Usnic acid is a lichen metabolite with antioxidant, anticancer, antifungal, antiprotozoal, and antimicrobial activities.Usnic acid (>50 mg/kg) reduces the number of indomethacin-induced gastric ulcers in rats through decreased lipid peroxidation and decreased myeloperoxidase activity. It is fungicidal against C. orthopsilosis and C. parapsilosis (IC₈₀s = 7.8 and 15.6 μg/ml, respectively) and bactericidal against seven vancomycin-resistant E. faecalis strains (MIC = 125 μg/ml). Usnic acid (25 mg/kg, injected intralesionally) reduces L. amazonensis parasite loads by 72.3% in footpads of infected mice. It also reduces growth of A2780, HeLa, MCF-7, SK-BR-3, HT-29, HCT116, HL-60, and Jurkat cancer cells in vitro (IC₅₀s = 48.5-199 μM).

Usnic acid is an antimicrobial, antitumor enzyme inhibiting agent shown to uncouple oxidative phosphorylation in mouse-liver mitochondria. Usnic acid induces necrosis in certain cells.

Usnic acid is a naturally occurring dibenzofuran derivative found in several lichen species with the formula C18H16O7. It was first isolated by German scientist W. Knop in 1844 and first synthesized between 1933-1937 by Curd and Robertson. Usnic acid was identified in many genera of lichens including Usnea, Cladonia, Hypotrachyna, Lecanora, Ramalina, Evernia, Parmelia and Alectoria. Although it is generally believed that usnic acid is exclusively restricted to lichens, in a few unconfirmed isolated cases. The compound was found in kombucha tea and non-lichenized ascomycetes. At normal conditions, usnic acid is a bitter, yellow, solid substance.It is known to occur in nature in both the d-and l-forms as well as a racemic mixture. Salts of usnic acid are called usnates (e.g. copper usnate). Usnic acid can be used in cosmetics material.

Usnic acid is a major metabolite from a number of lichen and fungal species and is perhaps one of the longest known microbial metabolites, first isolated in 1844. Usnic acid exhibits a broad range of biological activity, indeed it is easier to list the bioassays in which it does not exhibit activity. More often than not, potency is low and of limited value. Usnic acid is an important metabolite for dereplication in discovery research programs.