View photos of the medicinal plant Asclepias subulata (Desert milkweed), profiled in Medicinal Plants of the American Southwest.

A passage from Medicinal Plants of the American Southwest:

Desert milkweed (and other closely related species such as Asclepias erosa, A. albicans, and A. linaria) is known to have a high cardiac glycoside content. Therefore Desert milkweed will affect the heart more profoundly than other commonly used Milkweeds, i.e. Pleurisy root (A. tuberosa) and Antelope horns. Think of Desert milkweed as Antelope horns plus more ‘heart’. Unlike Antelope horns, the plant does not have a coherent history of traditional use, or at least one that has been recorded or revealed, so approach its use with prudence.