I just posted these replies on a greyhound forum about the same topic...
I would greatly prefer valium over Ace. I've seen dogs have a bad reaction to ace (the dog fights the drug) which is worse than any phobia reaction I've ever seen. I've also seen a greyhound sedated to a very scary level with only a small dose. Ace is a drug you can overdose them on and they don't wake up. Not to be used lightly with greyhounds, in my book. Ace won't necessarily make them less fearful...they may be just as afraid, but drugged to a point of being unable to actually display any fear behaviors.
I've never tried Xanax since valium has always worked great for our dogs that have needed it, but I'd look into that before giving ace. Basically, I would use ace in a non-fear/anxiety related situation where I needed the dog sedated/quiet and there were no other good options.
....and a follow up....
We do keep Ace on hand and have/will use it. I think you took the absolute right approach by dosing the drug at the lowest and working your way up to find the right treatment dose for each dog in a non-anxiety situation. I guess my biggest hesitance in making it my "go to" drug for anxiety (or anything, really) is the difference in reaction between dogs to the same amount. Our tiny 50lb Hanna can take a large dose and is fairly unaffected other than being a bit more 'chilled' and less active, while our much larger,70lb, boy Caesar was knocked on his @$$ for an entire day on a lower amount. Haley had such a bad reaction to ace (she was terrified...her eyes were bugged out, heart racing but she just laid there and wouldn't/couldn't move) that she had me in tears and I swore I'd never use the drug again (but...I've since come around to realize that there can be a time and place for every drug). I just don't like Ace's unpredictability...I've given it to, oh, 9-10 or so hounds over the years and seen quite a bit of varying reactions. Strong reactions to low doses and weak effects on larger doses. But, it can be helpful for some dogs, as you've experienced.
I think people have to talk to their vets and may have to try a few things until they find what works for their pup.
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Here's a good link:
http://veterinarynews.dvm360.com/dvm/article/articleDetail.jsp?id=136493&pageID=1&sk=&date=