There can be harm from taking D3 as a suppliment. If, that is, you don't get enough K2 complex.
D3 makes calcium more bioavailable (among things).
The K2's "tell" calcium where to go. It should be going to bones. Without enough K2, calcium goes to soft tissues, like arteries.
Therefore, too much D3 without enough K2 complex to tell the newly bioavaialble calcium where to go can send that calcium to soft tissues instead of bone. And yo don't want that.
I beleive this is one reason why women have caught up with men in terms of heart issues. Women tend to take lots of calcium, but being that they're not getting any K2 complex (which comes from things like raw milk and butter from cows eating green grass) that calcium tends to go to the arteries. Ooops.
So, I do take 5,000 -10,000U of D3 a day. I also take a K2 complex. I also drink raw milk and use KerryGold butter.