Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Phil vomited profusely overnight on Saturday, continued to vomit on Sunday, and despite treatment is still not himself.
Vomiting and nausea can be due to stress, a change in diet, parasites, a viral or bacterial infection, eating something he should not (like a piece of a toy or string), inflammatory bowel disease or even a food allergy. But it can also be related to metabolic organ disease (such kidney or liver disease) due to organ wastes irritating the gut or an obstruction in the gastrointestinal tract.
The good news is that his bloodwork and urinalysis this morning were within normal limits, he has not vomited anymore, and he did eat a small amount. Was a blood test called a feline specific pancreatic lipase (fel spec PL) done? Pancreatitis in cats can lead to horrible abdominal pain and significant vomiting.
Cats with nausea will often vocalize as a response to abdominal pain and discomfort, so I am not necessarily surprised that he was vocalizing. After 24+ hours of that sort of vomiting I can imagine his abdomen was very sore. I assume that he he has stopped that now?
I am not surprised that he isn't drinking. Cats tend to drink very little normally and since he received fluids his body likely isn't telling him to drink.
Appetite stimulants like Mirtazapine also have some effects on behavior. Some cats will vocalize more, act a bit drunk, and seem spacey after taking it. So that too may be playing a part in him not seeming to be himself.
Cerenia is an excellent anti-nausea drug, it works on the stomach to ease nausea as well as the nausea center in the brain. So it too can have an effect on the brain and behavior.
I understand your worry, but at this point I see many positive signs that he may come along.
The behavioral effects of Mirtazapine and Cerenia should lessen over the next 2-3 days. And he should start eating better and drinking more.
If he seems to be worsening again then I would test him for pancreatitis if that hasn't been done and consider an abdominal ultrasound to evaluate his abdomen further, looking for signs of an obstruction, pancreatitis, inflammatory bowel disease, or an abdominal mass, including infiltrative type intestinal cancer or a discrete tumor.
Please let me know if you have any further questions.