Like all medicines, Revolution has some side effects. The most common is hair loss at the site of application with or without inflammation (redness, flaking) in cats. Other side effects reported in cats and dogs include vomiting, diarrhea with or without blood, anorexia (decreased appetite), lethargy (sluggishness), salivation, rapid breathing, pruritus (itching), urticaria (welts, hives), erythema (skin redness), ataxia (incoordination), fever and rare instances of death. There have also been rare reports of seizures in dogs.
If you have additional questions about possible side effects, talk to your veterinarian.