Foxgloves produce digoxin, a chemical known as a ‘cardiac glycoside’, which acts to increase the heart’s output force and the amount of blood pumped on each beat. The entire plant (including the root and the seeds) is toxic. This toxicity is one of the plant’s natural defences against animals which would otherwise eat it.
Foxglove extract was first described as a treatment for heart conditions in the late 1700s. However, it was difficult to determine the quantity of the active ingredient in plant extracts, and there is only a small difference between the amount which is medically effective and the amount which will cause poisoning. Because of this, it is now used less frequently than other similar drugs.