For a long time they were telling people not to take more than 2,000 iu a day of vitamin D, but then more recent research showed that 5,000 iu and perhaps even up to 10,000 a day may be safe enough. I am convinced that ny vitamin D levels are low as I don't get much sunlight, and those with mercury toxicity tend to have low vitamin D levels. I also read today that vitamin D boosts NK cell activity which is low in CFS(mercury toxicity).