Comfortis is one of two brands of flea medications that contain the novel insecticide known as spinosad. Comfortis is duly approved by the FDA in the management of fleas in dogs and is available as a chewable beef-flavored tablet that has a maximum duration of clinical effectiveness of 1 month. It is only available with a veterinarian’s prescription.


Being an insecticide that contains the active ingredient Spinosad, Comfortis for dogs is indicated in the following.

Killing of Adult Fleas

Adulthood is one of the most prolific life stages of fleas, fully capable of laying tens of eggs per day for up to 3 weeks. Since adult fleas attach to the skin of their hosts, it is also here where they lay their eggs. Killing adult fleas will help stop the laying of eggs and effectively puts a stop to the life cycle of fleas.

comfortis for dogs

Killing of Larval Fleas

Flea eggs that hatch on hosts eventually turn into larvae. The pharmacodynamics of spinosad shows that it is most effective against the larval form of fleas, producing higher mortality rates among larval population than adult flea population. Since larva is considered the transitory stage before the pupa stage, this also helps put a stop to the flea life cycle.

Prevention of Flea Infestation

Killing both larvae and adult fleas can put an immediate stop to flea infestation as there are considerably less adult fleas that will lay eggs and fewer larvae that develop into pupae and then adult fleas.

Benefits of Comfortis

There are a variety of benefits that Comfortis can provide to both you and your pet. These can include the following.

  • Provides fast flea-killing action in as short as 30 minutes upon administration of Comfortis.
  • Provides month-long flea protection for your dog.
  • Kills both adult fleas and their larvae forms to help stop the flea life cycle.
  • The chewable tablet formulation allows for very convenient administration.

Related Post: Best Flea Collar for Dogs

How Comfortis Works

The active ingredient of Comfortis, spinosad, targets the binding sites of nicotinic receptors that accept acetylcholine. What spinosad does is that it blocks these binding sites so that acetylcholine will not be able to bind with their specific receptors. Since acetylcholine is needed in the generation of nerve impulse transmission, the blockage of these acetylcholine receptors lead to the cessation of nerve impulse propagation. In other words, spinosad causes paralysis which ultimately leads to the demise of the flea.

Potential Side Effects

One of the good things about Comfortis is that it is generally well-tolerated by dogs with the most common reported side effects being vomiting. Technically, if your pooch vomits within an hour of giving Comfortis, another dose should be given immediately. This is not to say that there may no longer be any other potential side effects. The following can be seen in a dog that has been given Comfortis.

  • Changes in appetite
  • Increased thirst
  • Decreased activity or lethargy
  • Excessive salivation
  • Cough
  • Skin redness
  • Diarrhea
  • Hyperactivity

If you observe the above manifestations, it is often wise to seek the advice of your veterinarian.

Things You Should Know about Comfortis

If your pet is concurrently being treated with ivermectin, caution should be exercised since the FDA has documented reports of ivermectin toxicity when used concurrently with Comfortis. Interestingly, these reported cases of ivermectin toxicity were all from off-label use of ivermection, usually in the management of demodectic mange. As such it is imperative that you keep this in mind.

Comfortis is a flea medication duly approved by the FDA for use on dogs and puppies with a minimum age of 14 weeks. It is a once-monthly dosing and as such scheduling should be strictly followed.

Related Post: Best Flea Treatment for Dogs

What You Should Tell Your Vet before He Prescribes Comfortis

Pregnant and lactating dogs should not be given Comfortis. While there are no studies showing that Comfortis is harmful to such dogs, there are no studies showing that they don’t have effects either. Also, if your pet has history of seizures, it is best that your vet knows about it.

How to Give Comfortis to Your Dog

Comfortis is available in color-coded boxes that represent the weight category of dogs as well as the corresponding dose of spinosad. Comfortis is a once-a-month medication and should only be given to pooches that are at least 5 pounds or at least 14 weeks old. The dosing is as follows.

  • 5 to 10 pounds – use the pink box with 140 mg of spinosad
  • 1 to 20 pounds – use the orange box with 270 mg of spinosad
  • 1 to 40 pounds – use the green box with 560 mg of spinosad
  • 1 to 60 pounds – use the blue box with 810 mg of spinosad
  • 1 to 120 pounds – use the brown box with 1,620 mg of spinosad
  • Over 120 pounds – use a combination of the different formulations

Comfortis is a chewable tablet in beef flavor so it can be safely chewed by your pet, included in its food, or given like any other tablet medications.

border collie

What to Do If You Miss a Dose

Missing a dose should not really be an issue since Comfortis is only given once a month. However, if you missed the date that you’re supposed to administer its monthly Comfortis, then give the dose the moment you remember it.

What to Do in Case of Comfortis Overdose 

While Comfortis overdose is a very rare phenomenon, it is not impossible especially if you don’t give the correct dose. If you suspect Comfortis overdose, bringing your pet to an emergency facility is a must.

Some Drug Interactions

Off-label use of ivermectin has been implicated in the increased incidence of ivermectin toxicity when used with Comfortis. It is often wise to consult your vet about the possible interactions of medications, nutraceuticals, and supplements that your pooch may be presently taking on Comfortis.

The once-monthly dosing, excellent safety profile, amazing flea-killing effects, and chewable formulation of Comfortis make it an excellent flea medication. Unfortunately, it doesn’t address other pests like ticks so you will still have to consider other medications for these pests.


  1. Oral Flea and Tick Treatment – PetMD
  2. Safer Ways to Kill Ticks and Fleas on Dogs and Cats – Pets WebMD


Olivia Williams
Olivia is our head of content for, mum of one and a true animal lover. With 12 different types of animal in her family, it's never a dull moment. When she isn't walking the dogs, feeding the cats or playing with her pet Parrot Charlie, you will find her product researching and keeping the site freshly updated with the latest products for your pets!


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