Researchers at the University of Chicago medical school found that American Ginseng is able to significantly lower blood glucose levels and improve glucose tolerence tests. This is very good news for diabetics as, not only does it lower sugar levels, it also improves energy. However, you need to be careful taking this herb if you have a history of hypoglycemia (low blood sugar). Interestingly, researchers could not identify how American Ginseng lowered blood sugar, but they found it was not through antioxidant activity alone. In this study they used a water extract so just making a tea is sufficient. In China, American Ginseng is felt to promote longevity.
 
Fitoterapia. 2009 Jul;80(5):306-11. Epub 2009 Apr 7
Am J Chin Med. 2002;30(2-3):187-94