usnic

Usnic refers to usnic acid, a secondary metabolite produced by many lichens. This compound is particularly common in species such as Usnea (old man’s beard) and is also found in other lichen genera including Cladonia, Evernia, Lecanora, and Ramalina. In lichens, usnic acid contributes to coloration, largely imparting yellow to yellow-green hues, and can be extracted as crystals for study and use.

Chemically, usnic acid is a lipophilic dibenzofuran derivative that occurs as two enantiomers. It is relatively insoluble in water but dissolves in organic solvents. The production of usnic acid occurs in the fungal partner of the lichen, via polyketide biosynthetic pathways, and its levels can vary with species, geography, and environmental conditions. Usnic acid is relatively stable in dry conditions but can be sensitive to light and heat.

Biological and practical aspects of usnic acid include its antimicrobial properties, with activity reported against certain bacteria, fungi, and some viruses in laboratory studies. It has historically contributed to the yellow pigmentation of lichens and has been explored as a natural dye and in various cosmetic and pharmaceutical contexts; however, many potential medical applications remain under investigation and are not established therapies.

Safety and regulation note that high doses of usnic acid can be hepatotoxic in humans and animals. Because of safety concerns, products intended for ingestion have been discouraged or regulated in many jurisdictions, and cosmetic or botanical products containing substantial amounts may be subject to regional restrictions. As with many natural compounds, usnic acid is primarily of interest as a biochemical and ecological object rather than as a widely accepted medicine.