Scientific and ethical issues in add-on designs for antidiabetic drugs.

This editorial describes the clinical trials related to antidiabetic drugs, most of them following an "add-on" design of where the new drug is added to metformin and the comparative arm is metformin plus placebo. Many drugs are already approved for therapy following this design; the authors believe that it is unethical to continue this trend because it makes it impossible to stratify the many antidiabetic drugs according to their efficacy and toxicity.