Phytotherapy is the study of the use of extracts from natural origin as medicines or health-promoting agents.

Traditional phytotherapy is a synonym for herbalism and regarded as alternative medicine by much of Western medicine. Although the medicinal and biological effects of many plant constituents such as alkaloids ( morphine, atropine etc. ) have been proven through clinical studies there is debate about the efficacy and the place of phytotherapy in medical therapies.

Modern phytotherapy, following the scientific method, can be considered the study on the effects and clinical use of herbal medicines.

In herbal medicine, plant material that has been processed in a repeatable operation so that a discrete marker constituent is at a verified concentration is then considered standardized. Active constituent concentrations may be misleading measures of potency if cofactors are not present. A further problem is that the important constituent is often unknown. For instance St John's wort is often standardized to the antiviral constituent hypericin which is now known to be the active ingredient for antidepressant use. Other companies standardize to hyperforin or both, although there may be some 24 known possible constituents. Only a minority of chemicals used as standardization markers are known to be active constituents. Standardization has not been standardized yet: different companies use different markers, or different levels of the same markers, or different methods of testing for marker compounds. Herbalist and manufacturer David Winston points out that whenever different compounds are chosen as 'active ingredients' for different herbs, there is a chance that suppliers will get a substandard batch (low on the chemical markers) and mix it with a batch higher in the desired marker to compensate for the difference.