Shi Tzu's Grooming anxiety

You may want to work on grooming him at home or getting a different cut. It could be that his hair is getting tangled and when they try to brush it out his skin is pulled. Try to identify what the problem is - does he hate the entire place or is he just a problem in the bath/when they clip him, etc and then work with the dog at home.

I also think that sometimes groomers in the big pet shops don't have the experience or the time to work with them you may want to consider a place where he can have specialized attention. My mom used to be a groomer and sometimes with a problem dog she would keep them for the entire day so that the dog learned to relax at her place. She would groom them a little at a time and then play with them / give them a treat or whatever to let relax. After the second or third time most dogs were fine to be groomed.

I agree with Doc Jill. And all meds have side effects. Have you thought of taking your dog to anothet groomer? I had a vet I liked very much who had taken care of several of my dogs, but for some reason, he and his staff couldn't handle a little fear-biting terrier of mine. I took her to the vet I have now and she behaved OK. Maybe your dog would warm up to another person or environment better. Maybe you could "fake groom" the dog a bit and get it more used to the procedures as a routine.

always check with your vet before giving any medication to your pet. Also, most sedatives are going to be prescription only.