Thymosin Alpha-1 side effects

How often side effects were reported

The safety record of Thymosin Alpha-1 (Ta1) comes from clinical studies, mostly in patients with infections or cancer, who usually received 1.6 mg under the skin.¹² In those studies, the most common side effects were local irritation, redness or discomfort at the injection site.¹ In most studies, Ta1 was not linked to significant adverse events at doses of 1 to 16 mg under the skin for up to 12 months.¹

Larger studies and label data point the same way:

• TESTS sepsis trial (2025), 1,106 adults, double blind: no safety outcome differed significantly between Ta1 and placebo.³
• Hepatitis B trial (1998), 98 patients, randomized: the authors observed no significant side effects.⁴
• Flu vaccine trial (1989), 90 men aged 65 to 99, double blind: no toxicity was seen in either group.⁵
• Metastatic melanoma trial (2010), 488 patients: adding Ta1 to dacarbazine and interferon alfa caused no extra toxicity.⁶
• Zadaxin label, Indonesia (as summarized by FDA): drug-related adverse events occurred in under 1% of more than 2,000 people.²
• Post-marketing estimate (2016 book chapter): authors working for or advising Zadaxin's maker reported over 17 million doses in an estimated 350,000 or more people, with infrequent, mild adverse experiences.² This is an estimate linked to the manufacturer, not a controlled study.²

Reported side effects by frequency

Most common

• Injection-site reactions: irritation, redness or discomfort where the injection was given, the complaint seen most across studies.¹
• Italy's Zadaxin label (as summarized by FDA) reports no clinically appreciable side effects apart from occasional modest, temporary pain at the injection site.²

Rare, per the Indonesian label

• Redness of the skin (erythema), temporary muscle wasting (transient muscle atrophy), joint pain with hand swelling (polyarthralgia with hand edema) and rash.²

Reported in single studies or case reports

• Liver enzyme flares: in chronic hepatitis B, the Indonesian label says ALT can rise temporarily to more than twice its starting level during treatment.² It says treatment is generally continued unless signs of liver failure appear.²¹
• Thyroid changes: TSH abnormalities were reported in a hepatitis C study.¹ One FDA adverse event report described anxiety, atrial fibrillation and a temporary TSH drop in a man also taking peginterferon, so the cause is unclear.¹
• Fever and nausea: in a 1982 phase 1 study of 14 people with advanced cancer given single doses into the muscle, 1 had fever (at 2.4 mg/m2) and 2 had mild nausea (at 4.8 and 9.6 mg/m2).²
• Nipple pain: reported in one liver cancer study of 25 patients.¹

Serious events

• Stem cell transplant recipients: graft failure (engraftment failure) and fatal immune hemolytic anemia, in which the immune system destroys red blood cells, were reported with Ta1.¹²
• Infections after transplant: a case series in these patients found more infections with Ta1.¹
• FDA's reading: Ta1 could cause or worsen graft-versus-host disease and engraftment failure in stem cell transplant patients.²¹

Risks FDA flagged in 2024

• Immune reactions: FDA says compounded Ta1 may pose a significant risk of immunogenicity, an immune response against the peptide, for certain routes of administration.⁷
• Poor characterization: FDA found key data on impurities, aggregates and bacterial endotoxin missing for both the free base and the acetate.¹ It called the substance sensitive to formulation, processing and environmental conditions that may lead to aggregation and degradation.¹
• Vaccines: FDA says safety data are too limited to judge the right dose or the risks of giving Ta1 with flu vaccines.¹ It adds that putting an immune modulator into any vaccine could raise unknown safety concerns.¹

Warnings on labels abroad

FDA's 2024 review summarizes these warnings from Zadaxin labels outside the US.²

• Deliberate immune suppression: the Indonesian label says Zadaxin should be considered contraindicated in people kept immunosuppressed on purpose, such as organ transplant patients, unless benefits clearly outweigh risks.²
• Children, pregnancy and breastfeeding: Italy's label says it should not be used in children and is contraindicated in pregnancy and breastfeeding.²
• Autoimmune disease and allergy: Italy's label says use in autoimmune disease must be judged case by case, with caution in atopic people or anyone with past allergic reactions.²¹
• Other immune medicines: Italy's label advises caution when Zadaxin is combined with other immunomodulatory drugs, because Ta1 acts on lymphocyte function.²

Animal safety data

According to the Zadaxin product monograph as summarized by FDA, single doses up to 20 mg/kg caused no drug-related safety signals in mice, rats and marmosets.¹ Neither did repeated doses under the skin of up to 6 mg/kg a day for 13 weeks, or 1 mg/kg a day for 26 weeks.¹ FDA could not review the underlying data, and it found no carcinogenicity studies and no published reproductive toxicity data.¹ These are animal results and say nothing direct about people.

Research vials are a separate question

The human data on this page come from Zadaxin and from trial drug given under medical care.¹² FDA notes that Zadaxin is a finished drug product that cannot be used interchangeably with Ta1 as a substance.² Health Canada warns that unauthorized injectable peptides, thymosin alpha among them, may contain high-risk ingredients, additives or contaminants that may or may not be listed on the label.⁸ The vial sold on this site is labeled: For research use only. Not for human or veterinary use.

Related pages

Read the full Thymosin Alpha-1 profile, the study schedules in Thymosin Alpha-1 dosage, and the trial results in Thymosin Alpha-1 benefits and uses. For general background on peptide risks, see Peptide safety basics.