Is there a governmental agency that monitors the usage of herbal medicines?

FDA to some degree: The fda (food and drug administration) monitors the usage of herbs in the us, though they are not monitored very strictly. Herbs do not require fda approval before being marketed, though the fda regulates health claims made for herbs. If the fda gets evidence that an herbal product is hazardous it intervenes, such as when it banned the herb ephedra in 2004.