Comfortis Side Effects

Please note - some side effects for Comfortis may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

Comfortis Side Effects - for the Professional


In a well-controlled US field study, which included a total of 470 dogs (330 dogs treated with Comfortis chewable tablets and 140 dogs treated with an active control), no serious adverse reactions were observed with Comfortis chewable tablets. All reactions were regarded as mild and did not result in any dog being removed from the study.

Over the 90-day study period, all observations of potential adverse reactions were recorded. Reactions that occurred at an incidence > 1% within any of the 3 months of observation are presented in the following table. The most frequently reported adverse reaction in dogs in the Comfortis chewable tablets and active control groups was vomiting. The occurrence of vomiting, most commonly within 48 hours after treatment, decreased with repeated doses of Comfortis chewable tablets.

Percentage of Dogs (%) with Adverse Reactions

a This number (n=139) is less than the total number of dogs in the safety population for the active control group (n=140) because one dog joined the study late and was only dosed at Month 3.

Month 1 Month 2 Month 3
Active Topical Control (N=139a) Comfortis
Active Topical Control (N=124) Comfortis
Active Topical Control (N=125)
Vomiting 12.7 12.2 7.8 3.2 5.8 4.8
Decreased Appetite 9.1 5.0 2.8 1.6 1.9 0.8
Lethargy 7.6 5.0 3.5 4.0 1.2 0.8
Diarrhea 6.7 5.0 4.3 0.8 1.2 0.0
Cough 3.9 5.0 0.4 2.4 0.0 0.0
Polydipsia 2.4 1.4 0.7 0.0 0.4 0.0
Vocalization 1.8 0.0 0.4 0.0 0.4 0.0
Increased Appetite 1.5 0.0 0.4 0.8 0.4 0.0
Erythema 1.5 0.0 0.4 0.0 0.4 0.0
Hyperactivity 1.2 1.4 0.0 0.0 0.4 0.0
Excessive Salivation 1.2 0.0 0.4 0.0 0.0 0.0

In US and European field studies, no dogs experienced seizures when dosed with Comfortis chewable tablets at the therapeutic dose range of 13.5-27.3 mg/lb (30-60 mg/kg), including 4 dogs with pre-existing epilepsy. Four epileptic dogs that received higher than the maximum recommended dose of 27.3 mg/lb (60 mg/kg) experienced at least one seizure within the week following the second dose of Comfortis chewable tablets, but no seizures following the first and third doses. The cause of the seizures observed in the field studies could not be determined.


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