Do you use flea preventatives, like Frontline, on your dog? Have you heard about the potential side effects?


I’ve been using the topical tick/flea preventative Frontline Plus on my dog Quinn for about 3 years now. I do this so that he won’t become infected with diseases carried by ticks or suffer the misery of flea bites – I use this medication to keep him healthy. Then why do I feel a bit nauseated and guilty when admitting to using Frontline? Because there are well-informed people out there telling me about serious and potentially fatal side effects associated with the use of topical tick/flea preventatives on dogs, including a whole host of cancers, liver disease, thyroid disease, and even death. But, to make matters more complicated, there are equally well-informed people on the other side of the aisle telling me there isn’t a causal link between topical tick/flea preventative treatments and the development of cancer (or other series diseases) in dogs. ARGH!

One thing I know for sure though is that the active ingredient in Frontline Plus is fipronil and the EPA categorizes fipronil as a carcinogen, which means that fipronil is absolutely involved in the development of cancerous cells.

I’m gravely concerned about using topical tick/flea treatments on our pets, but let’s take a look at some research. Here’s a link to results from a study done by the American Veterninary Medical Assocation looking at bladder cancer in Terriers titled “Topical flea and tick pesticides and the risk of transitional cell carcinoma of the urinary bladder in Scottish Terriers”, one being that that dogs in general are more at risk for developing cancer than humans, so maybe this means we need to be overly cautious about the levels of carcinogens they’re exposed to? Angel then states,

– “According to the Journal of Pesticide Reform, ‘In tests with laboratory animals, fipronil caused aggressive behavior, damaged kidneys, and ‘drastic alterations in thyroid function.’”

Lizi contacted me and provided some great information regarding the above study. Please see the Insecticide Factsheet for more information explaining the above conclusions. These results are enough for me to stop using the topical flea treatments, but I’ll continue for the sake of sharing as much information as I’ve found so far. Because the important things are covered by the link Lizi provides I’ll remove my concerns about the Journal of Pesticide Reform, because that’s not the focus here.

The National Pesticide Information Center provides results of research done on fipronil and its effects on rats, dogs, humans, and other animals. Here are some relevant excerpts:
– “Researchers fed dogs 0.2 mg/kg/day fipronil (length unknown) and observed no adverse effects. In the same study, researchers observed clinical signs of neurotoxicity at 2.0 mg/kg/day.2″. Not much help here though because the length of time dogs were exposed to fiponil is unknown.
– “Data from short-term and long-term toxicity studies with fipronil in rats, rabbits, mice and dogs ‘do not suggest any endocrine disruption activity’.” Okay, well this would mean that there was no casual evidence found linking fipronil to the development of cancerous cells in rats, mice, or dogs.

Back in 2009 the EPA began to look more closely at the risks associated with these topical tick/flea preventatives. From the American Veterinary Medical Assocation’s (AVMA) site:
– “In April 2009, the Environmental Protection Agency (EPA) issued an advisory concerning approximately 70 spot-on flea and tick control products because of an increase in the number of reports of adverse reactions to the products. Reactions reported included skin irritation, skin burns, seizures, and death. In May 2009, the EPA met with the manufacturers of the products to discuss the issue. In a July 2009 advisory, the EPA and the Food and Drug Administration (FDA) cautioned consumers to be cautious when using these products and to consult with their veterinarian. On March 17, 2010, the EPA announced it was taking steps to increase the safety of spot-on pesticide products for flea and tick control for cats and dogs.” The AVMA has a great list of FAQ regarding this investigation here: Clearly there are issues here that I hope are addressed soon, but until then I won’t be using a topical tick/flea preventative on Quinn the Dog!

I’m wondering if these studies are considering long-term exposure to these topical tick/flea preventative treatments? Is this whole fiasco a case of the drug companies dragging their feet on the results and we’ll be the ones looking back saying “Why didn’t we just stop using these topical preventives?” And, let’s not forget, the subjects that suffer during these studies are DOGS! For now I’m looking into tick and flea preventatives that are taken orally, like Trifexis, and I’ll let you know what I find. It would be great to hear your reactions, thoughts, concerns, questions!

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11 Responses to Do you use flea preventatives, like Frontline, on your dog? Have you heard about the potential side effects?

  1. Molly P says:

    Great topic Julia! I am very concerned about the long term side effects of using topical flea control products on my dog and cats. I am actually a bit more concerned about using it on my cats since they actually want to and can lick the site after application.

    I am constantly trying to find an alternative. I too have looked into the oral medications but have been hesitant to use them. Once ingested if the animal has a reaction there is not much to do other than induce vomiting. At least with a topical you can wash it off if there was an immediate reaction. But I realize the larger concern is the long term side effect of topical use like cancer or other tumors.

    I have tried Comfortis with Siegy once and he vomited a huge amount shortly after ingesting the pill. I chose not to dose him again directly after he vomited as I was directed to do by both the vet and the manufacturer of the drug. If this drug made Siegy sick once why would I immediately give him another dose? I have not tried another oral flea medication since this incident.

    Anyway, not much in the way of any answers here, but I await your research findings and am hopeful you come up with a great alternative for both dogs and cats that is safer!

    Thanks for the great work!

  2. Lizi Angel says:

    Julia, thanks for this, and for mentioning my article. For your information, here’s the link to the Journal of Pesticide Reform article on fipronil … … you will see that the article is lengthy and well referenced.

    I think it’s also worth raising a point concerning the testing methods that were used in the National Pesticide Information Centre: “Researchers fed dogs 0.2 mg/kg/day fipronil (length unknown) and observed no adverse effects. In the same study, researchers observed clinical signs of neurotoxicity at 2.0 mg/kg/day.2″. FED dogs fipronil? So it wasn’ a study of topical application, but even so, “clinical signs of neurotoxicity at 2.0 mg/kg/day.2″ were observed.

    Still not using it on my pets!

  3. Julia Julia says:

    Hi Lizi, Thank you so much for your excellent response and for reading my post! I went to the article and, you’re right, it’s very well researched. I’ll edit my post to include it and will edit the section on the Journal of Pesticide Reform.

    And, yes, testing methods are a whole separate issue. The National Pesticide Information Centre discusses the testing and feeding the the dogs/other animals fipronil, which is horrendous. As I mention above, this information was provided to show the laboratory studies testing the effects of fipronil on dogs and other animals, not looking at topical flea/tick preventatives.

    All this aside I agree with you that I absolutely will not be using Frontline or any other topical flea/tick treatment anymore. No question. Thanks again for your help Lizi and for providing the dog community with such a fantastic blog.

  4. Jacqueline Cecil says:

    What is the ‘trade off’? What are the diseases/inconveniences that Quinn, or any dog, is
    being exposed to by not using treatments that protect them from fleas, ticks, etc. while perhaps exposing them to more serious illnesses?
    Very well written article which I found informative and ‘easy to read’ in spite of the
    potentially scientific (over my head) bent.
    Thanks! Good work.

  5. Julia Julia says:

    Molly! Thanks for the wonderful response and for sharing your experience. Poor Siegy, I’m so sorry he had such a bad reaction. Apparently this initial bought of vomiting is the most common side-effect of Comfortis. I called my vet to ask her about it and she gave the same recommendation of giving a second dose. She told me that this initial vomiting isn’t anything to worry about and that giving further doses would not cause the same reaction – seems odd, but I’ll look into this, since it goes above my level of understanding.

    Lizi Angel posted a link on her blog to GreenPaws Flea and Tick Products, which I found very helpful: greenpaws

    This is what they say about Comfortis, which uses the chemical Spinosad.
    “Spinosad is administered to dogs in a tablet. It disrupts the nervous system function of insects, but is not neurotoxic to mammals. Its risk to humans is very slight. Veterinary reports do not indicate cause for concern, although long term studies have not been conducted. EPA classifies Spinosad as not likely to be a carcinogen. [source] Spinosad is listed on EcoWise Certified IPM Program Materials List.
    Toxicity: Administered as a pill and therefore very low risk to humans. However, all pesticides should be used with caution and in consultation with a veterinarian.”

    I was trying to find actual studies done on Comfortis, but haven’t yet. I’ll keep looking, because the following is referenced by a lot of sites, and can be read in full here:
    “In a well-controlled US field study, which included a total of 470 dogs (330 dogs treated with Comfortis chewable tablets and 140 dogs treated with an active control), no serious adverse reactions were observed with Comfortis chewable tablets. All reactions were regarded as mild and did not result in any dog being removed from the study.”
    Continues on, “Over the 90-day study period, all observations of potential adverse reactions were recorded. Reactions that occurred at an incidence > 1% within any of the 3 months of observation are presented in the following table. The most frequently reported adverse reaction in dogs in the Comfortis chewable tablets and active control groups was vomiting. The occurrence of vomiting, most commonly within 48 hours after treatment, decreased with repeated doses of Comfortis chewable tablets.”

    I was at Green Dog Pet Supply this morning and they set me up with a local (Oregon) company called Mad About Organics. The regiment required to maintain a flea-free dog with this company is a bit overwhelming, especially since the predictions are that this season with be BAD for fleas. Because Oregon doesn’t experience a good freeze like other parts of the country/world the consensus is that you have to keep up a regiment all year round.

    With the flea season looming so large this year I’m considering about putting Quinn on Comfortis, but until I decide I’ll be using Mad About Organics. I’ll continue researching and looking for studies on long-term usage of Comfortis along with other options, like Trifexis though and see what I find.

  6. Julia Julia says:

    Excellent question Jackie! You raise a great point for discussion. Yes, the issue is that your dog will be more likely to become infested with fleas or ticks, which can lead to serious health issues. First concern are fleas, which cause a great deal of discomfort in animals leading to scratching that can cause skin to become inflamed and, more severely, lead to hot spots. Hot spots are raised, swollen patches of skin that are infected and are very painful. Fleas bite and then feed on the dog’s (or animal’s) blood and can cause the animal to become anemic or even die. The home vet book I use says, “Some dogs develop a marked hypersensitivity to the saliva of fleas and experience intense itching which results in skin abrasions, hair loss, and secondary pyoderma. Fleas are also an intermediate host for tapeworms.” (Dog Owner’s Home Veterinary Handbook, 2007, 117). The big issue here is if a flea infestation is not taken care of immediately. Tapeworms are a big concern as well, as ingesting tapeworms can lead to serious illness and death in animals and humans.

    Flea infestations can be prevented, it’s just a matter of following a strict regiment and never slipping up, because once you’ve got a dog with fleas it can be very difficult to disinfect the house (especially carpeted areas). It’s not impossible though. The big draw with topical and oral treatments like Frontline and Comfortis is the ease of use and the fact that these treatments work so fast and so effectively. Aside from the high price tag there is no hassle involved, because all you need to do is apply (or feed) the preventative once a month and you’re set.

    When considering ticks there is more cause for concern because of the potential for the animal developing Lyme Disease. As you know Lyme Disease is very serious and, once the disease has progressed to a certain point, incurable. The big issue with Lyme Disease is that it’s notoriously difficult to diagnose. Humans and animals can undergo blood tests for Lyme Disease and have a negative result even when they are, in actually, infected with the disease. As a Mainer I’m sure you aware of all this of course! :)

  7. Helen Corlew says:

    The trade off is to use things that are not as toxic. Such as emu oil, citronella, cider vinegar, garlic–to name a few things that do work. And people must remember even if they use a top spot treatment they are not 100% and your dog still can get lyme disease. I have a good friend that uses Frontline on her dogs but still has had deer ticks on her dogs and 3 of her 5 dogs have had lyme.
    Another item that really helps in to control fleas and ticks, at least in your own yard, is use diatomaceous earth. You can also use this directly on your canine(s).
    Thank you to the link to the paper

  8. Julia Julia says:

    Helen thank you so much for visiting and for commenting! Great suggestions for non-toxic options. I’ve wondered though if the smells ever bother the dogs or other dogs around them? Especially the citronella.

    And thanks for bringing up diatomaceous earth – I’d never heard of this (here’s a link for those who aren’t familiar:, but it’s quite an incredible natural resource!

    I just ‘liked’ your FB page for Prairie Isle Dog Trekking – looks great

  9. Brenda Watts says:

    I am not an expert on this, very rarely have had to use a me nightmares doing it..i try very hard to keep on top of fleas and we do not have deer tick problem here so I feel blessed. However I have a friend in Spain who tried the no pesticide route with her labs went with a natural repellant…one contracted a tick borne disease tho she never saw many ticks….she said that with what the dog is now going through she wishes she had applied a topical..It is such a battle to know what to do……I guess it all depends on what the degree of tick exposure is.

  10. Julia Julia says:

    It’s true that a big issue is where you live – my husband and I are from Maine and whenever we’ve been back there with Quinn we use Frontline religiously. New England in general is very heavily infested with deer ticks we’ve had many family members and many good friends diagnosed with Lyme Disease. One friend in particular tried to treat herself homeopathically by going to a homeopathic doctor and she now has the disease permanently with constant and painful symptoms.

    I agree that the best defense is to be thorough by always checking your pets after they’ve been outside, especially in areas with tall grasses. And to check yourself as well!

    Comfortis is an option that I’ve now started with Quinn. The main ingredient in it is Spinosad, which is not an insecticide and therefore a better option that Frontline or Advantix. But it’s a tough situation all around because the studies out there don’t agree on the potential side effects. Something that I wish someone would address is the shorter lifespan of dogs vs humans when looking at susceptibility to cancers from toxins and insecticides.

    Thanks for the great comment Brenda :)

  11. Julia Julia says:

    CORRECTION: Oh wait, Spinosad is an insecticide, but it’s “is derived through the fermentation of a naturally occurring organism”, making it a natural product.

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