Taking exenatide, a drug that is a synthetic form of a substance found in Gila monster saliva, led to healthy sustained glucose levels and progressive weight loss among people with type 2 diabetes. Exenatide is a synthetic form of a hormone called exendin-4 that occurs naturally in the saliva of the Gila monster, a venomous lizard native to the American Southwest. Gila monsters only eat about twice a year, so their insulin has to be very efficient. The lizard hormone is about 50 percent similar to a human hormone called GLP-1 that increases the production of insulin when blood sugar levels are high.
More information: http://www.sciencedaily.com/releases/2007/07/070709175815.htm 
How does it work?
“Researchers at the Salk Institute have discovered how a hormone turns on a series of molecular switches inside the pancreas that increases production of insulin.” http://www.sciencedaily.com/releases/2011/09/110926173123.htm