Products can also have additional ingredients that are not necessarily listed on the label. The first such study in the United States of St. John's wort is currently under way. Until there is more control of the administration of the herb, the test results are like comparing apples to oranges, or maybe even apples to zucchini or cows. An active ingredient has been identified in some herbs. This has been the portion used in some of the more definitive studies.
Active ingredients vary with plant growing conditions.
Drying, mincing, and otherwise changing the original fresh herb also affects the active ingredient. Extracts, tinctures, and powders have varying effectiveness. If the form of the herb a person uses is different from that used in the study of the herb, there is no guarantee that the herb will produce the same effect. Some herbs may have more than one active ingredient. The multiple active ingredients may be much more effective than any one alone.