If a person has too much calcium, why would taking vitamin d be harmful?

It increases calcium: Vitamin d increases calcium absorption, and if it gets too high, it can cause lots of problems such as stomach upset, muscle weakness and even depression. This is why taking too much vitamin d is a problem for most people - 800 units daily is plenty for most. If you have high calcium already, though, you might want to avoid taking extra, and limit sun exposure (raises active vit d).