Monk fruit is small dark green melon, which contains a very sweet compound called mogroside, which is now being used as a sweetener. In its purest form, it is 300 times sweeter than sugar. Mogroside is now being converted into a natural zero calorie sweetener, and it is in a number of foods and beverages.
In 2012, the Academy of Nutrition and Dietetics reported that monk fruit sweetener was safe. The FDA put it on its GRAS list (Generally Recognized as Safe). However it has not been studied extensively, although it has been used in Asia for centuries with no apparent negative health outcomes.