Toxicity studies of Usnea lichens containing (+/-)-usnic acid administered in feed to F344/N Nctr rats and B6C3F1/Nctr mice.

Usnea lichens and purified usnic acids have been used historically in traditional herbal medicine as bactericidal and antimicrobial agents. Usnea lichens contain 1%-3% (+/-)-usnic acid and extracts of these lichens are currently marketed in the United States as herbal antimicrobial agents. (+/-)-Usnic acid exhibits membrane proton uncoupling activity, which not only forms the mechanistic basis of its bactericidal action, but also has provided a rationale for its use as a fat burning, weight-loss agent. Purified (+)-usnic acid has been marketed in the United States for this purpose either alone or in combination with other chemical agents. Use of some of these fat burning products that contain (+)-usnic acid has resulted in serious liver damage. This study investigated the potential toxicity of ground Usnea lichens containing (+/-)-usnic acid in male and female Fischer 344/N Nctr rats and B6C3F1/Nctr mice that were exposed via feed for 3 months. F344/N Nctr rats were administered 0, 30, 60, 120, 360, or 720 ppm in feed, while B6C3F1/Nctr mice were administered 0, 15, 30, 60, 180, or 360 ppm in feed. (Abstract Abridged).