I have taken 5000IU of vitamin D3 daily for the past year and a half. This is significantly above normal dietary recommendations, but has been suggested to be safe in one study cited below. I do not recommend taking such doses without a doctor’s supervision and checking blood levels.
As is the case with many vitamins, there are strong correlations between deficiency and poor health. However, association does not prove causation. With vitamin D supplementation, many studies suggest benefits for bone strength in the elderly, and better glucose tolerance. However, a few studies suggest that supplementing with vitamin D also allows more calcium to get into atherosclerotic (“hardening of the arteries”) arteries, contributing to heart attacks and/or stroke.