Side Effects and Safety of Comfortis Flea Tablets in Dogs

Side Effects and Safety of Comfortis Flea Tablets in Dogs
Category: Comfortis

Summary: Switching to Comfortis for Dogs? Find out if there any side effects with this new product and how can it be used safely in your dog...

Side effects occur in a small number of dogs with all medications, and flea medicines are no exception. However, when compared with products currently on the market, Comfortis Flea Tablets for Dogs have a good safety profile with few side effects.

Comfortis Safety Summary

  • Comfortis has a high degree of safety, and compares favourably with other brand name flea control products.
  • Side effects are rare and usually minor in severity if they occur i.e. intestinal upset (see side effects information below).
  • Side effects can be reduced or eliminated entirely by following the administration guidelines listed below.

How to use Comfortis safely

Always make sure that you follow the label directions precisely - these provide a guide to using the medication within safe limits. Follow the guidelines below to use Comfortis safely in your dog and dramatically reduce the risk of side effects occurring:

  • Administer your dog's dose (as per their bodyweight) with a FULL MEAL.
  • Only use Comfortis in adult dogs and puppies over 14 weeks old.
  • Do not use if your dog is a breeding animal i.e. pregnant, nursing etc.
  • Do not use if your dog has been diagnosed as epileptic or has a history of seizures/tremors.
  • Avoid if your dog has been prescribed ivermectin by your vet for a condition.

By following the above guidelines and being aware of the potential side-effects you will be able to use Comfortis to provide safe, effective flea control for your dog.

Comfortis side effects

The majority of side-effects for Comfortis relate to the fact that it is an oral medication. Whenever you are giving oral medications there is always a chance that a dog with a sensitive stomach may have a reaction such as diarrhoea or vomiting.

The initial studies and post-approval data for Comfortis list the side effects for a small number of dogs (in decreasing order of frequency) as: vomiting, depression and lethargy, anorexia, ataxia, diarrhea, pruritus (itching), trembling, hypersalivation and seizures.

Vomiting and Diarrhoea

Comfortis is an oral medication, which means that some dogs with a sensitive stomach may vomit shortly after administration. It is therefore no surprise that this was the most common side effect reported in an FDA-approved study.

What may be surprising to some is that Comfortis dogs were only 0.5% (half a percent) more likely to vomit than dogs receiving a non-oral (spot-on) brand-name flea treatment. Lose stools were also slightly more likely in dogs treated with Comfortis.

To avoid this reaction, always make sure that the dose of Comfortis is given with a full meal. If vomiting occurs within one hour of administration, redose with another full dose the following day. It has been demonstrated that the incidence of vomiting is reduced dramatically or eliminated in sensitive dogs after they have received several monthly treatments.

Reduced Appetite and Lethargy

A small number of dogs may experience reduced appetite after administration or lethargy. If your dog experiences such symptoms please contact your vet, however the majority of such reactions are considered mild, and will decrease with continued use. Conversely, a percentage of dogs actually experience increased appetite, which is testiment to the palatability of the product.

Other Side Effects

Other side effects are considered extremely rare. Avoid Comfortis in dogs which have been diagnosed with epilepsy, or have a history of seizures or tremors. Avoid in dogs who have been prescribed ivermectin by their vet i.e. off-label use, although you may wish to avoid using ivermectin based heartwormers i.e. Heartgard in conjunction with Comfortis.

Comfortis can be combined safely with all common veterinary medications and pet products. If you have any concerns regarding the administration of Comfortis please contact your primary care veterinarian.

More Comfortis safety information

If you have further queries about this product please contact our vet. Also, please note that latest product information can always be found by visiting the manufacturer's website:


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Important: The information provided in this article is for general information only and is subject to change. Always verify the accuracy of the above first with your primary care veterinarian or in the case of a product, by reading packet and prescribing information before administering it to your pet. The specific needs and circumstances of your pet have not been taken into account. Please refer to our Terms & Conditions for further information.

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