Shakespeare called them “cuckoo-buds”, while in English rural dialect, buttercups were often referred to as “crazies”, both of which tapped into an ancient folk belief that buttercups caused insanity.
While native buttercups won’t make you go mad, they are certainly toxic to both humans and livestock if eaten – symptoms include burning of the mouth and gastrointestinal irritation.
Applied topically, buttercups can also cause blistering, a fact that wasn’t lost on early physicians, who relished any opportunity to “draw out” illness through the skin using buttercup salves and poultices. Some varieties of buttercup, however, are more toxic than others.
In Sardinia, the Ranunculus sardous, or hairy buttercup, is deadly and may have given rise to the phrase “sardonic grin”. Anyone foolish enough to eat the flower, or have it forcibly administered, would find that the toxins in the buttercup caused their facial muscles to contort into a horrible smile, usually followed by a lingering death.