According to the passage, ----.

Many herbs contain hundreds or even thousands of
chemicals that have not been completely
catalogued. While some may ultimately prove
useful as therapeutic agents, others could well
prove toxic. Most herbal products sold in different
parts of the world are not standardized, which
means that determining the exact amounts of their
ingredients can be difficult or impossible. Moreover,
many herbal practitioners are non-physicians who
are not qualified to make appropriate diagnoses or
to determine how herbs compare to proven drugs.