You might have brushed by it in the forest, where this hairy-looking symbiosis between algea and fungi perches on tree limbs. The look of the lichen usnea explains its nicknames: "old man's beard," "tree's dandruff," "women's long hair," and "beard lichen." For centuries, it's been considered a handy medicinal. People grab some to dress wounds, or take it internally for infections or oral inflammation. But in the 1990s, when manufacturers of weight-loss drugs started adding sodium usniate (usnic acid) to their formulas, several cases of liver damage emerged. Usnic acid was implicated, and people were warned away not just from the drug LipoKinetix, but from internal use of usnea, too. Most traditional uses of usnea are either topical, or short-term, and it's unclear whether usnea itself is risky, or if the problem lies with long-term consumption of large amounts of the isolated chemical constituent instead of the whole plant.
(Podcast: "The Plant Detective," 11/29/14)
