The Aztec Classic Codex of 1552 recommended the root as a treatment for cough, and the leaves do have demulcent, expectorant qualities. The dried leaves contain hernandulcin, a sesquiterpene compound stated to be 1000 times sweeter than refined sucrose. Tea made from the leaves has been used externally on cuts and abrasions and as a mouthwash to fight gum disease and tooth decay.
Toxicity
But, the leaves should not be used as sweetener because they also contain camphor, which makes this herb quite toxic.

Part used::
Leaves

Origin:
Mexico

Hull, Kathleen; Photog. Hull, Meredith /Indiana Medical History Museum: Guide to the Medicinal Plant Garden./ USA: Indiana Medical History Museum. 2010. -- p. 58.