Box of ComfortisComfortis is a once-monthly flea fighting formula based on the active ingredient spinosad, a natural substance derived from soil microbes. It is an insecticide which works by attacking a flea’s nervous system and comes in the form of chewable tablets, avoiding the discomfort of topical solutions and the risks associated with pets accidentally licking topically applied products.

It is considered one of the most effective flea treatments, and owners who haven’t had success with other remedies have been able to rid their dog of fleas with this medicine. However, on the down side it has been known to cause harmful side effects in a small number of animals.

Does it kill ticks?

Comfortis is currently only advertised as being effective at killing fleas. However, it may also work against ticks, but the company is not allowed to advertise this until studies have been completed which prove its effectiveness. If you need to eliminate a tick infestation we recommend using a different medicine such as Frontline Plus for the time being.

Recommended Dosage

Note: You should never administer Comfortis without speaking to your vet first.

There are five different color-coded boxes of Comfortis, each made to suit a particular size of dog. The box you pick should match the weight of your pet. The table below will help you.

Weight of dog

Colored to use

3.3 – 4.9 lbsYellow box (90 mg spinosad)
5 – 10 lbsPink box (140 mg spinosad)
10.1 – 20 lbsOrange box (270 mg spinosad)
20.1 – 40 lbsGreen box (560 mg spinosad)
40.1 – 60 lbsBlue box (810 mg spinosad)
60.1 – 120 lbsBrown box (1620 mg spinosad)

Being a once-monthly treatment you will only need to administer one dose per month. In most cases a “dose” is a single tablet from the colored package matching the weight of the dog. For example, a 30 lb dog would need just one tablet from the green box per month. There is a special section on the back of the packaging where you can write the dates you administered the medicine, we recommended filling this in so you don’t forget when the next dose is due.

If vomiting occurs within the first hour the manufacturer recommends administering another full dose.

Note: If a dog weighs over 120 lbs you will need to use an appropriate combination of pills. As an example, a 130 lb dog would need one 1620 mg pill (brown box) and one 140 mg pill (pink box) for a total spinosad dosage of 1760 mg.

Is It Safe?

Many owners find Comfortis both easier to use and less stressful for their pets than topical treatments like Vectra 3D, but it may not be suitable for all dogs. There have been cases of bad reactions to the drug, and vomiting is somewhat common (though you can help to reduce the risk of vomiting by giving the tablets just after a meal).

Dogs under 14 weeks of age or those who are pregnant should not be treated with this product. The main ingredient spinosad has been shown to cause abortion in pregnant females, and also anorexia in puppies. Giving spinosad to dogs who are prone to seizures is thought to increase the risk of seizure activity, and dogs with a pork allergy could suffer a bad reaction due to the pork proteins contained within the tablets.

FDA Status: This product is FDA-approved for veterinary use under application number 141-277.

Guidelines Of Use

Because of how the drug works it can be unsafe when used incorrectly. You should always talk to your vet before using this medicine, and tell the vet about:

  • Any medical conditions your pet has
  • Other supplements or medication your dog is taking
  • Bad reactions your pet has had to flea treatments in the past

Remember to administer the tablets just after a meal to help avoid vomiting.

Important: Never use high doses of ivermectin (the anti-parasitic drug contained within Heartgard and Ivomec) in conjunction with spinosad, this combination could result in a severe reaction leading to seizures, blindness and tremors amongst other possible effects.

What Is It Used For?

Comfortis is used to prevent flea infestations or to kill existing adult fleas before they can lay eggs. Spinosad is very fast acting and begins working within 30 minutes. Just 4 hours after the medicine has been taken, most (or all) fleas will be dead.

Although the drug kills adult fleas, problems can persist as a result of pupae still being present. This is usually a sign that the home environment contains a large number of fleas. If you have other household pets suitable for treatment with this product you will be advised to give the medicine to them too, as treating all flea-prone animals in the home helps to control the insect population and prevent re-infestation.

Ridding your home of fleas:

If for some reason you suspect your home has a flea problem, there are several steps we recommend taking. The first is to put your dog’s bedding and any other washable items they commonly rest on into the washing machine. It is even better to buy new bedding if this is an option for you. The second step is to vacuum the house thoroughly, paying extra attention to darker areas where fleas are more likely to gather. Finally we recommend that owners invest in a dehumidifier for their home. Because fleas require humidity to survive these machines can effectively rid an entire room of these annoying parasites within a period of just 48 hours.

Comfortis Side Effects

A dog could experience the following effects during treatment:

Most Common

  • Vomiting
  • Lethargy
  • Loss of appetite

Less Common

  • Diarrhea
  • Loss of coordination
  • Behavioral changes
  • Increased hunger
  • Increased thirst
  • Skin redness
  • Trembling
  • Drooling
  • Seizures
  • Itching

Unconfirmed Reports

  • Temporary paralysis
  • Slow heartrate

Adverse effects should gradually stop occurring with repeated use. If unwanted effects persist or if your dog experiences any serious reactions you should contact the vet immediately.

More Information

Owners may find the following review of the product to be helpful:

When it comes to alternatives you have several options. Topical products such as Vectra 3D and Revolution are the most common but can be irritating to the skin. Topical products can also lead to itching as well as serious problems if the product is licked. If you’d rather avoid topicals you could try Sentinel Flavor Tabs, another oral medicine which is given once per month. Sentinel is able to break the life cycle of fleas by eliminating the eggs but it is not able to get rid of adult fleas.


Comfortis website
Dr. Becker
Plumb’s Veterinary Drug Handbook (sixth edition)