Stevia (/ˈstiːvɪə/, /ˈstiːvjə/ or /ˈstɛvɪə/) is a sweetener and sugar substitute extracted from the leaves of the plant species Stevia rebaudiana. The active compounds of stevia are steviol glycosides (mainly stevioside and rebaudioside), which have up to 150 times the sweetness of sugar, are heat-stable, pH-stable, and not fermentable. These steviosides have a negligible effect on blood glucose, which makes stevia attractive to people on carbohydrate-controlled diets. Stevia's taste has a slower onset and longer duration than that of sugar, and some of its extracts may have a bitter or licorice-like aftertaste at high concentrations.