As with any medication, sometimes side effects occur. See the Comfortis label for complete safety information.
For cats: In some cases, cats vomited after receiving Comfortis. If vomiting occurs within one hour of administration, redose with another full dose. Additional adverse reactions observed in studies were lethargy, decreased appetite, weight loss, and diarrhea. Use with caution with concomitant extra label use of ivermectin. See the Comfortis label for complete safety information.
For dogs: The most common adverse reaction reported is vomiting. Other adverse reactions reported in decreasing order of frequency are: depression/lethargy, decreased appetite, incoordination, diarrhea, itching, trembling, excessive salivation and seizures. Following concomitant extra-label use of ivermectin with Comfortis, some dogs have experienced the following clinical signs: trembling/twitching, salivation/drooling, seizures, ataxia, mydriasis, blindness and disorientation. See the Comfortis label for complete safety information.