Plant species in the genus Asclepias have been used medicinally for millennia. The figure of Aesculapius, the Greek god of medicine, may have been based on a legendary Greek physician.
Milkweeds contain cardiac glycosides, naturally occurring drugs that increase the force of heart contraction and have been used to treat heart conditions. Cardiac glycosides also have potential anti-cancer application; in the lab, they exhibit properties toxic to cancer cells.
Butterflies pollinate all milkweeds, and two have developed an interesting relationship with the plants. The milkweeds’ cardiac glycosides are potentially poisonous to livestock and humans. However, monarch butterflies, which rely on milkweeds as food, are able to eat the poisonous compounds without being affected, and thus gain protective advantage: Birds that eat the butterflies become sick and learn to stop. The viceroy butterfly cannot eat milkweeds but is protected from birds by its similar appearance to the monarch. This is considered a classic example of Batesian mimicry and the co-evolution of the two species.
In many Native American cultures, mothers unable to produce milk drank a tea of the whole plants from various species of milkweeds. This probably is an example of the Doctrine of Signatures, the belief that certain characteristics of a plant signify its uses; in this case, the plant’s milky sap would correspond to lactation. All milkweeds except A. tuberosa produce a milky sap, and this species is not reported to have been used for this purpose.