Often people ask me how do you know herbs work? I often wish I had a one sentence answer. Sometimes I feel this answer is equal to explaining the human genome. Not every answer can be that simplified. Whether I'm being asked about the herbs in the Pac Herbs packets or about Chinese herbs or Western herb or South American herbs, the first clarification is what type of herbs are we discussing?
Because my background in Traditional Chinese Medicine and herbs of Asia, I will refer to only "Chinese herbs" here. Plants or Chinese herbs have naturally grown in the wilderness of Asia since earliest recorded history. Because many are not the same species of plants as those found in North America many of these herbs are new to Americans. Mother Nature gives us an abundance of plants both for food and medicine on every continent. History tells us man has always used local plants for medicine and we still do. Even pharmaceutical drugs are based on chemicals found in nature.
Regardless, where a plants origin is, it's the historical use of herbs for healing that make the difference between knowing which herbs work and which do not. When we view hundreds of years of a specific plant or herb usage for a certain condition then we have a collective experience that is replicated over time. How do we know an herb works? Generations of mankind using the same plants or the same combinations of plants for the same illness with successful results proves this.
This is the short answer, the long answer gets more technical about the various chemical markers, constituents in each plant specie and processing and packaging that all impact the final product. This is why I mention, which is easier to answer, questions on the human genome or questions describing how and why herbs work. If your not satisfied with this short answer here. Check out a few other posts here and here which go into a bit more depth. Whatever answers you are looking for, it's clear that even modern medical analysis will never have all the answers on herbal medicine that history holds.