Dear Monica Ennis,
Thank you for writing to JA with your question.
Metformin is the most frequently prescribed oral anti-diabetic medication in the U.S. It works in a different way from Glyburide. Metformin is often used in diabetics that also have obesity, and can help with weight loss as well as decrease cholesterol. Glyburide has been found to have the effect of reducing strokes. There is actually a drug called Glucovance, which is a combination product of metfromin and glyburide. My guess would be that either this combination product, Glucovance, does not come in the most perfect dose needed for your control, or it may be that it is still under patent and would be more expensive than buying the two drugs separately.
These two articles in wikipedia, a free online encyclopedia explains more details about how each drug works. There is the possible side effect of hypoglycemia which is increased when you take both of these drugs together. So it is important to monitor you blood sugar as directed, and to notify your physician if you start to see that your blood sugar is getting too low. If you are working on your lifestyle changes, such as diet and exercise, it is possible that you will not need both of these drugs at some point, so keep a journal to monitor your blood sugars so that you can report them to your doctor if needed.
If you have any more questions or comments, please ask so that I or another expert can address them.