Anything in excess can make a dragon, or a human sick. But not giving enough can also make them sick. I have never had a dragon get D3 poisening in the 10 years that I've owned dragons, and they have all used the same calcium supplement with D3 their entire lives. The only way it is possible they will get D3 poisoning is if your dusting with d3 everyday, and they are living in a tank with an MVB or living outside.
For comparative examples since I keep repeating the same thing,
A reptisun 10.0 at 6 inches away produces a reading of 75. If there is a screen between the reptisun and the animal, at 6 inches it produces a reading of 56.
A new powersun at 6 inches produces a reading of 185.
a new T-rex spotlight can produces a reading of upwards of 400 depending on its angle!
And a mega ray produces readings of around 330 at 6 inches.
I believe last time I read about it, In Australia, the sun at noon produces a reading of 600+ something of uvb. But the thing is, its not noon all the time and the level of uvb varies throughout the day. Another research project I read had beardies in different tanks all exposed to differen't uvb bulbs and powers. They found that the beardies seemed to like to stay in the areas where they were getting a reading of 30-150. So more isn't always better.
Uh im' rambling. All that was to explain why D3 with a reptisun, and why no D3 with an MVB.
-Brandon