ZADAXIN (thymalfasin) is well tolerated. During clinical experience involving over 2000 individuals with various diseases distributed over all age groups, no clinically significant adverse reactions attributable to thymosin alpha 1 administration were reported (see table below).
| Disease | Adverse Event Rate | |
| Viral Infection | Chronic hepatitis B Chronic hepatitis C Human immunodeficiency | < 1% drug related adverse events for all indications |
| Cancer | Non-small cell lung cancer Melanoma | |
| Vaccine adjuvant | Hepatitis B vaccine Influenza vaccine | |
| Immune disorders | Autoimmune liver disease Primary immune deficiency |
Adverse experiences have been infrequent and mild, consisting primarily of local discomfort at the injection site, and rare instances of erythema, transient muscle atrophy, polyarthralgia combined with hand edema, and rash.