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.

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