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Hello, this is Dr. Jess. I am sorry to hear of Simon's issues. Prazosin is a human labeled product that is produced by pfizer and as a generic, so it isn't that it is an experimental drug or anything, the issue is that it only comes in 1mg, 2mg, and 5mg sizes, which are too large for cats. This is why your vet needs it to be compounded. There are not any veterinary labeled products, but veterinarians often use products that are not labeled for veterinary use all the time because if we did not, we would have less than half the medications we use available. Getting a medication labeled as a veterinary product is costly and so many drug companies won't go through the bother if they don't think it will be worth it for them.
That being said, side effects of prazosin can include hypotension (low blood pressure), dizziness, lethargy. These are often only short term and once the pet is used to the drug they wear off. We can also see GI side effects (nausea, vomiting, diarrhea). These are all the side effects listed in our drug formulary. I haven't seen any cats with these issues.