A slightly increased number of basophilic hepatic foci were observed in chronic rat studies with budesonide and in carcinogenicity studies an increased incidence of primary hepatocellular neoplasms, astrocytomas (in male rats) and mammary tumours (female rats) were observed. These tumours are probably due to the specific steroid receptor action, increased metabolic burden on the liver and anabolic effects, effects which are also known from other glucocorticosteroids in rat studies and therefore represent a class effect. No similar effects have ever been observed in man for budesonide, neither in clinical trials nor from spontaneous reports.
The drug's most common side effects are poor or reduced appetite, vomiting, lack of energy, diarrhea, and weakness. Occasionally, more serious side effects, including bloody diarrhea, collapse, severe sodium/potassium imbalance, and destruction of the adrenal gland may occur, and may result in death. In 2014, with input from CVM, the manufacturer updated the information about patient monitoring and side effects on the package insert. Although not proven to be caused by Vetoryl, some additional side effects reported to CVM and now included on the package insert are adrenal insufficiency, shaking, elevated liver enzymes and elevated kidney tests.