Yes, many herbs will lose their effectiveness if you use them all the time or too frequently, which is why they're best saved for when they're really needed. Medicinal plants are often dilute forms of natural drugs, not foods or dietary supplements. For that reason, you should not take them casually or for no good reason, any more than you would take a pharmaceutical drug casually or for no good reason. When using medicinal herbs, bear in mind that individuals vary in their responses, so it is best to let your experience be your guide and use only remedies and brands that give you consistent results.
The exception to the general rule about using medicinal herbs only when needed for a specific purpose are the tonic herbs, or adaptogens, such as ginseng, that can strengthen or invigorate the healing system, and Asian mushrooms that enhance immunity. I use and recommend a number of tonics including garlic, ginger, turmeric, and green tea for toning the cardiovascular system, reducing inflammation and lowering risks of cancer. Ginseng comes from several species in the genus Panax, whose name comes from the same root as "panacea" meaning "all-heal." Used regularly, ginseng increases energy, vitality and sexual vigor, improves skin and muscle tone, and helps build resistance to stress. I often recommend ginseng, especially the American species, to chronically ill patients and to those who are debilitated or lacking in vitality.
I also frequently use mushrooms, such as the Chinese caterpillar fungus (Cordyceps sinensis), as tonics for improving athletic performance and increasing energy, and species like reishi (Ganoderma lucidum), shiitake (Lentinella edodes), and maitake (Grifola frondosa) to boost defenses against infections and cancer. Clinical research on medicinal mushrooms suggests that taking more than one species at the same time often gives better results.
Be sure to give any tonic a reasonable trial - take it regularly for at least two months. With some, you'll experience obvious effects - perhaps an increased sense of well-being and more energy. Or you may find that, over time, a tonic improves your response to both physical and emotional stress and decreases your susceptibility to illness.
Andrew Weil, M.D.