Stevia is a genus of about 240 species of herbs and shrubs in the sunflower family (Asteraceae), native to subtropical and tropical regions from western North America to South America. The species Stevia rebaudiana, commonly known as sweetleaf, sweet leaf, sugarleaf, or simply stevia, is widely grown for its sweet leaves. As a sweetener and sugar substitute, stevia's taste has a slower onset and longer duration than that of sugar, although some of its extracts may have a bitter or licorice-like aftertaste at high concentrations. With its steviol glycoside extracts having up to 300 times the sweetness of sugar, stevia has garnered attention with the rise in demand for low-carbohydrate, low-sugar food alternatives. Because stevia has a negligible effect on blood glucose, it is attractive as a natural sweetener to people on carbohydrate-controlled diets. The availability of stevia varies from country to country. In a few countries, it has been available as a sweetener for decades or centuries; for example, stevia is widely used as a sweetener in Japan where it has been available for decades. In some countries, stevia is restricted or banned. In other countries, health concerns and political controversies have limited its availability; for example, the United States banned stevia in the early 1990s unless labeled as a dietary supplement, but in 2008 approved rebaudioside A extract as a food additive. Over the years, the number of countries in which stevia is available as a sweetener has been increasing. For centuries, the Guaraní peoples of Paraguay used stevia, which they called ka'a he'ê ("sweet herb"), as a sweetener in yerba mate and medicinal teas for what they believed to treat heartburn and other ailments. Current research has evaluated its effects on obesity and hypertension. Stevia has a negligible effect on blood glucose, and may even enhance glucose tolerance; it may be useful as a natural sweetener for diabetics and others on carbohydrate-controlled diets.