What it is
An extract of the stevia leaf — used for centuries by the Guaraní — concentrated to 200–400× the sweetness of sugar. The purified glycoside (rebaudioside) is what's approved; crude whole-leaf extract the FDA still won't clear.
Why your biology objects
On the actual health evidence it's among the most benign non-caloric sweeteners there is: gut bacteria cleave it to steviol, which is glucuronidated and passed in urine — no calories, no glucose or insulin rise — and it was cleared of the old genotoxicity worries (JECFA ADI 4mg/kg). Its effect on the microbiome at real doses is minimal and far milder than saccharin or sucralose. The honest caveats sit *beside* the molecule, not in it: most retail "stevia" is mostly erythritol (judge it by that line), and WHO's caution on sweeteners is low-certainty, class-level, and confounded.