Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 16 years of experience. I look forward to working with you. I apologize for the delay in an expert responding, but there are often not many available overnight.
You first mentioned Revolution and then Advantage. So, assuming you did give Revolution full dose a week apart, there would not be any significant reaction expected if the proper sized dose was administered. Perhaps, we may get a topical reaction but the drug is generally very safe, even though it is absorbed systemically. As for Advantage (as long as this is not the dog product which can be toxic to cats and was the cat product) the same is true and this drug is not absorbed systemically. For the topical reaction it is usually hair loss, maybe a small scab, in the location that it was administered. Rarely we may see twitching immediately after application when cats get an odd sensation but it resolves very rapidly (this is highly unusual for Revolution or Advantage), and if he has always done fine in the past with the drug, then it is highly unlikely to happen this time.
So, I would be more worried there is something else going on with him. Perhaps, he has a skin infection that is causing irritation/itching and the twitching. IF, however, he has full body trembling, then this is a medical emergency and you need to take him in to be seen, although I see that he is very tough to get into the vet. :( But if you do get him out from under the bed and he is twitching and profoundly lethargic, then it is an emergency and I doubt he will put up much of a fight to be seen today.
I am at a point I need to know what questions you have. I hope that the information I provided has been helpful.
Please let me know if for any reason you need further clarification, have more questions, or were expecting a different type of answer.
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