If your doctor tells you to take vitamin D can you take vitamin D3?

Yes, actually you SHOULD take Vitamin D3.

You have two choices of Vitamin D, Vitamin D2 and Vitamin D3.

Vitamin D2 is not absorbed as well as Vitamin d3 and it has a shorter half life.

Even though foods are still fortified with Vitamin D2, many researchers believe that Vitamin D2 is "no longer suitable for supplementation or fortification".