Herbs are considered dietary supplements, and basically part of the food chain. Prescription drugs, to come to market, must undergo hundreds of millions of dollars of clinical testing to prove their efficacy and measure their toxicity. Herbs are nature's medicines that have sometimes 5,000 years of historical use for many conditions. Only in recent times have universities and private industry taken these traditional uses and run clinical studies to determine how they might be working or have worked in the past and predict what they might be useful for in the future. However, they are still dietary supplements, and really are subject to claims that are just associated with structure and function of the human body.
You have to understand that there is a very strong political aspect to herbal medicine. Between 25% and 50% of pharmaceutical drugs on the market today are derived directly from, or from a model of, a plant chemical. This plant chemical cannot be used directly as it comes from nature because it cannot be patented and protected financially, so the chemical must be altered into a new entity, an analog, and then tested for efficacy and toxicity. If this new chemical works and isn't too poisonous, it is entered into the hundreds of millions of dollars of testing to bring a new drug to market.
The herbs in themselves, as whole herbs, are generally much milder, generally not toxic, take longer to work, and are much more complicated than pharmaceutical drugs. Pharmaceuticals are monostructures; that is they contain one single chemical. Herbs can contain hundreds and hundreds of chemicals, each doing something different in the bodies, oftentimes offsetting toxicities; that is if there is a chemical that might raise blood pressure there may be a secondary chemical that offsets chemical A's ability to act in this manner.
In terms of manufacturing, many medicinal herbs today are manufactured by pharmaceutically licensed laboratories. All herbs and dietary supplements must comply with government GMPs - good manufacturing procedures. This is not to say that some companies don't use inferior ingredients.
