The potential side effects of Ivermectin (the active ingredient in Heartgard) include liver problems, vomiting, diarrhea, loss of appetite, depression, lethargy,
skin eruptions, seizures, tremors, paralysis, autoimmune disorders, thyroid problems, fever, weakness, dizziness, coughing, nose bleeds, difficulty breathing, pneumonia, irritability,
sudden aggressive behavior, nerve damage, fertility problems, and
sudden death.