What it is
The bitterness in tonic water — a cinchona-bark alkaloid that started life as colonial malaria medicine.
Why your biology objects
A genuinely pharmacologically active drug, dose-capped as a flavor: the FDA limits it to 83mg/L and requires a label declaration because at higher doses it causes cinchonism, blood and cardiac effects. The mandatory cap and declaration are the tell — this is more than a benign flavoring.