[moondog]

For a dog that's had a day or two of diarrhea, depending on how sick they are when the vet sees them (lethargy, weakness, etc), taking a "try this and let's see" may be the best first course of action. However, when you have a dog that has had diarrhea for a MONTH, well, it's apparently more than just a tummyache and I'd think bloodwork (including a chem panel) is needed to begin ruling in or out a number of conditions that can be implicated with this type of testing.

Diagnosing a dog with IBS is not really a diagnosis...it's a symptom secondary to a cause....but it's often the best a vet can do unless the underlying cause of the upset can be determined. A blood panel helps eliminate or reveal some serious illnesses that can cause chronic diarrhea, so I consider it a necessity....especially after such a long bout with diarrhea.

I hope your previous vet can help get some answers for you and your dog.