Drugs that combine two classes of medications are called combination drugs. Combination drugs available in the United States as of early 2008 included Glucovance (glyburide and metformin), Avandamet (rosiglitazone and metformin), Metaglip (glipizide and metformin), and Janumet (sitagliptin and metformin).

Metformin is the common ingredient in all these drugs because it safely suppresses glucose production in the liver without the risk of hypoglycemia. And it also doesn't promote weight gain and improves lipid profiles in some patients. The same risks and contraindications that apply to these drugs separately are also applicable to their combination formulation.

However, even if you are familiar with one component of a combination drug (i.e., metformin), you should always read the drug's directions for use thoroughly before taking it to be aware of any unknown side effects or warnings.