Stevia is plant belonging to the sunflower family, a genus with hundreds of different species, all of which are native to Central and South America. Out of all of these small perennial shrubs, only one species has been used for hundreds of years to sweeten tea and food. This species is stevia rebaudiana, and its leaves are considered to be 30 times sweeter than sugar, while extracts are reportedly 300 times sweeter. Although the plant itself has been used for hundreds of years by Native Americans of Brazil, Paraguay and Bolivia, the potent extract called stevioside has only been used for 25 years and has been the subject of numerous conflicting studies. Some studies found stevioside to have numerous alarming side effects, including contraceptive properties, harmful reproductive effects, toxicity of the kidneys, hypoglycemia or hypotension and cancer. These potential dangers were challenged by other studies that found stevioside to have a very low toxicity and little to no side effects. What we do know is that stevia extract has been used in Japan and Canada as a no-calorie food additive and sweetener, without any adverse reactions or side effects reported.
Studies done on the contraceptive properties of stevia were prompted by persistent rumors that indigenous women of South America were using the herb as a contraceptive. In the online magazine Science (Vol. 162, Nov 1968), a study was published that found stevia rebaudiana decreased fertility in female rats, and continued to do so for nearly two months after it was stopped. In direct opposition, a more recent study published in the Journal of Endocrinology and Reproduction (Vol. 12, 2008) concluded that stevia rebaudiana had no adverse effects on the fertility of adult female mice. Whichever study turns out to be correct, the real question is whether this information translates to humans. Some researchers point out that if stevia did have contraceptive properties, then the native population of Brazil and Paraguay would certainly be affected, which is not the case.
Along with the conclusion that stevia decreases fertility in female rats, some studies have found that it also affects the reproductive system of male rats. A study published in the Journal of Ethnopharmacology (Vol 67, Nov 1999) found that administering stevia extract decreased levels of testosterone while lowering sperm concentration. For each study that claims such, there is another that produces the opposite results. One study done by researchers at the Primate Research Center of Chulalongkorn University in Bangkok, Thailand studied not only the effect of high dosages of stevioside on a group of hamsters, but two subsequent generations. They found that health, mating and fertility were not affected at all, in any of the generations, even with a dosage of 2,500 mg/kg. The recommended daily dose for humans is 2 mg/kg.
Toxicity of the Kidneys
Like every other danger associated with stevia, toxicity of the kidneys is both supported and unsupported. Some sources say that large amounts of stevia can damage the kidneys, especially if used over a long period of time. More recently, studies have shown that long-term use of stevioside has numerous health benefits, one of which is improved kidney function. The one thing that researchers agree on is that more research and information is needed to fully understand the herb's effect on humans.
Hypoglycemia or Hypotension
In Brazil and Paraguay, the herb stevia is used to treat diseases and disorders such as hypoglycemia, hypotension, obesity, hypertension, depression and fatigue. So it is interesting that in the United States some nutritionists are advising against treating hypoglycemia with stevia, citing the possible dangers of the herb. In the case of stevia and hypotension, there is a significant amount of data saying that stevia lowers blood pressure and as such can adversely affect hypotension, which is a potentially life-threatening condition. Health-care providers suggest exercising extreme caution with the herb if you have hypotension or symptoms of the condition.
There were more studies done on the herb that suggested stevioside could potentially cause the mutation of human DNA, thereby causing cancer. Due to these studies, stevioside was labeled as a carcinogen and mutagen (cancer-causing agent), and it wasn't until a review of those studies more than 20 years later that it was found that 14 out of 16 had concluded that there were no known harmful effects of stevioside. Some studies were also questioned as to the validity of their claims, because they were sponsored by corporations that produce the artificial sweetener aspartame.