Generally speaking, if you use more formal deworming drugs, whether it is deworming inside or outside the body, the side effects are very small. The only obvious reaction may be that the corpses of parasites will appear in the feces. Some cats with weak stomachs may experience mild vomiting and loss of appetite, but they will recover in 1-2 days. If the cat continues to have severe vomiting and diarrhea after being dewormed, you need to go to the pet hospital for examination and treatment in time to see if the cat has licked the external deworming medicine. Some external deworming medicines are toxic. .