Does anybody know anything about Phenoxybenzamine?

My vet prescribed this to help with Simba's symptoms for his urinary tract issues. I'm wondering if Phenoxybenzamine is the type of medication that takes effect immediately (like a person taking an aspirin) or, is it the type of medication that needs to be in the cat's system for a while before it starts to work. My vet never really gave me a straight answer and I've read some contradictions on the internet.

I don't have any experience with it, but my Pill Book Guide says it's a smooth muscle relaxant...that kind of thing usually works pretty quickly. Also says it should be taken with food and an overdose can cause low blood pressure.

Thanks for your reply! The vet did say it is supposed to relax the bladder.