This is the information I found as to ingredients; it seems that the dog version actually has LESS of some products in it:

cats: 9.8% Fipronil (100 g/L) & 11.8% (S)-Methoprene (120 g/L)

dogs: 9.8% Fipronil (100 g/L) & 8.8% (S)-Methoprene (90 g/L)

Thanks Donna.

I guess what I'm wondering is if there's something not "read in the numbers" that would make it harmful to divide dog frontline for cats. The box DOES say not to use on cats, but would that just be for the dummy that would put a large dose on a small cat? Or for the dummy that doesn't know why dividing a dose would be a bad idea?

Thanks, Meghane. That was exactly the kind of info I was needing. Although I don't understand the reasoning for a needed difference in concentrations for cats vs dogs, I'll just take your word for it. Its the reason I asked, so thanks. :)

Are there 2 sizes of frontline plus for cats? I can't remember now. If so, they double the volume for each incremental size increase, so if your cats are small and there is a big cat size, you could divide the large cat dose in half. That would still save some money.

The methoprene is the insect growth regulator. I guess cats metabolize it differently and some is lost due to that, hence the larger dose. That's my guess anyway. I wasn't so good at the pharmacokinetics of anti-parasital drugs.

Well, I didn't realize I had bought so much of the frontline for dogs and none for the cats. I had tried the spray for cats because it would have been cheaper. My cats weren't happy at all with the spray. Thus, the reason I asked about dividing the dog frontline. Oh well.

Thanks for the explanation about the methoprene. (I'm sure you did well in all your classes. :)

And make sure you buy this stuff in quantity, and online. It's so much cheaper that way! I usually pay about $50 for the 6 pack of Frontline Plus (the green package). I look for the website that is currently selling it the cheapest (inc. shipping costs) and then before I actually buy it I look for coupon codes for that company. I'm usually pretty successful using this technique. It turns out there are three or four companies that are usually cheapest... and one is in Australia. They mail it to me in the US and it's the same ingredients and formulation as the US-version.

When in doubt call your vet. A good vet will not try to con you into buying a separate package but will explain the difference. This is really silly to chance it when you can get a definite answer in a 1 minute or less phone call tomorrow morning. Or call the 1-800 number on the package.

Quirky.. yes it WOULD have been silly to chance. That's why I came here for the information. I had my answer before the day was out. You must not have read all the posts. :)

Xine.. I have a friend who also buys from a company in Australia. Now, she can't seem to find the website.

I have divided the dog dose for the cats. The only difference is the Methprene is not as strong in the dog version. This is the ingredient that stops the flea/tick reproduction capability. So far I haven't had any problems with the exception the cats come home with a tic or two around 21 days post application. They were waiting until around 26 days to attack my critters, when I used the actual cat vial. Since you can only use every 30 days anyway, I don't see the problem. They will still get tics with it on, my vet says they die quickly. I can get 4 cat doses from one medium dog dose. I have to find my notes to remember the proper dosage. NO!!! you can't use a whole vial of the dog one on a cat!!!!

be careful with dog frontline on cats, a ingredient in the dog is permethrin, it can kill a cat, might want to look into that

If anybody's interested large dog doses are divided up in syringes and used on cats in rescue facilities all the time. The ingredients are exactly the same ingredients in both with the exception of the percentage of S-methoprene.

Just as described in the very beginning 11.8% in cats / 8.8% in dogs. S-methoprene stop the maturing of flea eggs and flea larvae preventing them from reaching sexual maturity and reproducing. To be happy with the results I had to and was able to research this all the way back batch mix. I was able to do this because I had access to Epidemiological Health Department resources.

Sorry forgot to include the fact the percentages are different in different countries

Personally, I wouldn't trust doing this! I realize this is an old thread but someone could be google searching and find this. I've heard too many horror stories of cats being killed by doing things like this. You'd have to know what you were doing and be able to know the exact measurements. You'd probably have to use a syringe to measure the exact amount of liquid.

i agree with not doing this! there is a poster on the wall in my vet's office that names poisonous substances for both dogs and cats. in the cat column, it specifically mentions "spot on flea/tick treatment for dogs". you could be playing with fire and it just isn't worth it!

also, be careful buying online. the only place i trust is chewy.com. other places could be selling practically anything and oftentimes, there are knock off products that have identical packaging. also, if you buy from somewhere other than your vet, the guarantee will be void.

"Sorry forgot to include the fact the percentages are different in different countries."

Could you provide one example of this supposed difference?

" I was able to do this because I had access to Epidemiological Health Department resources."

I'd love to see the "resource" that says the lesser drug is satisfactory on the cat.