Re: building up immunity to herbs.....????
It really depends on the herb. I have several books on many herbs as well as research on the internet before I consider taking a new herb. Many books will have warnings if any are needed for a particular herb. Also, herbs can do things to one person that will not happen to most people. For instance, I tried fever few for my ostio arthritis. It has to be taken for a while before its anti-inflammatory properties kick in. After 2 or 3 weeks of taking it, I got very dry chapped skin on my legs. I didn't connect it to this at first because it is winter and I tend to have dry skin in winter anyway. When I developed a rash on my legs also, I suspected the feverfew was causing it and stopped taking it. The rash a very dry skin cleared up. I read one warning that if you are very allergic to poisin oak and poisin ivy, you might get a reaction to feverfew also. I got a reaction to feverfew, but different from the poisin rash and blisters.
Herbs I take regularly are turmeric for my ostio arthritis, hawthorne berry for peripheral circulation, dandilion root for my hemroids, and I know for a fact from personal experience that these work and continue to work over very long periods of time.