Comfortis for Dogs | Wag!
Comfortis for Dogs | Wag!

Comfortis is a once-monthly oral medication used as a flea killer and preventative for dogs and cats. This medication is a flea neurotoxin. When the flea bites a dog treated with Comfortis, they ingest the drug, which takes effect within 30 mins. Comfortis's active ingredient spinosad overwhelms the flea's nervous system, eventually causing paralysis and cell death. 

Spinosad is a natural byproduct of bacteria S. spinosa that colonize within certain types of dirt. This compound has a variety of applications and is commonly used as a pesticide to control populations of biting and crop-destroying insects. Though toxic to fleas, spinosad is a safe and natural solution for canine flea infestations at the appropriate dosage.


Comfortis doses range from 14 mg to 28 mg per pound and come in 5 strengths for weight groups from 5 lbs to 120 lbs. This medication is only approved for flea prevention in dogs over 3.5 months old and 5 lbs.  

Dosage amounts according to weight group

  • 5 to 10 lbs: 140 mg

  • 10 to 20 lbs: 270 mg

  • 20 to 40 lbs: 560 mg

  • 40 to 60 lbs: 810 mg

  • 60 to 120 lbs: 1,620 mg

Dosage instructions

Comfortis is a convenient way to treat fleas for dogs because it's only one pill a month. Since it's flavored, most pets will gobble it down without much persuasion. Make sure your dog eats a sizable meal before giving this medication.

Simply remove the Comfortis tablet from the blister pack and give it to your dog like a treat — then repeat in 30 days. Make a note in your calendar on the date of their dose and when the next pill is due.  


A controlled study of Beagles found that Comfortis killed over 50% of adult fleas within 30 minutes and 100% of fleas after 4 hours of ingestion. Further investigation found that this medication lowered egg production reinfested dogs by nearly 99%. 

Active ingredients in Comfortis



A 6-month supply of chewable Comfortis tablets for dogs costs $100 on average. This evens out to less than $20 per month.

Side effects


Giving Comfortis to pregnant dogs increases the risk of early and late-term miscarriage and neonatal demise, especially in high doses. Researchers also noted low birth weight, trouble with temperature regulation, and low activity levels in neonatal puppies born to canines given Comfortis.

Comfortis passes through the mother’s milk and may pose a risk to nursing puppies. Ask your vet about safe flea preventative options for lactating dogs. 

Research shows Comfortis increases the risk of hepatotoxicity when taken with ivermectin — a dewormer used to treat mange. Research indicates that combining these two anti-parasitics can cause poor motor function, confusion, seizures, loss of eyesight, 

This medication may induce seizures in epileptic dogs. Monitor your dog very closely at the start of treatment and make a vet appointment if your dog exhibits any pre-epileptic symptoms. 

Drug interactions

The anti-parasitic ivermectin is one of Comfortis's few contraindications. Don't use this medicine in addition to other drugs containing spinosad, since this may cause an overdose. Drug interactions include:

  • Ivermectin

  • Trifexis

  • Topical and oral medications containing spinosad

Allergic reactions and sensitivity

Do not give Comfortis to dogs with allergies to pork or medications containing spinosad, like Trifexis. Most dogs tolerate Comfortis well, and adverse reactions are uncommon. The most common side effect of a spinosad sensitivity is vomiting. This is typically not a major concern, and nausea resolves in a few hours. 

Call your vet if your dog experiences any of these symptoms with or without nausea:

  • Scratching

  • Confusion

  • Swelling of the face, mouth, throat, tongue, or extremities

  • Loss of consciousness

  • Seizures

  • Struggling to breathe

Frequently asked questions

What does Comfortis taste like?

These tablets have an artificial beef flavor.

Is Comfortis safe for dogs used for breeding?

This medication hasn't been studied in canine studs. Research shows Comfortis is a high-risk medication for pregnant canines. Look into other flea treatment options if you think your dog is pregnant.

Should I give Comfortis for dogs who are allergic to pork?

No. This medication has ingredients that are pork-derived. Look into alternative treatments such as topical flea gels and sprays. 

Should I use a flea collar or topical flea treatment with Comfortis for dogs?

No, topical flea treatments are unnecessary since Comfortis breaks the reproductive cycle and is 100% effective at eliminating adult fleas. Combining Comfortis with certain other medications may cause serious side effects. 

Is Comfortis still effective for dogs after a bath?

Yes! Since Comfortis isn't a topical treatment, you can bathe your dog right after treatment. 

Is it okay to give food with Comfortis? 

It's recommended that you give this medication with food since it can cause stomach issues. 

How do I know if my dog has fleas?

Signs of a flea infestation include scratching and vocalization. You may see the adult fleas moving about on shorthaired or light-colored dogs, particularly on the belly where the hair is thinner. You may also observe flea eggs, larvae, and dirt-like debris caused by the digestion of your pet's blood, but not always. Tapeworms can also indicate an infestation since fleas sometimes transmit them. 

Comfortis Questions and Advice from Veterinary Professionals

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