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Side Effects of Dog Flea Treatment

| Updated September 26, 2017

When it comes to protecting your dog, you want to make sure you have the best possible prevention against fleas, ticks and other parasites. However in some dogs, small dogs especially, problems can arise.

Fleas are a consistent annoyance to dogs; evident by their constant itching. An allergic reaction to flea saliva can cause problems for the most stalwart pup. Even with an ability to manage past the itch of fleas, these parasites can cause anemia through blood drinking, and hair loss through scratching.


  • If you are ever concerned that your dog may be experiencing a negative reaction to flea treatment, stop the treatment immediately, and take him to a qualified vet.

Most flea and tick products are monitored through the Food and Drug Administration to prevent harm to your dog, but depending on the size and potential allergens, a reaction is possible. Spot-on pesticides -- or those applied directly to the dog's skin -- tend to have more severe side effects than others.

Mild Reactions

When side effects happen, they primarily attack your dog's nervous system. Symptoms that indicate a mild poisoning include:

  • Itching at the spot the medication was applied.
  • Respiratory issues.
  • Paw flicking, ear twitching, vomiting and diarrhea.

If any of these occur, it may be a case of allergic reaction to the pesticides included in flea treatment, "pyrethrin" and "pyrethroid." Dogs who have recently undergone anesthesia -- such as during a spay or neuter -- and dogs with low body temperatures from a bath or cool weather are more predisposed to reactions.

More Serious

If your dog continues to vomit after a flea treatment, you will want to take him to your vet immediately. Other severe symptoms include:

  • Lack of coordination.
  • Salivating.
  • Dilated pupils.
  • Muscle tremors.
  • Depression.
  • Refusal of food and water.

Each of these symptoms indicate a more severe problem than simply an allergic reaction. The worst reactions to spot-on flea treatment were to dogs between 10 and 20 pounds. The ingredients used in flea treatments are effective insecticides, and can affect pets that stronger dogs can absorb.


  • Check the weight of your dog before administering any topical treatment; more is not better in these cases, as insecticide poisoning can severely affect the nervous system of your pup.

Alternatives for your Dog

Although over-the-counter flea medications work for many dogs, if your dog proves allergic, there are options. Groom your pet first, with a fine-toothed comb. Remove every flea you see, and completely change your pets bedding. After a long bath and deep bed decontamination, you can set a trap for remaining adult fleas. Close to where your dog sleeps, place a night light and add a dish of soapy water beneath it. There are commercial electric traps for fleas that provide the same outcome: a source for the fleas to head toward at night, and then become trapped.

Once cleansed, you can protect your dog's space by placing diatomaceous earth around the house. This dirt, which is actually made up of fossilized remains, is a desiccating substance to most parasites, including fleas. In most cases, you can brush this into your allergic dog's fur without a reaction, as it is nontoxic to humans and pets.

In all cases, if your dog is experiencing an inordinate reaction to fleas or helpful flea medication, take him to your local veterinarian for a checkup and more information. It may be a weight issue -- an overdose of pesticides are hard on any little guy -- or it may be something more serious.