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How to Choose the Safest Flea Treatment for Dogs

Topical and oral flea treatments are considered equally safe as long as the recommended dose is followed. Topical treatments containing methoprene and pyriproxyfen are considered safer than treatments containing other substances.

Flea prevention is vital, and finding a safe flea treatment is an important dog-owner task.

Veterinary antiparasitic products are widely recommended by vets to kill and prevent fleas on dogs. There are many different types of flea treatments, such as spot-on pipettes and collars to chewable tablets or sprays. 

But these products are not always safe for dogs.

In this article, you will find out the safest treatments for fleas on dogs, if flea medication is bad for dogs, and more.

How Do the Main Active Substances in Flea Treatments Work?

Some active substances act as neurotoxins, while others affect the developmental stages of fleas. These substances are called insecticides (or acaricides), and some are more dangerous than others for dogs.

The majority of active substances in antiparasitic products act as neurotoxins, blocking the nerve impulses of fleas. This blockage leads to flea paralysis and death.

Nerve impulse blockage is produced through:

  • Impaired neuromuscular system[1]
  • Disruption of normal nervous system function[2]
  • Depolarization of the sodium channel on nerve axons[3]

The active substances that work by blocking the flea’s nerve impulse are:[4]

  • Pyrethroids of the second generation, dimethylcyclopropane carboxylic acid derivatives: permethrin, phenothrin, and etofenprox.
  • Neonicotinoid compounds: imidacloprid and dinotefuran.
  • Phenylpyrazole group: fipronil and pyriprole.
  • Organophosphorus group: diazinon.

These antiparasitic substances only affect adult fleas. They have no effect on flea eggs or larvae. 

S-methoprene and pyriproxyfen affect flea larvae instead of adult fleas. These substances regulate the developmental stages of these parasites and are called insecticides-juvenoids. They inhibit larvae from transforming into adult fleas

S-methoprene and pyriproxyfen are analogs of a special juvenile insect hormone, which prevent flea metamorphosis.[5]

Safest Flea Treatment for Dogs

How Do the Main Active Substances in Flea Treatments Work

The safest substances for flea and tick control are S-methoprene or pyriproxyfen. Always read the label of an antiparasitic product carefully, as some manufacturers combine these substances with more harmful insecticides and acaricides.

All chemicals in antiparasitic products are potentially toxic to pets to some degree. Although no medication is 100% safe, leaving dogs untreated for fleas and ticks is much more dangerous. Before applying flea treatments, you have to consider which type to use.

Flea treatments fall into two categories:

  1. Topical treatments: Spot-on pipettes, collars, powders, shampoos, or sprays.
  2. Oral treatments: Chewable tablets and pills.

Is Topical Flea Treatment Safer Than Oral?

Both methods are equally safe, although some say oral treatment is more dangerous. If treatments are applied or administered according to your dog’s weight, they should not be harmful. Some pets are more sensitive to antiparasitic substances and can experience allergic reactions or poisoning.

Safest Topical Treatment for Dogs

The safest substances in topical antiparasitic treatments for dogs are methoprene and pyriproxyfen. These are considered to be less toxic than other substances. They are both insect growth regulators, inhibiting insects’ development. They are widely used in topical flea treatments for pets.[6]

Spot-on Pipettes

Methoprene and pyriproxyfen inhibit the expression of certain genes involved in the metamorphosis of insects (including fleas). This results in various biochemical and cellular effects on insects’ bodies, disrupting their development.

In other words, these substances stop insects’ growth and development.

These processes do not occur in vertebrates, which is why methoprene and pyriproxyfen are considered relatively safe for dogs.[7]

Combination of Multiple Active Ingredients

Methoprene and pyriproxyfen are often combined with adulticides (substances to combat adult fleas) or acaricides (substances against ticks). They are added for their effect on the development of fleas in the egg and larva stage.

Adulticides are inefficient for fleas in the developmental stages. They are considered potentially harmful to pets due to various potential side effects.

Because methoprene and pyriproxyfen are mainly used in mixtures with other insecticides/acaricides, side effects are more likely to be caused by the other active ingredients.

If methoprene poisoning is suspected, assessment of renal and hepatic function is recommended as this substance is metabolized in these organs.

Safest Oral Treatment for Dogs

Safest Oral Treatment for Dogs

Unlike topical products that are absorbed into the sebaceous glands, oral treatments are distributed via your dog’s blood.

Oral treatments are easy to administer and are generally recommended for dogs sensitive to topical antiparasitic substances.

Oral treatments are considered excellent for households with other small pets or children, as they don’t leave a residue.

Oral treatments in dogs are represented by:

  • Chewable tablets
  • Pills

Antiparasitic Chewable Tablets as Flea Treatment

Depending on the product, you can find the following active substances in chewable tablets for flea treatment and prevention:

  • Afoxolaner
  • Fluralaner
  • Sarolaner


Studies on the safety of afoxolaner in eight-week-old puppies have shown that it has no clinically significant side effects. Vomiting and diarrhea were the only symptoms observed sporadically. 

Afoxolaner has been shown to be safe for puppies when given repeatedly up to five times the maximum exposure dose.[8]


Studies suggest that fluralaner is safe for pets. It rarely causes side effects, which are not considered clinically relevant.[9]


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Sarolaner is well tolerated by dogs at a dose of 2-4 mg per body weight. Care should be taken for dogs with a history of convulsions, as it can cause abnormal neurological signs such as tremors, instability, or seizures. 

It is also recommended to avoid its administration to pregnant or lactating females and breeding dogs as it has not yet been evaluated.[10]

Antiparasitic Pills as Flea Treatment

Two of the most common active substances used in oral products for the treatment and prevention of fleas include Spinosad and Milbemycin oxime.


Spinosad is considered safe if administered at the recommended dose but may cause trembling, salivation, seizures, ataxia, blindness, and disorientation, especially when used in combination with ivermectin.[11]

Milbemycin Oxime

Milbemycin Oxime
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Milbemycin oxime is considered safe if administered at the recommended dose. Using a higher dose than necessary in dogs with the MDR1 mutation* (Multi-Drug Resistance 1) can cause toxic reactions[12]:

  • Hypersalivation
  • Uncoordinated walk
  • Fever
  • Seizures
  • Death

*MDR1 is a mutation that causes sensitivity to certain drugs, including ivermectin and milbemycin oxime. Dogs that have this gene will react to those drugs, but it all depends on the dose.

Is Flea Medicine Bad for Dogs?

Flea medicines are not considered harmful as long as the recommended dose is followed. Most dogs tolerate the active substances in flea medication well, but there are exceptions. Pets can suffer severe side effects.

sick dog on vet

Although most conventional dog flea products are approved by the EPA (Environmental Protection Agency), they have been linked to serious human health problems, such as cancer and respiratory and neurological problems.

These products can also cause skin allergies (allergies) or intoxication in dogs.

Dogs can generally become intoxicated if the product used was too concentrated for the dog’s weight or if the pet ingested the product.

Symptoms of poisoning in dogs include the following:

  • Loss of appetite
  • Vomiting
  • Diarrhea
  • Weakness
  • Incoordination
  • Hypersalivation
  • Abnormal heart rhythm
  • Difficulty breathing
  • Fever
  • Low body temperature
  • Seizures
  • Death

Avoid products that contain imidacloprid and dinotefuran. These are nicotine-based chemicals and are toxic to children whose brains are still developing.

Non-Insecticide Flea Treatment for Dogs

Non-insecticide flea treatments include natural remedies, washing and combing your dog regularly, and preventative measures. Insecticide-free treatments are not as effective as insecticides, and some of them can cause intoxication.

Non-insecticide flea treatment for dogs may be considered safer than applying or administering antiparasitic products. Some natural flea treatments can still cause poisoning in animals, as in the case of essential oils.

The most common methods for treating fleas on dogs without insecticides are:

If you don’t want to or can’t use one of these methods, regular soap and water can kill adult fleas on dogs. Comb your dog’s fur with a flea comb for greater effects. Soak fleas picked up with the comb in soapy water. This causes them to drown.

Essential Oils Flea Spray

Keep Your Home Clean

Besides your dog, you need to make sure you keep your dog’s resting area clean. Wash your pet’s bedding every week. Vacuum and wipe the areas frequented by your dog daily to make sure you are decimating the flea population in your home.

You can use a steam vacuum cleaner on your carpets and furniture, such as a sofa, bed, or armchairs. These are the most common places where flea eggs can be found.

Regarding your yard or garden, preventive measures should be taken there as well. Fleas live outdoors, and your garden can make a good home for them. Use diatomaceous earth or beneficial nematodes in your yard to get rid of fleas. They are safe for plants.

Diatomaceous earth is also considered safe for dogs and people. It can be dangerous for pets if it gets in their eyes or if they inhale it (DE can cause breathing problems). Use in areas where dogs and children do not have access.


All flea treatments can cause side effects. Some dogs are more sensitive than others. Particular attention should be paid to the concentration you apply or administer to your dog as active substances can lead to intoxication and death if the dose is too high.

Leaving fleas and ticks untreated poses a greater risk to your dog’s health than using antiparasitic substances and intoxicating your pet. In most cases of antiparasitic poisoning, the dog recovers if the vet intervenes in time. 

In the case of fleas or ticks on dogs, the risk of getting sick is much higher than the risk of poisoning from pesticides.

Consult your veterinarian for the best treatment for killing and preventing fleas on your dog.

About Iulia Mihai (DVM)

Dr. Iulia is a certified veterinarian with more than 10 years of experience in the field. With extensive knowledge of diet, care, and medication, she helps Misfit Animals provide readers with accurate knowledge on technical topics.

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