Most people are quite low in Vit D it has been found recently in US and other countries. This has lead to screening for it at many
doctor's offices.

The general rule for supplementation is 2000IU Vit D3 for those who have not had testing.

OR

1000IU D3 daily for every 10 pts needed to raise to a target result. This is an average rule, as some people may need more.

D2 from RX version is not very efficient and is no longer recommended by the experts.

Here is a good thread with 2 videos on it explaining in detail:
http://neurotalk.psychcentral.com/thread92116.html

The only serious consequence of very high supplements, is a possible hypercalcemia (elevated calcium) in the blood. But this is not common and can be prevented by not using high dose calcium supplements when Vit D is given in high dose. Vit D will enhance your absorption of calcium from foods eaten.