It is to the detriment of many that because a "medicine" or "herb" comes from a plant that the safety of it is never questioned. Many herbs that were originally used in the culinary sense such as garlic, onions, ginger, parsely, sage, rosemary, and thyme have been found to be beneficial to the body. On the other hand, is it well known that nutmeg can be toxic when used in amounts of more than one whole nutmeg? (Freeman, 2009)
Scientists do not know much about herbs, much to the dismay of several people. There are many conditions that make the study of herbs so difficult. One condition is that there are so many herbs and the scientific study of herbs is in it's infancy. Herbs often contain so many different chemicals and compounds, it is a tedious process of study. Other issues that intervene with research include how each and every herb interact differently, based on the content of the soil in which they are grown, the time of year it is harvested, and the portion of the plant that is used.
When looking for herbal supplements, make sure to read the labels carefully. You should see "standardized extract" on the label. This means that there is a guaranteed amount of the herb within each pill or capsule. Remember to discuss with your doctor the use of herbal supplements. And always be cautious when using them. Just because they come from a plant, are called "natural" (as many people will tell you!), they can be helpful, but may still carry potential danger. Be safe when using herbs!
Freeman, L. (2009). Complementary and alternative medicine(3rd ed.). St. Louis, MO: Mosby Elsevier.