The biggest drawback about Confortis to me is the expense. Almost $100 for a six month supply is steep, but worth it. I tried switching out topical ointments, but that was only temporary. It's the fleas that build an immunity, not the dog. I also use Sentinnel heartworm meds because it also has a flea fertility inhibitor. It's all very expensive, but worth it to me. With 3 dogs and 2 cats, a flea infestation is not an easy matter to deal with.