Quick answer: reported thymosin alpha-1 side effects are mostly mild. The most common is discomfort, redness or irritation at the injection site.[1] Less often, people have had rash, joint pain with hand swelling, or, in hepatitis B, a temporary rise in liver enzymes.[1][3] In the largest placebo-controlled trial, 1,106 adults with sepsis, side effects were no more common than on placebo.[2]
Thymosin alpha-1 (thymalfasin, TA1) is a 28-amino-acid immune peptide sold outside the US as Zadaxin. For its uses and evidence, see our thymosin alpha-1 profile and the longevity and immune peptides hub.
Side effects listed on Zadaxin labels
FDA’s 2024 review summarized two Zadaxin labels from outside the US.[1]
| Label | What it says about side effects |
|---|---|
| Italy (flu vaccine add-on) | No clinically noticeable side effects except occasional mild, short-lived pain at the injection site[1] |
| Indonesia (hepatitis B) | Under 1% drug-related adverse events in over 2,000 people; mainly injection-site discomfort, with rare redness, temporary muscle wasting, joint pain with hand swelling, and rash[1] |
The Indonesian label also warns about liver enzyme flares: the enzyme ALT can rise to more than twice its starting level. It says treatment should generally continue unless signs of liver failure appear.[1]
Side effects in clinical trials
| Trial | People | Safety findings |
|---|---|---|
| TESTS, sepsis (2025) | 1,106 | Adverse events in 66.4% on thymosin alpha-1 vs 67.6% on placebo; serious events 26.8% vs 29.3%; no unexpected serious related events[2] |
| ETASS, severe sepsis (2013) | 361 | No serious drug-related adverse event recorded[5] |
| Hepatitis B, Japan (2005) | 316 | All drug reactions mild; most were liver enzyme changes; similar at 0.8 mg and 1.6 mg[3] |
| Hepatitis B, Taiwan (1998) | 98 | No significant side effects observed[4] |
| Melanoma (2010) | 488 | No added toxicity when combined with chemotherapy and interferon[6] |
In TESTS, anemia was the most common adverse event overall.[2] Overall, FDA concluded that in most studies thymosin alpha-1 was well tolerated at doses of roughly 1 to 16 mg under the skin for up to 12 months, with injection-site irritation, redness or discomfort the most common reactions.[1]
Liver enzyme flares in hepatitis B
In the Japanese trial, 22 of 316 patients had an ALT flare, a temporary jump in liver enzymes, and treatment was paused in 16. All recovered, and none died of liver failure.[1][3] The authors linked the flares to the drug’s immune effect, and noted that flares also happen in the natural course of hepatitis B.[1]
If you have hepatitis B, liver tests during treatment are part of standard care. A flare is a reason to call your clinician, not to stop on your own.
Other effects reported less often
- Fever and nausea: in an early dose-finding study of single injections in 14 people with advanced cancer, one person had fever and two had mild nausea at higher doses.[1]
- Thyroid changes: in a hepatitis C trial, two people taking thymosin alpha-1 plus interferon had temporary drops and then rises in thyroid-stimulating hormone. Interferon alone can cause this, but the combination may raise the risk.[1][7]
- Heart rhythm: FDA’s adverse event database held one US report, a man on thymosin alpha-1 and peginterferon hospitalized with anxiety and atrial fibrillation. FDA noted these are known peginterferon effects, so the cause is unclear.[1]
Side effects in vaccine and combination studies
Many trials gave thymosin alpha-1 together with other treatments, which makes it hard to tell which drug caused a side effect:
- With interferon: hepatitis B and C trials often combined it with interferon, a drug with well-known side effects of its own. FDA noted that the thyroid changes and the heart rhythm report above are both listed as possible interferon effects.[1]
- With chemotherapy: in the melanoma trial, adding it to dacarbazine and interferon did not add toxicity.[6]
- With flu vaccines: the studies FDA reviewed concluded it was well tolerated, but several described side effects only briefly, some naming fever as the most common one. FDA said this was not enough to judge safety when combined with US vaccines.[1]
When to call a clinician
Contact a clinician promptly for a spreading or very painful injection-site reaction, a rash, joint swelling, fever, a racing or irregular heartbeat, or yellowing of the skin or eyes. Seek emergency care for trouble breathing or swelling of the face or throat, which can signal an allergic reaction. People with hepatitis B should keep their scheduled liver tests.
Who should avoid thymosin alpha-1 or use caution
The labels abroad list these warnings:[1]
- Children: the Italian label says it should not be used in children.
- Pregnancy and breastfeeding: contraindicated on the Italian label.
- Deliberate immune suppression: such as organ transplant recipients; treated as a contraindication unless the benefit clearly outweighs the risk.
- Autoimmune disease: use evaluated case by case.
- Allergies: caution in people prone to allergies or with past allergic reactions.
- Other immune drugs: caution when combined with other medicines that act on the immune system.
FDA added a specific concern for stem cell transplant patients: an immune-boosting drug could worsen graft-versus-host disease or affect engraftment.[1]
Immune reactions and product quality
Thymosin alpha-1 is a peptide, so the body can in principle make antibodies against it, a risk FDA raises for injected peptides generally.[1] In the US, products sold online are not approved medicines, so their purity, dose and sterility are not checked. For general advice on redness, lumps or pain after an injection, see peptide injection site reactions.
Is it safe long term?
Trials have lasted up to 12 months.[1] Authors affiliated with its maker estimate more than 17 million doses and over 350,000 people treated after approval, with infrequent, mild side effects.[1] That is reassuring for the approved uses and doses, but independent long-term data are limited, and the off-label, open-ended schedules promoted online have not been studied.
Thymosin alpha-1 dose and side effects
Side effects did not clearly rise with dose. In the Japanese trial, adverse events were similar at 0.8 mg and 1.6 mg.[3] For label and trial doses, see our thymosin alpha-1 dosage guide.
US status
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, mainly because of weak effectiveness evidence.[8] It does not appear in any of FDA’s 503A compounding categories as of May 14, 2026.[9] For a comparison with another immune peptide, see thymosin alpha-1 vs LL-37, and for the wider field, the best peptides for immune support and peptides for autoimmune disease.
Questions to ask a clinician
- Do I have an autoimmune disease, a transplant or a condition that makes immune stimulation risky?
- If I have hepatitis B, how often will we check liver enzymes?
- Could it interact with my other medicines, especially interferon or immune drugs?
- Is there an approved treatment with stronger evidence for my condition?
This is general information, not medical advice.
