Heartgard is quite potent and effective against heartworms and hookworms in cats. However, it must be administered according to the recommendation by a licensed veterinarian after a heartworm test as prescribed by the vet. There's a reason why the federal law of the US required a prescription for the medication before administering it to cats. Several reports have revealed some severe adverse reactions of Heartgard in cats if administered wrongly. These side effects include depression, lethargy, diarrhea, vomiting, mydriasis, anorexia, convulsions, ataxia, hypersalivation, and staggering.