The public portrayal of herbal medicine usually falls along the lines of folk remedies that are useful for mild conditions, with effectiveness that borders on placebo. Doctors will blandly suggest that taking an herbal supplement won't likely hurt, but it may not help much either.
As professional, medical herbalists know, this is far from the whole truth. I've had oncologists marvel at how it could be possible that their ovarian cancer patient could have gone 8 or 9 rounds of chemotherapy without red and white blood cell counts becoming dangerously low, and tell their client, 'whatever you're doing, keep doing it'. In a case like that, herbal medicines aren't treating a distinct disease, but they are assisting with the health and function of the fast dividing cells in the bone marrow that would otherwise be suppressed by chemotherapy. This is far from folk medicine (I would certainly never disparage folk medicine either!).
The dosages, and herb selection used in serious illness are obviously much different than an herbal blend one can pick up at a health food store. Sometimes the daily dose of an herb may be equal to an entire bottle of an encapsulated herbal supplement on the shelf. We can prescribe these herbs correctly and safely in large, effective doses because of traditional use, but also because of a great deal of pharmacological research that has been done on medicinal herbs over the last century.
Traditional guidance on which herbs to use for which condition are a treasure trove, and the accumulated clinical experience of over 2000 years of clinical practice should never be ignored. Scientific study of individual herbs and herbs in formulae have validated and expanded our understanding of how they work. Importantly, they have also given us a more precise understanding pharmacologically than was possible in pre-modern times. An excellent example is Huang Qi, Astragalus. Once thought to be an booster of energy and immunity (which is certainly is), we have a more precise understanding now of its immune modulating properties. We know that it enhances T Helper 1 cells, and suppresses T Helper 2 cells, making it useful for boosting certain types of immune response, while actually suppressing some chemical drivers of autoimmune diseases. This expanded understanding helps us more precisely prescribe in serious conditions.
There are herbs that have properties that are also quite outside traditional uses. Sang Bai Pi, or the bark of the mulberry tree, was classically used for lung conditions as an anti-inflammatory. However modern pharmacological study has shown us it has profound abilities to regulate blood sugar levels, assist with kidney excretion of uric acid to assist with gout, and is a CCR5 ligand antagonist that not only blocks a number of unhelpful inflammatory pathways, but could potentially block the entry point of the HIV virus into white blood cells. The complexity and effectiveness of many traditional herbs is gradually coming into better focus through pharmacological research, which will only benefit more patients in the future. Unfortunately, relatively few alternative and natural practitioners are fully aware or trained well in the pharmacology of herbal medicines, and it is my sincere hope the integration of traditional and modern practice accelerates for the benefit of our patients.