- If I am treated where it is approved, how long should the course last and what tests will we track?
- What would tell us it is not working?
This is general information, not medical advice.
