I use Multidophilus Plus from the refrigerator in the health food store to my refrigerator. I have a Standard Poodle and I found out through trial and error that he does a lot better if he has it every day, with food. The powder inside the gel cap tastes great, and once a dog figures that out they tend to smunch it right down as a treat. His stools are perfect, and that is not normal for a Standard Poodle. Also, the belching and passing gas they do so liberally, he doesn't.
I've been using this for 3 years with Worthy, and it has been perfect. It's fairly expensive, but with my previous dogs I only needed to use it for a time after antibiotic treatment. Dogs and cats both, I learned through trial and error, they always needed it. But Worthy needs it all the time, because of his short gastro-intestinal system. For the other dogs and for him, it's been great.
One unknown about probiotics for dogs is that we don't know exactly which bacteria they need. Multidophilus provides a variety of friendly bacteria, and apparently the right ones are in there somewhere!
I suspect that some cases of bowel disorder are due to lack of friendly bacteria in the dog's intestines going on for too long. So kudos to you for looking into this for your dog. It will make a great difference.
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Kathy Diamond Davis, author, "Therapy Dogs: Training Your Dog to Reach Others," 2nd edition, and the free Canine Behavior Series articles at http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47