Herbal remedies have good evidence for efficacy.
They are effective because they contain chemicals that interact with receptors or enzymes. Just as aspirin, morphine and cocaine are herbal derivatives with both good and bad effects, we should not be surprised that other herbal remedies may also have associated harm. Adverse reactions to herbal medicines are not just due to the herbs themselves. It gives information on the regulatory framework for herbal remedies. Most usefully it describes mechanisms of action, cautions, drug interactions and adverse effect information, and is well referenced.