Comfortis® for Dogs and Cats

PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet.

What Is Comfortis® for Dogs and Cats?

Comfortis® is a chewable prescription medication for dogs and cats to treat and prevent flea infestations. This flavored chew is given once a month to help combat fleas. 

Comfortis® For Dogs is approved for use in dogs and puppies 14 weeks of age and older and 5.0 pounds or heavier.

Comfortis® For Cats is approved for use in cats and kittens 14 weeks of age and older and 4.1 pounds or heavier.

Remember, preventative flea medications for pets only work when given on time as instructed, based on the medication’s directions.

Comfortis® Considerations

Comfortis® should be used with caution in breeding female dogs and in dogs with pre-existing epilepsy.

At the time of this publication, studies had not been performed to evaluate the safety of Comfortis® in breeding male dogs and in breeding, pregnant, or nursing cats.

Giving Comfortis® with certain medications can result in health risks to your pet, so it’s important to discuss your pet’s medications, including vitamins and supplements, and medical conditions with your veterinarian first.

How Comfortis® Works in Dogs and Cats

Once administered, Comfortis® works by disrupting the fleas’ nervous system within 30 minutes. Comfortis® kills 100% of fleas on dogs and 98% of fleas on cats after four hours. 

Comfortis® Directions for Dogs and Cats

Follow the directions on the drug label or as provided by your veterinarian. This medication should be given on a full stomach. Refer to the package label and your veterinarian to determine dosing directions. 

Missed a Dose? 

If you forget to give a dose of Comfortis®, give it when you remember and resume your normal dosing schedule. Do not give extra or double doses.  

Comfortis® should be given once a month. If you are late giving a dose of Comfortis®, give it when you remember. The next dose should be given one month from the day it was last given.  

If vomiting occurs within an hour of administration, give the full dose again. 

Possible Side Effects of Comfortis® in Dogs and Cats

Vomiting is the most common side effect seen with Comfortis®. This side effect can be lessened when Comfortis® is given on a full stomach, which also helps the body absorb the medication. 

Other less frequent side effects include: 

• Fatigue 

• Lack of appetite  

• Weight loss 

• Diarrhea 

• Cough 

• Increased thirst 

• Itching 

• Lack of coordination 

• Seizures 

Human Side Effects 

If you accidentally ingest a pet medication, immediately call your physician or the national Poison Control Center hotline at 800-222-1222. 

Monitoring 

No specific monitoring is required for this medication, but your veterinarian may recommend routine testing depending on your pet's individual needs if: 

• Severe side effects are seen (see above) 

• Your pet’s condition worsens or does not improve with treatment

• You see or suspect an overdose 

• You have additional questions or concerns about the use of Comfortis® 

Comfortis® Overdose Information in Dogs and Cats

Symptoms of a Comfortis® overdose are very similar to the side effects listed above. Vomiting is the most frequent symptom of an overdose. 

If you suspect an overdose, immediately contact your veterinarian or an animal poison control center. Consultation fees often apply. 

Pet Poison Helpline (855) 764-7661 

ASPCA Animal Poison Control (888) 426-4435 

Comfortis® Storage for Dogs and Cats

Comfortis® should be stored at controlled room temperatures from 68 to 77 F. Brief exposure to temperatures from 59 to 86 F are acceptable. Keep the medication in the provided blister packs until ready for use in order to protect the chews from moisture and light. 

Keep out of reach of children and pets.