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Spinosad (Comfortis)
Revised: April 23, 2023
Published: April 17, 2008

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Comfortis, Trifexis

Available in 140 mg, 240 mg, 560 mg, 810 mg, and 1620 mg chewable tablets 


Historically, flea control was a labor-intensive process involving sprays, dips, foggers, yard sprays, powders, and more. As technology progressed, products became safer and more convenient culminating in a whole “next generation” of products starting with Program® in 1995, and Advantage® and Frontline® not long after. For the first time, flea control could be performed once a month in the simple form of a chewable tablet or smear of topical oil. These products were extremely effective and still are but there was room for improvement yet.

The best flea product would kill fleas so quickly that they would not be able to bite nor lay eggs but would still be safe for the pet and convenient to use. Unfortunately, no flea product is fast-killing enough to kill a flea before its first bite but pet safety has been mastered and it is certainly possible to kill fleas before they can lay eggs.

Comfortis®, with active ingredient spinosad, was released in late 2007 by the Eli Lilly veterinary division (now Elanco) to fill the need for an ultra-fast flea-killing tablet that lasts an entire month. The pill was originally released for dogs only but as of 2012, it has since been approved for cats. Spinosad is also available in another Elanco product called Trifexis®. Trifexis combines spinosad and milbemycin oxime, a broad-spectrum dewormer so as to provide heartworm prevention, intestinal parasite protection, and flea control in one tablet.

Spinosad works by activating nicotinic acetylcholine receptors in the flea nervous system, causing an overload in it. The flea seizes to death. 

Spinosad is derived from bacteria and is non-synthetic. It is considered to be a "green" insecticide and is approved for use on produce to be labeled as organic.

How This Medication is Used

Spinosad is given once a month orally to kill fleas. It is not helpful against other parasites though at higher (non-commercial) doses it will also kill ticks. Spinosad is used topically in humans to kill lice. As mentioned, spinosad is also available in a tablet called Trifexis® which combines spinosad with a broad-spectrum dewormer (milbemycin oxime) to cover fleas as well as heartworm, roundworms, hookworms, and whipworms

Oral spinosad should be given on a full stomach. This lessens the chief side effect (nausea) and also helps absorb it into the bloodstream for the most rapid possible flea kill.

Spinosad is approved for dogs or cats 14 weeks of age and older. The product combined with milbemycin oxime is approved for dogs 8 weeks of age or older as long as they weigh at least 5 lbs. The product is beef-flavored so as to be accepted by the pet as a treat.

Fleas begin to die 30 minutes after the pill is given and all fleas should be dead by four hours.

Spinosad is a prescription drug. It is not available without a valid doctor-patient relationship or at retail outlets.

Side Effects

During initial testing, 12% of dogs vomited after taking spinosad the first time with an additional 5% showing reduced appetite and another 5% having diarrhea. These numbers dropped with subsequent uses of the medication. Obviously, if a given pet seems to be intolerant to it, use another flea control product. That said, one way to mitigate the upset stomach is to give half of the dose in the morning and the other half in the evening.

Interactions with Other Drugs

Spinosad is felt to be safe in conjunction with all other flea control products and heartworm preventives. There is one interaction that bears mentioning: spinosad can increase the risk of ivermectin side effects when ivermectin is used at the high doses required to treat skin parasites such as demodectic mange. Low doses of ivermectin used in commercial heartworm prevention products are not problematic for this interaction.

It should be mentioned that numerous non-FDA-approved ivermectin doses have been circulated on the Internet allowing people to save money by using large animal formulations of ivermectin rather than those manufactured for dogs and cats. This practice rarely involves ivermectin doses comparable to those in the small animal products and involves doses that could be toxic to ivermectin-sensitive individuals, and most certainly involves doses that could be toxic if combined with spinosad. 

Concerns and Cautions

Comfortis® tablets are beef flavored but contain pork protein. They should not be a problem for pets with beef allergies but could be a problem for a pet with a pork allergy, rare as that might be.

Spinosad is best not used in pregnant or nursing females. During initial testing, spinosad was given to pregnant and nursing mother dogs at doses slightly above the recommended dose. Problems observed included loss of litter during pregnancy, poor weight gain of puppies, and vomiting. Problems did not occur after the puppies reached age 14 weeks.

Trifexis® (spinosad in combination with milbemycin oxime) is approved for puppies aged 8 weeks and older. Comfortis® (spinosad alone) is approved for puppies and kittens 14 weeks of age and older. 

Initial testing included use in dogs with pre-existing seizure disorders with doses higher than the recommended dose. Some of these dogs ended up with seizures. It is unknown whether or not this was caused by the product. For this reason, dogs with seizure disorders should probably use a different flea control product.

The dose for spinosad is slightly different for cats than dogs so the strength of the tablet needed for a cat does not always correspond to the tablet for a dog of the same weight.

When spinosad was approved for cats, Elanco did not make a new formulation; they simply changed the labeling of the boxes to reflect which size tablet should be given to which size cat. This means that the tablets were designed as chewable for dogs. They are large tablets as far as a cat is concerned; it may be necessary to give the tablets in pieces.

Vomiting is the most common side effect of spinosad. This side effect can be made less severe by breaking the pill in half and giving each half 12 hours apart. If vomiting occurs within one hour of taking the pill, the dose should be repeated. After one hour, enough spinosad should have been absorbed and re-dosing is not necessary.

Keep away from light and store at room temperature. Do not refrigerate.

If you miss a dose, give it when you remember it.

It is our policy not to give dosing information over the Internet.

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