From online magazine The Scientist:
“For more than 5,000 years, herbs and other natural ingredients have been used for medicinal purposes. Today, people use such concentrated natural products as supplements to help combat various diseases, from depression to cancer, as well as to boost health, including immunity and memory. Based on Natural Standard research, in the United States alone more than $40 billion is spent each year on these products. An estimated 60 percent of cancer patients try natural products, and 40 percent take vitamins or other dietary supplements.
Just because herbal products are developed from plants, they cannot necessarily be deemed harmless. Like prescription drugs, herbs and supplements may cause unwanted side effects and can interact with prescription drugs, other natural products, or foods, and may even alter diagnostic and laboratory test results. Unlike regulated drugs, however, dietary supplements can be marketed without approval from the US Food and Drug Administration. As a result, herbal products are often not thoroughly evaluated by the FDA unless there is sufficient evidence to prove that they are unsafe. Partly due to this regulatory freedom, as well as to a lack of available clinical research, interactions between herbs and conventional drugs are often overlooked…”
They can and do however. Read more: