It's a hormone and best taken with calcium for absorption.
Great resource: [link to www.vitamindcouncil.org]
If you live in the north my doc recommends 10,000/day in winter - 5,000 in
the Fall and Spring - 500 in the summer June-Aug If you are getting at least 15 minutes of sun (without sunscreen).
You need to have your blood tested if you take 5,000+ for more than 3 months.

It is wonderful for your body - keep taking it. Just get informed first.