6 application of the flea treatment was $97 for dog 40 - 88 lb. $99 for 6 applications for a dog 89 - 123 lbs. My dogs weigh 40 and 50 so I just bought the large dog and split the application. Ingredients are the same medium and large. Is there any harm in splitting application?

Sentry Large Dog 89 - 132 lbs
Fipronil 9.8%
(s) - methoprene 8.8%
other 81.4 %
4.02 ml

  • "ingredients are the same" -- what are the dosages? And "inactive" ingredients do sometimes matter in terms of how the drug distributes itself and how side effects are managed.
    – keshlam
    Commented Oct 12, 2016 at 1:56
  • 1
    We use Revolution for dogs and do split dosage on rabbits. In addition to the ingredients being the same the dosage should be the same. For example the Cat and Dog versions of Revolution are different strengths. What brand/drug are you using. Is the strength listed on both packages? Something like 2.0 ML contain 240 mg. Commented Oct 12, 2016 at 10:49
  • @JamesJenkins It is Sentry. The dose was not twice as big but like 80% bigger. I will look it up and add it to the question
    – paparazzo
    Commented Oct 12, 2016 at 11:36

2 Answers 2


The following is for the use of Revolution (Selamectin) for dogs, as used on house rabbits. This is prescription medication administered following the guidance of our Veterinarian. You should contact your Vet before splitting dosages.

Note we monitor our pets weight in kilograms (kg), while the medication is prescribed by pounds (lbs) so we have an additional calculation.

Revolution for Dogs 20.1 - 40lbs; 1.0 ml per tube

  • Convert the range to a single number 20+40=60, 60/2=30, patient weight per tube is 30lbs
  • 30lbs divided by 2.2kg equals 13.6KG of patient per tube (per ml)
  • 1ml divided by 13.6kg equals 0.08ml per kg
  • 4kg patient times 0.08ml equals 0.32ml per dose

  • Same as above in pounds

  • 1ml divided by 30lbs equals 0.04ml per pound

  • 9lbs patient times 0.04ml equals 0.36ml per dose

    Test math

  • 13.6kg times .08ml equals 1.08ml per tube
  • 30lbs times .04ml equals 1.2ml per tube

Revolution for Dogs 85.1 - 130lbs; 3.0 ml per tube

  • Convert the range to a single number 85+130=215, 215/2=107, patient weight per tube is 107lbs
  • 107lbs divided by 2.2kg equals 48.8KG of patient per tube (per ml)
  • 3ml divided by 148.8kg equals 0.06ml per kg
  • 4kg patient times 0.06ml equals 0.24ml per dose

Why calculate based on the middle weight?

As you can see from the test math, some variation enters during rounding. Using the middle weight, any rounding errors are less likely cause a problem.

How to do it?

Trying to guess an amount while squeezing some on a live wiggling patient, is likely to cause issues. To little and the fleas are not controlled, to much and the patient can be injured.

The tube is opened by pressing down on the top cap. When you remove the cap, there is a small hole in the top of the tube. Use a syringe with needle attached to suck out the correct amount of medicine. Remove the needle, and apply the medicine as directed by the packaging.

Syringes can be purchased at your local farm supply store. As long as you keep it clean, you should be able to use the same syringe multiple times (applying to skin not injecting).

Note that some packages (i.e. Revolution for cats) have different strengths in the tube. Do the math with every new package of medicine, don't bet your pets life that nothing has changed.

  • Revolution for 15 to 22 pound cats contains 60mg of Selamectin per ml
  • Revolution for 20 to 40 pound dogs contains 120mg of Selamectin per ml >>twice as strong<<

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If you have the dosing and a few syringes you can split it no problem, I would contact the company or your vet clinic to ask how many mls each dog needs.

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