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I take 5,000 a day when I remember to do it. I have chronic fatigue syndrome and I have noticed that when I take the D on a consistent basis I feel a bit better. Nothing has made me feel as good as taking the junk out of my diet, but I do think the D helps.I take mine in liquid drops. I got them at a local vitamin store for about 10 dollars.
I live in Portland, Oregon where we rarely get sun 9-10 months out of the year and started taking 12,000 IUs a few years back to start with. Over the years I have stayed consistent but dropped the dosage significantly down to 5,000 IU's per day. I think I paid $10-12 for a 6-8 month supply well worth every penny.
In studies of Vitamin D and cancer, 400 IU per day did not discernibly decrease cancer, but 1100 IU per day did. I've seen one article that said 10,000 IU per day was like "not quite getting a sunburn" each day. Personally I target 4000 IU per day, but I sometimes forget.If you are from a sunny Mediterranean country, though, and you get outside a reasonable amount, you may not need the supplement.