As others have advised, every living thing is different. Not all medications agree with all who take them. My experience with Cerenia injections is that the anti-nausea effect lasts a bit less than 24 hours in an adult cat.
According to this page on drugs.com the average half-life of the drug in adult cats when administered as a subcutaneous injection is 17.2 hours. According to the section of the article on adverse reactions:
Alternative nausea treatments include ondansetron (generic is
much cheaper than branded Zofran version). Slippery elm bark is a natural, soothing remedy which can help with GI upsets and nausea. It might be worth trying these alternatives; cat nausea is a problem that needs to be addressed ASAP because they must keep eating (as I know only too well from painful experience).
There is very useful information on nausea and its treatment on this page.
Has Catee eaten anything since you posted? I certainly hope that his appetite has improved; it's awful when we can't get our little ones to eat. Try to see whether you can tempt Catee to eat a little mashed up food from your hand. Warming food helps, too. Other things to try at following links:
Persuading cat to eat
Suggestions on how to stimulate appetite
Another thing to check for is whether a kitty may be constipated; this can make them very nauseated, even make them vomit. Sometimes diarrhoea can leak around a stuck stool. The site
www.felineconstipation.org is very helpful. If constipation is a cause of nausea a very short period of treatment with metoclopramide (Reglan) can help to improve gut motility. (Course needs to be short because metoclopramide has some nasty neurological side effects, some of which may be irreversible if the medication is given over too long a period.)
A course of B12 injections can be a help with appetite issues, too.
Mogs
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