Usnic acid is a polyphenolic compound that is found in lichens. It has been shown to have potent antioxidant and anticancer properties. Usnic acid inhibits mitochondrial respiration by binding to the complex I of the electron transport chain, thereby preventing proton pumping. This leads to a decrease in the mitochondrial membrane potential, which may result in apoptosis or necrosis. Usnic acid also inhibits cancer cell proliferation by binding to fatty acids and inhibiting their synthesis. Usnic acid also protects liver cells from oxidative damage through its antioxidant activity, thus preventing liver cirrhosis. The neuroprotective effect of usnic acid may be due to its ability to increase the production of glutathione and other antioxidants in the brain, which can reduce neuronal death caused by excitotoxicity or oxidative stress.