Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting for a response, but your requested expert isn't online which delayed your question coming up on the list for all to answer. I would like to help if you are still interested in an opinion.
We do not prescribe Rimadyl for cats because they have trouble metabolizing this nonsteroidal anti-inflammatory.
It is used in cats in some other countries in a limited fashion. The usual dose is 2 mg per pound (4 mg/kg) subcutaneously or intravenously as a one time dose before surgery. That would be 20mgs for a normal 10 pound cat. So if your kitty is the usual 10 pound kitty she ate about a double usual dose.
Side effects seen in cats that have ingested Rimadyl include nausea, vomiting, diarrhea, a decrease in appetite, possible stomach ulcers, kidney and liver damage.
Since it has been longer than 2 hours it is likely that the Rimadyl has been absorbed and it is too late to induce vomiting. Eve with a double dose though if your kitty is otherwise healthy and not on another nonsteroidal or steroid drug she may tolerate this one time dose just fine.
To help avoid stomach upset you can give her an acid reducer as a preventative. Either:
1) Pepcid-ac (famotidine) at one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at one half of a 10mg tablet per 10 to 20 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle his stomach and decrease the chance of ulcers forming. These can be used for several days as they are very safe.
If she has a history of a sensitive stomach I recommend offering a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it more palatable and get additional fluids in, which will support her organs.
If things go well and she eats the bland diet and doesn't vomit feed her the bland diet for 3 to 4 days then slowly start to mix back in her regular food, a little more at each meal. It should take about 5 to 7 days to slowly convert her back to her regular diet.
If she vomits even with the acid reducers, runs a fever (more than 103F rectally), or has a lower than normal temperature (less than 99F), has a tense painful belly, or if she refuses to eat even after the acid reducer is given she should see a veterinarian for an examination, diagnostics, injectable anti-nausea drugs, intravenous fluids and supportive care.
Please let me know if you have any further questions.