Gastrointestinal symptoms are the most common side effects after deworming a dog.The dog may vomit, have diarrhea and a lack of appetite.
When the veterinarian suspects a parasitic infection in a dog, he or she will give a deworming treatment, usually in the form of oral pills. The treatment either kills or forces the parasites to leave the dog's system. Depending on the level of infection, the side effects that occur in response to the treatment may be quite strong.
Most owners will see their dogs vomiting or having diarrhea or both with live or dead parasites in the mix. The dog will likely have a lack of appetite and may have some gastrointestinal distress. More severe side effects from the medication may include salivation, muscle twitching and seizures. If the dog is being treated for heart worms, it could be at risk of developing a thromboembolism from a blockage in the heart.