A study performed at Auburn University tested 4 common heartworm preventatives: Heartgard Plus, Interceptor Flavor Tabs, Revolution, and Advantage Multi. The study found that only Advantage Multi was 100% effective against dirofilaria immitis (heartworm). Read the full story here.
I must disagree with you that Heartgard is a safe drug for all dogs, even those with mutant genes/drug sensitivities. I have 2 Chihuahua/Terrier mix rescue dogs who weigh about 15 pounds each. Both dogs were given Heartgard chewables in 2010 and within 1 hour of ingestion, I was at the emergency vet clinic because both dogs (not related) became violently ill. Both dogs had high fever, profuse drooling, tremors, and staggered so badly, they couldn’t walk. Both dogs were in a catatonic state and appeared to be blind. Both had respiratory distress. The ER vet administered a cortisone shot that stopped the severe allergic reaction in both dogs. Both dogs were ill for several days after the Heartgard incident with recurrent vomiting and loose stools. Both dogs have different genetic background, and probably not of the collie/shepherd genesis. Best guess is they are Chihuahua/Terrier/Pomeranian, and Heartgard made them so violently ill, I feared they might have died if not for ER intervention. I don’t care what the experts say, I lived through a horrendous ordeal with two small dogs in acute medical crisis due to Heartgard. They take Interceptor for heartworm prevention, and have no problems. Please do not assume that Heartgard is safe for all dogs, because I can prove that statement is false. My dogs almost died from Heartgard.