Hi Molly, Prilosec is now an OTC drug and has also just come out in the generic form Omeprazole as well (in the US). Please take your cavalier to the vet or neurologist to discuss using this drug and getting the correct dosage for his weight. Depending on other medications, he may not be able to take it, so it is necessary to check with your vet first.
Also Prilosec/Omeprazole is a proton pump inhibitor (as are other similar drugs) which lessens the amount of CSF produced by the body. There are other drugs that can have a similar effect in decreasing CSF, like Lasix the diuretic. So if your cavalier is already taking Lasix he may not even need Omeprazole.
It is always best for your dog's medications to be managed by the vet, who has access to information about all known drug interactions. And any time your dog needs an additional medication, such as an antibiotic, the drug interaction must be considered beforehand.
As far as side effects, as long as I give Geordie his Omeprazole 2 hours before or after any other drugs, he seems to do quite well. I can tell it is working, because he sometimes starts scratching when it's time for another dose. I have noticed that he seemed a little happier when he was on Lasix for 10 days due to pulmonary edema, and I stopped giving him Omeprazole. But then the vet wanted him off the Lasix to make sure the pulmonary edema didn't return, so he's taking Omeprazole again. Soon I will be asking if we can try something else, just to see if he seems a little happier.