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Tuesday, July 19, 2016

Is It Okay To Split Flea Or Heartworm Prevention?

Regular reader Lynn recently emailed me with the following question.

I have 2 cats (12# and 15#) and one small dog (a 6# Yorkie). I have been using the same Frontline Plus formula for all of them. Years ago, my old country vet told me to buy the XL size dog vials and pour them into a dropper bottle, then apply 9 drops to each cat and 13 drops to the Yorkie.  This has saved me a tremendous amount of money and has always worked well.

My current Frontline Plus product (purchased over a year ago and which is almost gone) lists the active ingredients as Fipronil 9.8% and (S) methoprene 8.8%.  Now when I look online
The CAT’S formula says Fipronil 9.8% but (S) methoprene is 11.8%.  I don’t know if that has always been the case, or if Frontline has changed their formula? Now I’m nervous about using the same product on all of them!

Do you know which flea/tick products would be safe for me to use on both my cats and my dog, please? (I am willing to buy any brand recommended!) I understand why it could be considered “risky” to have your clients measure out their own doses, but trust me, I am meticulous!!!  Please help!

This is a good question and brings up multiple points which I will try to address.

Let's start with the challenge of a single product safe for both dogs and cats.  This is very difficult to come by for several reasons.  First, there are products safe for dogs that are dangerous or even fatal for cats.  Though most people see the two species differently, canines and felines actually differ significantly in several aspects of their physiology.  You should never use a product on a cat that is for dogs only, especially over-the-counter products.  Many flea preventatives clearly state "do not use on cats", and this is not a casual phrase.  Ignoring this warning could result in a cat dying or having to be hospitalized for extensive treatment.  Additionally the physiologic differences mean that sometimes the dosage between dogs and cats can be different, and therefore difficult to use interchangeably.  Dosages and warnings are there for a reason, and you shouldn't try altering them on your own.

Next, despite what many think, it's actually difficult to accurately measure doses from a split applicator.  You will inevitably lose a small amount in the syringe or dropper, and while this shouldn't cause any harm, it could result in under-dosing.  Even someone who is very careful cannot always ensure that you will divide the dose properly.  I'm also curious as to where the vet got the calculation for the specific number of drops. Again, this shouldn't result in harmful dosing as most of these products have a very wide safety margin.  I would worry about the more likely possibility of not using enough of the product to effectively prevent fleas.

As a follow-up to the last statement, everyone needs to be careful not to equate "no fleas" with "product efficacy".  I have many clients here in Georgia who have never used any kind of flea prevention and yet have never had a flea on their dogs or cats.  It always surprises me when I see the dogs who spend time outside yet never have any fleas despite a lack of prevention.  If we use the logic of "I've not had a flea problem when splitting a dose" then we must apply similar logic to "I've never used flea prevention and have never seen fleas".  Just because someone hasn't seen fleas doesn't mean that their particular method (or lack thereof) is actually effective.  Now, a split dose may indeed provide sufficient protection, but we can't automatically make that assumption with a single anecdotal story.  Correlation is not the same as causation.

Lastly, there is the broader consideration of product warranty.  Many flea prevention products contain some sort of warranty on their efficacy and safety.  However, that warranty is dependent on the consumer using the product as specified.  If a person splits a dose or uses a dog product on a cat, and there is some kind of subsequent reaction or toxicity, the manufacturer is unlikely to support the product or compensate the client.  When you use any product (not just flea prevention or in veterinary medicine) against manufacturer recommendations, you are often voiding the warranty and eliminating your chance for any recourse if it doesn't meet expectations or causes a problem.

Lynn, I hate to say it, but I don't think you're going to find that "magic bullet" you're looking for.  It will be difficult to find the same concentration and ingredients that are equally safe for both dogs and cats.  I also don't recommend splitting products that weren't designed to be used that way.  One of the consequences of having multiple pets is the increased costs that go along with that situation.  With three dogs, three cats, and multiple other pets I know this first-hand.


  1. Hi Dr. Bern,

    This is Lynn! I am so appreciative (and honored!) that you took time out of your busy practice (and life!) to answer my question. I cannot thank you enough.

    So now I am rethinking flea/tick prevention for my 3 pets, and exploring options. I'm wondering what you think of the new collars (like Seresto)? I see they are available for both cats and dogs.

    And one last question... my dog goes in and out (1,000 times a day!) but the cats are totally indoors. Do you think I have a fighting chance of just flea/tick treating the dog and being OK?

    Again, many humble thanks for all your help!


    1. Glad to help Lynn. And questions like this keep me from always having to come up with my own topics. ;)

      I have seen a lot of good results from Seresto, and it's the only collar that I recommend. I've had clients and veterinary colleagues tell me how well it works, so I do include this in the products I recommend to people.

      If you're really diligent with your dog, the chances of your indoor cats getting fleas is low but not impossible. Fleas have been known to hitch rides on shoes and socks, leading to fleas in the house. But it really depends on where you live as to the risk factor. The western US has very low humidity so fleas are much less of an issue than in the southeastern part of the country. Talk to your local vet about what is recommended in your area and how big the risks might be.

    2. Dr. Bern,

      Thank you so much for your reply!

      Ohmyword, just one more question, please -- if you don't mind! 😳

      Do you feel comfortable with your clients buying flea/tick products online from places like Drs. Foster and Smith, 1-800-PetMeds, etc? My vet is very against that, questioning the authenticity of the products. In fact, for Rx items like heartworm prevention pills, he will not provide an Rx to a requesting online pharmacy. But of course, since Seresto is OTC I can get that anywhere, and prices can be much less online than in his office. Please advise?

      Again, many many thanks!

  2. If you request a prescription, it's illegal for your vet NOT to write one. You are completely within your rights to ask for one, and he/she must provide it. You cannot be legally required to purchase any medication only from your local vet. In fact, the US Congress has debated a bill to require veterinarians to provide written prescriptions even if not asked for them. Most of the veterinary profession opposes this since we are already required to provide a written prescription if it is requested by a client (assuming that the product is medically appropriate).

    I get requests from online pharmacies every day, and always fill them. I do caution people to make sure the packaging is correct, as I've heard of stories where someone in the US ordered a medication based on the pet's weight in pounds, but the packaging was from overseas so it was listed in kilograms, thus resulting in the wrong dose.

  3. Dr. Bern,
    Thank you again for the reply. So interesting and informative, as always.
    I really appreciate your time and expertise. I am a better pet parent because of you and your blog!


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