Incurin is a relatively safe medication for dogs and shouldn't cause aggressiveness in them. However, a study on 224 dogs by drugs.com revealed that on prolonged dosage duration between 14 to 42 days, around 1.8 percent of dogs showed aggressiveness. A total of three dogs had to be euthanized because of uncontrolled aggressiveness. While the number is pretty small, there are still chances that the medication might cause aggressiveness in some dogs. It's imperative to consult your veterinarian if you see any signs of aggressiveness in your dog when on Incurin.