I went through the exact same thing. I'd been taking the same brand of suppliments over and over for a very long time. First of all my adrenals were so bad off, they could not handle vitamins. So, on the advise of Dr. Lam, I slowly tapered down and than I did not take hardly any vitamins for 2 weeks. In the meantime I did some research and found that synthetic vitamins really are not good for you. I found a high quality supplimental line called "Garden of Life" they are all derived from "whole foods," all organic. Than I started up very gradually again, first with Garden of Life "Living C" taking only one a day for 3 days, than adding two a day, taking them 4 hours apart. Than added a small amount of Vit. B complex C0-enzyme from "Wellness Resources.." I do good on this brand. I think one can build up a tolerance if anything is taken for to long a time, especially if it is the same brand. Over 2 months, I noticed a gradual improvement in my symptoms. I also found a sublingual Panathene, so it goes directly into the blood stream, bypassing the liver.
But, just wanted to share the info. as it has been working for me. I also take a day off every two weeks and don't take any suppliments, to give my body a break.