The foxglove, Digitalis purpurea, is a tall biennial of European woods and banks bearing a one-sided spike of drooping purple thimble-shaped flowers. Its beauty must be met at once with a warning: the foxglove is deadly poisonous in every part, containing potent cardiac glycosides that act powerfully on the heart, and it has killed people and livestock, a fact stated here plainly and not as any invitation to use it. Its name is usually explained as the folk's glove or fairy folk's glove, the flowers being the gloves or bells of the little people, and its many country names, fairy thimbles, witch's gloves, dead men's bells, tie it closely to the fairy world and to death.
The specific compounds responsible, cardiac glycosides including digitoxin and digoxin, work by interfering with the sodium-potassium pump in heart muscle cells, altering the force and rhythm of the heartbeat, the same mechanism that makes purified digoxin a genuine prescription medicine for certain kinds of heart failure and irregular heart rhythm today, used under close medical supervision because the safe dose and the dangerous dose are dangerously close together, a narrow margin that makes the raw plant especially hazardous to anyone attempting to use it without exact pharmaceutical measurement.