Content is the most difficult one to control, since in most herbal drugs the active constituents are unknown.
Sometimes markers can be used which are, by definition, chemically defined constituents that are of interest for control purposes, independent of whether they have any therapeutic activity or not.
Herbal drugs are usually mixtures of many constituents.
The active principle(s) is (are), in most cases unknown.
Selective analytical methods or reference compounds may not be available commercially.
Plant materials are chemically and naturally variable.
The source and quality of the raw material are variable.
The methods of harvesting, drying, storage, transportation, and processing (for example, mode of extraction and polarity of the extracting solvent, instability of constituents, etc.) have an effect.