Although many people, especially the elderly who stay mostly indoors, are not getting enough vitamin D, and supplementation could be helpful in many medical conditions, there are concerns with high dose use. At the annual meeting of the American Academy of Dermatology, Dr. Martin Weinstock of Brown University, Providence, R.I., mentioned findings of significantly increased risks of falls and fractures associated with vitamin D megadoses. Studies examined very-large intermittent doses of the vitamin given to elderly patients. For example, a single, annual intramuscular injection of 500,000 IU increased the risk of fracture by 50% over 3 years. And oral doses of 60,000 IU per month, given once a month, increased the chance of both falls and fractures.
Comments: I am not sure how to interpret this yet, and do not know if smaller, but still high, daily doses lead to the same risks, but I mention this as a reminder that people be a little cautious. I do not think most adults need more than 2,000 units a day as a supplement. Read more about vitamin D.