Studies have found highly variable amounts of active ingredients in products randomly tested. A 1995 Consumer Reports investigation reported finding huge variations in ginseng concentration in the ten different brands they tested. The Los Angeles Times investigated St. John's wort in 1998. Other issues to consider when purchasing dietary supplements in the United States are ones of cleanliness, proper identification of the plant, and purity of the supplement. If the plant was gathered in the wild, there is no way of knowing if the pickers correctly identified the plant each time they picked.
Manufacturers are unlikely to check each plant portion for validity.
Often, volatile oils-containing herbals have their aromas enhanced by the addition of quantities of essential oils from other sources. 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.