If you said food, you d be right - sort of. Just because herbs are natural, remember that they work in the body like drugs. You may not be getting what you pay for. There are no required standards so the herb or food may or may not contain what the label states it contains. One example is ginseng, an expensive herb.
Studies test the effectiveness of certain parts of the plant.
One part of the plant may not be as effective as another but may be sold anyway. Echinacea is one example. In addition, the pills you buy may not contain the recommended amount of herbs that work. Herbs and drugs may not mix. There is much we do not know about herbs.