fractious dog

Question
I have an 8 yr old male long-haired chihuahua with CaOx bladder stones and
a lick granuloma.  He is a rescue and I do not know the details of his previous
life.
I have had him for two years and he is much better as far as aggression goes
except when he goes to the vet or when I need to do any kind of medical
procedure for him such as applying antibiotics or getting a muzzle on him.  
Do you have any suggestions other than drugging him.  I tried to slowly
introduce the muzzle and reward him and associate the muzzle with food but
he knows what it is for and freaks if I try to put it on him.  I am also trying to
figure out a way to put an Elizabethan Collar on him so he won't lick.  
However, I worry about leaving him alone for any time when he has this on
especially as he needs to drink more because of his uroliths.

Answer
I am going to assume this is a neutered dog, yes?
You have a dilemma that is more breed related than just his. Since he has had this issue for longer than you have had him and no one has ever tried before (most likely) to change this behavior, it will be very hard to break him of it.

I don't do behavior training but the site that Matty Margolis has is full of wonderful tricks and tips. I can't even find the time to explore the whole site it is so big. You may want to take some time and do a search on the site for a problem such as yours. It is not uncommon, believe me!

As a tech, we had to muzzle or drug dogs such as yours for ours as well as their safety. I have a two and a half inch scar on my left thumb from a Chihuahua that wasn't muzzled and was insane with fear and used my thumb to express it.

So I don't really have any suggestions except to explore training sites.
Matty's site is at: http://www.unclematty.com/training/allabout.php

Let me know if you find something that works well for you.