From what it sounds like you have to MASS quantities of Vit D for it to be harmful. I always take a lot during the winter, and I have felt better in the gym, in my head, in the bed, whereas usually in winter life blows. I think I was meant to live in the sun, and who knows when you'll get that in the Midwest. I saw some research recently that low vitamin D is one of the body's responses to chronic disease or infection, thus supplementing could be debilitate this response. However, I thought I saw later they had trouble linking low Vitamin D to any chronc disease.... go to Ergo-Log.com and type in vitamin d and there's a relatively extensive list of some of Vitamin D's purported benefits.