Side effects can include hyperactivity or lethargy, excessive panting, itching, vomiting, nervousness, diarrhoea, seizures, and trembling. Other side effects that cats may exhibit include pupil dilation, increased heart rate, vocalization, fever, and salivation. Both cats and dogs may also experience an allergic reaction to the treatment that is characterized by hives, difficulty breathing, and swelling of the lips or tongue. There is more information from drugs.com about Capstar here and Comfortis here.