(for veterinary information only)


AVAILABLE IN          
90 mg, 140 mg, 240 mg,     
560 mg, 810 mg, and 1620 mg


In older times, flea control was a very 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 topical products require time to spread over the fur and skin oil of the pet, typically needing a day or even two depending on the amount of skin oil present. The flea is killed when it moves through the toxin treated hair. After a couple of days all the fleas that had been on the pet at the time of the product application are dead but new fleas will have arrived from the environment and they will not be dead for yet many hours (exactly how many hours depends on which topical is being used). The relative slow kill of the topicals meant that it was normal to see some fleas on the pet even if the product is working perfectly. The number of fleas on the pet is determined by the number of fleas jumping on the pet from the environment versus how fast the topical product can kill them. In the time it takes for fleas to be killed by the product, they are still biting the pet and possibly laying eggs.

What the public really wanted was a product that killed fleas fast, so fast that even with a heavy flea emergence from the environment, fleas would not be seen on the pet. While no product can kill a flea instantly, it seems that the oral products that reach fleas through the pet's bloodstream are the fastest acting.

The first ultra-fast flea product on the market was Capstar®, a tablet that began killing fleas in 30 minutes and could clear every flea on a pet’s body in a matter of hours. The problem was that this pill did not have residual activity. It could be given daily if needed but it would be expensive to do so. If only there could be a pill that worked as fast but lasted the whole month.

Comfortis®, with active ingredient spinosad, was released late 2007 by Eli Lilly and Company 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 been approved for cats.


Spinosad is given once a month to kill fleas. It does not kill other internal or external parasites. It is for dogs or cats 14 weeks of age and older. The product is best given on a full stomach and is beef-flavored so as to be accepted by the pet as a treat.

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


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 the medication, another flea control product should be selected.


Spinosad works by activating nicotinic acetylcholine receptors in the flea nervous system causing an overload of the flea nervous system. The flea seizures to death. 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 heartworm prevention 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, certainly involves doses that could be toxic to ivermectin-sensitive individuals, and most certainly involves doses that could be toxic if combined with spinosad. For more details on circumstances leading to ivermectin toxicity, please see the ivermectin page in the pharmacy library.


Comfortis® tablets are beef flavored but contain pork protein. They should not be a problem for pets with beef allergy 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 puppies reached age 14 weeks. Spinosad has not been evaluated for pregnancy or lactation in the cat.

Spinosad should not be used in puppies or kittens under age 14 weeks.

Initial testing included use in dogs with pre-existing seizure disorders with doses higher than the recommended dose. Some of these dogs broke 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 versus dogs so that 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 feline use, Elanco did not make a new formulation for cats; 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 chewables for dogs. They are large tablets as far as a cat is concerned; it may be necessary to give the tablets in pieces.

Page posted: 12/20/07
Page last updated: 8/9/2013