I'll answer your question about milk thistle first, and then discuss tonics. As you know, I recommend taking extracts of the herb milk thistle (Silybum marianum) for liver problems. Milk thistle is non-toxic, and European research has shown that it stimulates regeneration of liver cells and protects them from toxic injury. I think it is unlikely that milk thistle is causing your headaches or is responsible for your feeling unwell, but there is always the possibility of an unusual individual reaction to any substance. I would suggest omitting it for a couple of weeks and then adding it back to see if you feel better when you stop taking it and, if so, whether your headaches and other physical complaints return when you resume. You might also try switching to another brand: Thisilyn, from Nature's Way is a good choice.
As for tonic herbs, you certainly can take more than one at a time. I'm a great believer in natural tonics to strengthen or invigorate the healing system. The word tonic comes from a Greek word meaning "stretch," and tonics stretch or tone the healing system the way physical exercise tones muscles. I use and recommend a number of tonics - among them, garlic, ginger and green tea for toning the cardiovascular system, reducing inflammation and lowering risks of cancer. Ginseng is another famous tonic product. It comes from a 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 builds resistance to stress. I often recommend ginseng to chronically ill patients and to those who are debilitated or lacking in vitality.
I also frequently use mushrooms as tonics, such as the Chinese caterpillar fungus (Cordyceps sinensis) for improving athletic performance and increasing energy, and species like reishi (Ganoderma lucidum), shiitake (Lentinella edodes), and maitake (Grifola frondosa) to boost immunity. Research on medicinal mushrooms suggests that taking more than one at the same time gives better results. I take a mushroom tonic called Stamets 7™ containing seven different species. Visit www.fungi.com for more information.
Be sure to give any tonic you use a reasonable trial. That means taking it regularly for at least two months. With some, you'll experience obvious effects - for example, an increased sense of well-being or more energy. Even if you don't notice any immediate effects, pay attention to changes in how you respond to stress, both physical and emotional, and in your susceptibility to illness.
Andrew Weil, M.D.