Cardenolide is a type of steroid. Many plants contain derivatives, collectively known as cardenolides, including many in the form of cardenolide glycosides (cardenolides that contain structural groups derived from sugars). Cardenolide glycosides are often toxic; specifically, they are heart-arresting. Cardenolides are C(23)-steroids with methyl groups at C-10 and C-13 and a five-membered lactone (specifically a butenolide) at C-17. They are aglycone constituents of cardiac glycosides and must have at least one double bond in the molecule.Some plant and animal species use cardenolides as defense mechanisms, notably the milkweed butterflies. Species such as the monarch and the queen the cardenolides contained in the milkweeds (Asclepias) that they mostly feed on as larvae. The cardenolide content in butterflies deters most vertebrate predators, except a few which have evolved to become cardenolide-tolerant.
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