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Good morning, I'm Dr. Sara :) I'm a licensed veterinarian who works exclusively with cats and dogs. I'm sorry to hear about the mix up with your kitties' flea preventive.
While K9 Advantix can be very toxic to cats, the regular Advantage II is not. This is because it does not contain permethrin, as the K9 Advantix does. In cases of permethrin toxicity in cats, what we see is a whole body twitching/shaking progressing to possible seizures. I'll attach a video at the bottom. The treatment is to wash the kitty as quickly and thoroughly as possible with a good degreasing soap like original Dawn dish soap and controlling the tremors and seizures in the hospital setting.
If you're not seeing any of the typical signs of permethrin toxicity (or you know you used Advantage II rather than K9 Advantix), then you're not likely to need ER care. Especially based on your description of them still eating drinking and acting well.
Your kitty with the rash may require some anti-inflammatory treatment at the vet - I have seen some cats have local reactions to all sorts of topical products. Some needed my intervention and some did fine on their own.
Please let me know what other questions I can answer for you!
Here is a video of a cat showing classic permethrin toxicity:
Hope this helps :)
If it's Advantix, the suggestions I made are still the same - you did right by washing the product off. If you're seeing any of the signs of permethrin toxicity like the kitty in the video, this merits a trip to the vet.
What questions can I answer for you?