Oral medications are not used to attempt to regulate dogs, only as an adjunct to insulin therapy. The most commonly used adjunct in dogs is Acarbose (Glucobay, Precose, Prandase), which slows down digestion of starches and therefore moderates post-prandial glucose levels. 
There are unpleasant side effects, one of which is that attempting to use complex carbohydrate containing "starchy" foods for a hypoglycemia attack won't work. The drug is designed to inhibit the digestion of them. Only the use of simple carbohydrates like sugar will successfully bring blood glucose levels up. 
The reason oral diabetes medications are not successful in dogs is because some of them are designed to stimulate the beta cells of the pancreas into producing more insulin. Others are meant to allow the body to effectively use the insulin it produces. Most dogs have insulin-dependent diabetes, which means their beta cells are not capable of producing insulin--with or without medication stimulations. Basically, there's nothing in the endocrine pancreas able to be stimulated or any insulin being produced for the system to make use of. 
- ↑ Acarbose. Wikipedia.
- ↑ Nelson, Richard (2000). Oral medications for treating diabetes mellitus in dogs and cats. Journal of Small Animal Practice.
- ↑ Acarbose side effects. Wikipedia.
- ↑ Hypoglycemia Treatment. Wikipedia.
- ↑ Effectiveness of Oral Medications in Canine Diabetes. MarVista Vet.
- ↑ S. Schachter, R. Nelson, C. Kirk (2000). Effect of Oral Chromium Picolinate on Glycemic Control in Insulin Treated Diabetic Dogs-Abstract #71. ACVIM.
- ↑ Better Medicine-E-Newsletter. Intervet (June 2006).
- ↑ Schall, William (2009). Diabetes Mellitus-CVC Proceedings-Drugs other than insulin and insulin analogs used to treat diabetic human beings. DVM 360.
- Diabetes Mellitus-Treatment Options Bruyette, David, 2001, WSAVA
More about Acarbose as an addition to insulin for dogs.
- Controlling Diabetes Mellitus Without Insulin Brooks, Wendy C., Veterinary Partner
Article covers use of Acarbose as an adjunct to insulin in dogs.
- Oral diabetes medications in cats and dogs MarVista Vet