What are the effects of overdosing on diabetes medication?

Hypoglycemia. Overdose of either Insulin or some oral agents for diabetes causes hypoglycemia or low blood sugar. This can lead to altered mental status or coma in some cases. It should be treated rapidly with glucose containing foods or liquids to bring blood sugar up.
Depends on which med. The effects of an overdose depend on which medication and how much. Insulins, glimeperide, glyburide, Glipizide and those containing these medications can lower sugar below usual, most likely for a short periods of time. An overdose of others such as metformin, actos/avandia, januvia/tradjenta/onglyza are less likely to cause this problem.